COMlMUNITY CONSEQUENCES OF HABITAT USE AND PREDATION BY

COMMON EIDERS IN THE INTERTIDAL ZONE OF PASSAMAQUODDY BAY

A thesis

Presented to

The Faculty of Graduate Studies

of

The University of Guelph

by

DIANA JEANNE HAMILTON

In partial fulfilment of requirements

for the degree of

Doctor of Philosophy

November, 1997

0 Diana J. Hamilton, 1997 NationaI Library Bibliothèque nationale du Canada Acquisitions and Acquisitions et Bibliographie SeMces services bibliographiques 395 Wellington Street 395, nie Wellington OttawaON K1AW OttawaON KtAON4 Canada canada

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COMMUNTY CONSEQUENCES OF HABITAT USE AND PREDATION BY

COMMON EIDERS IN THE INTERTIDAL ZONE OF PASSAMAQUODDY BAY

Diana Jeanne Hamilton Advisor:

University of Guelph, 1997 Professor T. D. Nudds

1 investigated interactions between Common Eiders (Somaterza mollissima) and

the intertidal invertebrate comrnunity in Passamaquoddy Bay. New Brunswick. 1

determined that young duckiings were dependent on rockweed (AscophyfIumnodosum), a

perenniai macro aiga, to obtain food, as indicated by their feeding methods and timing, but

that feeding by older duckiings was unaffected by tide level. Commercial harvest of

rockweed may therefore be detrimental to the youngest duckhgs, but have little effect on

older birds. 1 also found that care of ducklings constrained female behaviour and habitat

use. This may be a local effect caused by very high predation pressure exerted on

duckiings at this site. Addt eiders were site-selective predators of blue mussels (Mytzhs

eduiis), and preferences varied depending on season and prey availability. Selection of

prey corresponded with predictions of the sheil mass minimization hypothesis and also with the nsk-averse foraging hypothesis. Using a series of predator exclusion cages, 1 found that in musse1 beds, ducks had little effect on species richness or diversity, but sigdicantly reduced total biomass by feeding primarily on blue mussels. Biomass effects persisted throughout the experiment in sites subjected to abiotic disturbance, but disappeared in undishirbed sites within a year.

Exclusion of ducks led to an increase in dogwhelks (Nucella hpzllus) under natural site cages, and by feeding on blue mussels they obscured the effect of eider predation. This indirect effect is an example of compensatory predation by a formerly redundant predator.

Effis of eider predation on rockweed bed invertebrate biornass were small, and again duck predation did not influence species nchness or diversity. Harvest of rockweed had no effect on invertebrate abundance, and little eff'on predation by ducks.

DiBerences in results between the two areas can be attributed to habitat heterogeneity and relative abundance of whelks. Although the effect on biomass was not measurable, predation by ducks may have contnbuted to maintenance of rockweed habitat. When ducks were excluded, substrate cover by blue mussels increased and rockweed declined.

These redts indicate that ducks are signifiicant predators in this community, and that waterfowl should not be neglected as predators in community studies. ACKNOWISDGEMENTS

1 am gratefid to mam, people for helping me with various aspects of this research.

First, 1 would like to th& my advisor, Tom Nudds, for his advice, support,

encouragement, and patience during the past four years. 1am grateful to my advisory

committee members, Dave Ankney, Eiizabeth Bodding, and Peter Yodzis for advice and

suggestions throughout rny degree, and for helpfbl comments on my thesis. I would also

like to thank John FryxeLI for helpful comments and insightfbl questions at various times,

which ultimately improved the thesis. 1 am gratefid to members of my examination

conmittee, Sandy Middleton, Tim Wootton, John Fryxe11, Peter Yodzis, and Tom Nudds

for helpfid changes to the thesis. Thanks to Claudia Schubert-Kuener, Mark mer, Carey

Bergman, Brent Gurd, and Chris McLaughlin for quizzing me before my quakfjmg exam,

making usefbi suggestions, and providing much needed distractions when 1 was in Guelph.

I received a great deal of help with the field and lab components of this research.

Co~aBrdar, Andrea Cox, Danielle Downing, Jim Godfiey, Jennifer Neate, Kim Smith,

and especiaily Cindy Doherty provided excellent field assistance. Biii Hogans provided

adult ducks, and Glyn Sharp, Ian Barkhouse, and Pat Kehoe provided ducklings for food

habits analysis. Discussions with GIyn Sharp and Pat Kehoe early in the study were

helpful. The Engineering department at the University of New Brunswick (Saint John)

aiiowed me to use the tensometer to cmsh blue mussels and Cedric Boone provided assistance. 1 am grateful to ail these people. 1 would also keto thank the staff of

Huntsman Marine Science Centre for assistance while conducting research there, and the biology department at the University of New Brunswick (Saint John) for aliowing me to use slide making equipment.

1 wish to acknowledge and thank the foiiowing organkitions for providing research funding for this project: Institute for Wetland and Waterfowl Research (IWWR),

Delta Waterfowl Foundation, Natural Sciences and Engineering Research Council

(NSERC)(research grant to T. D. Nudds), New Brunswick Department of Fisheries and

Aquaculture, Canadian Wddlife Service University Research Support Fund. Personal fiinding was provided through: NSERC Postgraduate Fellowship, Ontario Graduate

Scholarship, Bomeycastle FeUowship (MrWR), Elgin Card Avian Ecology Scholarship

(University of Guelph), Hunt sman Graduate Scholarship (Huntsman Marine Science

Centre), Faculty of Graduate Shidies Scholarship (University of Guelph) .

FinaUy, 1 would like to thank my husband, Matthew Litvak. He helped me with the field work that nobody else wanted to do, read and commented on my thesis, gave me many helpfùl suggestions, and supported and encouraged me throughout.

LIST OF TABLES

1.1 .Redts of MANOVAS of duck age and tirne of day on behaviour ...... 15

1-2 O Results of MANOVAS of duck age and tide level on behaviour ...... -22 1-3 .Results of ANOVA of duck feeding time...... -29 2.1 - Redts of x2 analyses of prey seleetion experiments...... -51 3.1 - Effects tested in ANOVA and MANOVA models ...... 84 3 -2- Results of ANOVAs of total biomass, species diverzity, and species richness ...... 93 3 -3 - Results of MANOVA of cornmon species dry tissue biomass ...... 96 4.1 - Results of ANOVAs of total biornass and biomass of periwinkles and whellcs ..... -141 4.2 - Resuks of analyses of blue musse1 and rockweed cover in cages and controls...... -157 4.3 - Cornparison of rockweed and musse1 bed habitat ...... -163 LIST OF FIGURES

Figure Page

1.1 .Behaviour of ducklings and adults at the four tide levels ...... 18 1 -2 .Behaviour of eiders at different times of the day ...... -21 1 -3 - Behaviour of ducklings and adults at dBerent hesof the day ...... 25 1.4 - The spent feediag for ducks in each age group ...... 31 2.1 - Tiles used in prey selection experiments...... -46 2.2 - Size fiequency distributions of mussels collected at Indian Point and Barr Road -34 2.3 - Tissue content of mussels collecteci at different times of the year ...... 36 2.4 - Regressions of sheli thickness and cmsbg resistance on shell length ...... 59 2.5 - Shell mass eaten by ducks and profitability of mussels of difEerent sizes...... 61 3.1 - Diagram of an experimental site...... -79 3 -2- Dry tissue biornass in cage and control areas ...... 87 3 -3 - Species richness in cage and wntrol areas ...... 90 3 -4- Species diversity in cage and control areas ...... 92 3 -5 - Dq tissue biomass of blue mussels in cage and control areas...... 100 3.6 - Dry tissue biomass of wheks in cage and control areas...... -102 3 -7- Average musse1 length in cage and control areas...... -105 3.8 - Size fiequency distributions of mussels collected from cages and wntrols ...... 107 3 -9 - Dry tissue biomass in cage and control areas afler reversal of exclosures...... 110 3.1 0 - Percent cover by blue mussels in cage and control areas ...... -113 4.1 - Dry tissue biomass in rockweed cage and control areas...... 143 4.2 - Species diversity in rockweed cage and control areas...... 145 4.3 - Species richness in rockweed cage and control areas...... -147 4.4 - Dry tissue biomass of blue mussels in rockweed cage and control areas...... 149 4.5 - Dry tissue biomass of periwinkles in rockweed cage and control areas ...... 151 4.6 - Dry tissue biomass of whelks in rockweed cage and control areas ...... 153 4.7 - Percent cover of mussels and rockweed in cage and control areas ...... 159 4.8 - Size frequency distributions of rockweed throughout the study ...... 162 GENERAL INTRODUCTION

A primary goal ofmany ecological studies is to understand how Werent species interact with each other and their environment. Biotic interactions such as predation and cornpetition, and abiotic factors such as physical disturbance are known to be important in structuring commdties (e-g. Comell 196 1, 1978, Paine 1966, 1969, Dayton 1971,

Lubchenco 1978, Lubchenco and Menge 1978, Sousa 1979a,b, Wootton 1992, 19934b,

1994%and many others). Numerous models of community regdation which relate these factors have been developed (e-g. Hairston et al. 1960, Menge and Sutherland 1976,

1987, Oksanen et al. 198 1, Schmitz 1992).

Although early theories of community structure (Elton 1927, Lindemann 1942,

Hairston et ai. 1960) were developed based primarily on terrestrial systems and lakes, that focus quickly expanded, and to date much of this research has been conducted in the rocb intertidal zone. This region is easily accessible and has many slow moving or sessile hvertebrates with short generation times, and interactions among species develop on a relatively short tirne scale (Menge 1997). Predator and prey abundance are easily manipulated throrigh use of exclosures and enclosures (HaU et al. 1990), aliowing researchers to rapidly gather detailed information.

Even though the marine intertidal system has been extensively studied, vertebrate predators have seldom been included in intertidal food webs (Pimm et al. 1991), and the significance of excluding these top predators is not well understood (Yodzis 1993).

Waterfowl (Anatidae), in phcular, have been negiected. In a synthesis of 65 food webs by Menge and Farrell (1989), only nine contained vertebrate predators (seven fish and two birds), and none considered waterfowl. They concluded that although cornpetition and predation are the most shidied animal interactions, very little is known about them at higher trophic lwels in the rocky intertidal zone. Similarly, Sih et al. (1985), in a 20-yr survey of predation studies, uncovered no intertidal studies involving birds, and ody three studies which involved marine birds, none of them waterfowl.

However, studies by Quammen (1984), Marsh (1986a,b), Hahn and Demy (1989),

Good (1992), Székely and Bamburger (1992), Dumas and Witman (1993), and Meese

(1993), among others, have shown that birds can have signincant effects on their invertebrate prey, and sometimes influence community structure. Therefore, it may be a rnistake to exclude them fiom intertidai community studies. Recently, Wootton (1992,

1993a,b, 1994% 1997) showed that predation by birds had significant direct and indirect effects on the invertebrate community in the Pacific Northwest.

Only a few researchers have studied the auence of predation by waterfowl in communities (e-g. Smith et rrl. 1986, Raffaelii et al. 1990, Nudds 1W2), and to my knowledge, there have been no attempts to examine the interaction of predation by waterfowl and abiotic disturbance. This may be a simcant omission, because abiotic factors are known to influence effectiveness of intertidal invertebrate predators

(Lubchenco and Menge 1978, Lubchenw 1980, Menge and Sutherland 1987, Menge

1991, and others), resulting in changes in community structure.

In most cases, community-oriented studies of predation by waterfowl have been of short duration, and showed either no effect or effects on only the main prey species (e.g.

Raffaelli et ai. 1990, Nudds 1992). These short-term experiments may not have been long enough to detect possible indirect effects in the system ( Yodzis 1988, Wootton 1992, but see Menge 1997 for an aiternate view). In one long term study, Bazeley and Jeffenes

(1986) found that exclusion of snow geese (Chen cc~emlescens)fiom a sdt marsh changed vegetation composition and species nchness.

To study the effects of a particular species on a community, it is often necessary to perform manipulative experiments in which that species is excluded and the response of the community is monitored (Menge et QI. 1994). However, it is also important to understand the behaviour and food habits of the species in question. Lubchenco (1978) suggested that the key to understanding complex interactions among trophic levels is to develop good knowledge of consumer feeding preferences.

Thus, 1exarnined the effects of predation by Common Eiders (Somateria molZissima) on invertebrate communities of the intertidai zone of Passarnaquoddy Bay,

New Brunswick. Common Eiders are large sea ducks which are present year-round at this location, suggesting that they may have significant effects on the prey community (sem

Marsh 1986% Hamilton et al. 1994). They are known to feed heavily on blue mussels

(Mwlus eedlis) (Milne and Dunnet 1972, Baird and Milne 198 1, Goudie and Ankney

1986, RafTaelli et al. IWO, Gorman and Raffaeiii 1993, Guillemette et al. 1993, 1996,

Hilgerloh 1997), the dominant invertebrate in much of this system.

The intertidal zone in my experimental area extends more than 500 m fiom shore at low tide, offering an excelient oppominity for rnanipulative experiments. The upper intertidal is relatively barren, with primady barnacles (Semibalanus balanozdes) and rough periwuikles (Litmina ~Iis)present. The rnid intertidal is dominated in roclq areas by rockweed (primarily AscuphyiIm nodosum but also Fucus vesiczîIosis), perennial macroalgae that provides three dimensionai structure to the system and is habitat for many species of invertebrates (Lubchenw 1983), as wel as juvenile fish (Rangeley 1994). The lower intertidal is dominated by blue musse1 beds in both rocky and muddy areas. Mussels occupy most of the primary space (substrate), though barnacles are also present in rocky areas. A wide variety of other species are also found living among mussels (see appendix 1 for a complete List of species that I found in my study plots).

Eider ducklings and associated females feed predominantly on invertebrates found in association with AscophyfIum (Cantin et al. 1974, Minot 1980, Bustnes 1996). This alga is commercialiy important, and is now being harvested in New Brunswick. In Chapter

1,I describe behaviour and habitat use by eider ducklings and females. 1 use observations of eider crèches coliected throughout the brood rearing period to determine whether duckling and female behaviour varies with tide Ievel and theof day, and whether it changes as ducklings mature. 1 assess duckling dependence on rockweed habitat by comparing feeding behaviour when rockweed availability differs, and 1 determine whether this dependence persists throughout development, or is restricted to certain ages. 1 discuss the possible effect of rockweed harvest on eider duckiings in light of these results. Finaily,

1 compare behaviour of females accompanying ducklings of dflerent ages, and test the assertion of Bustnes (1996) that femaie eiders caring for young are probably not constrained by parental duties, and that their behaviour with ducklings is similar to that when alone during summer. 1 conclude that eider ducklings are dependent on rockweed at an early age. but that this dependence disappears quickiy. However, in this system, activities of female eiders appear to be constrained by ducklings, suggesting that duckhg

rearing may have a larger effect on them than previously thought.

In Chapter 2,1 examine size-selective predation of mussels by Cornmon Eiders.

Adult eiders in this area feed primarily on blue mussels. Using a series of controiîed prey

selection experiments, 1 detemine fint whether eiders are size-selective predators, which

sizes of prey they prefer, and if preferences Vary with season and location. Then, 1 attempt

to determine the mechanisms behind prey selection in this species by examinhg a senes of

musse1 characteristics and size-fiequency distributions of mussels in the daerent

experimental areas. I test predictions arising fiom both the sheil mass minimization

hypothesis (Bustnes and Erikstad 1990) and the nsk-averse foraging hypothesis (Draulans

1982, 1984). Finally, 1 compare results fiom my study with those in other systems. 1

conclude that changing eider feeding preferences may result fiom seasonal variation in

relative tissue content and fiom differences in relative abundance of unprofitable prey.

In Chapter 3,I examine the effect of predation by eiders on an intertidal community. Using a series of exclosures and simulated abiotic disturbance in an intertidal musse1 bed, 1 examine the effect of eiders on blue mussels and the rest of the community under disturbed and undisturbed conditions. 1 compare total biomass, species richness, and diversity across treatments using analysis of variance, and 1 examine biomass of individual species in a rnultivariate analysis. Based on differences in biomass between cages (where ducks are excluded) and wntrols, size of the local duck population, and estimated food requirements for eiders, 1 calculate both observed and anticipateci musse1 loss due to eider predation. Agreement between the two indicates fkst whether eiders could have been responsible for observed changes, and second, what proportion of the change was attributable to them. Fïy,1 qdtatively compare prefemed sizes of prey for eiders, as deterrnined in chapter 2, with sizes apparently missing fkom the experimental area as a

Mertest of whether ducks were responsible for changes in mussel abundance.

In this chapter, 1 also discuss the interaction of predation and disturbance in terms of behavioural responses of vertebrate and invertebrate predators and growth responses of their prey. 1 pay padcular attention to dogwhelks (NücelZa IàpiZIus) (predatoty gastropods which also feed on blue mussels), which exhibit compensatory predation

(Robles and Robb 1993, Navamete and Menge 1996) in this system, and eventually obscure the effect of predation by eiders. I describe both direct and indirect effects of eider predation on the comrnunity, and compare my results to other -dies involving ducks, as weli as those in which invertebrate predators are responsible for community effects.

FinaUy, 1 disaiss the possibility that eiders are keystone predaton (sew Paine 1969) in this system.

In Chapter 41continue to examine the effiof predation by eiders on the invertebrate community with experirnents dedout in a rockweed bed adjacent to the musse1 area used in Chapter 3. In this experiment, haif of the sites were subject to a simulated rockweed harvest, ailowing me to determine the effects of harvest and predation by eiders on total biomass, species richness, and diversity of invertebrates. 1combine results £tom the harvest experiment with findings fiorn Chapter 1 to Mercomment on the possible effect of rockweed West on Common Eider duckiings. As in Chapter 3, whelks had a strong effm on the community and are given special attention. 1 end Chapter 4 with a cornparison of results in the rockweed bed with those in the adjacent

musse1 area (Chapter 3). Habitat heterogeneity is assessed in both locations, and the

relative effectiveness of ducks and whelks as predators in each system is discussed. 1

conclude that both ducks and wheks feed heavily on blue mussels, and the contribution of

each to total predation in the system is deteded by a combination of environmentai

heterogeneity and feeding behaviour of both species.

This study is the first long-term assessrnent of the effect of predation by ducks on

intertidal cornmmities, and the fïrst tirne in which predation by waterfowf and abiotic

disturbance (or rockweed harvest) have been considered jointly. Studies of behaviour of eiders (Chapters 1 and 2) are considered jointly with experimental manipulations (Chapters

3 and 4) to help to clarify the actions of ducks in this system. Results demonstrate that eiders are sigmficant intertidal predators which have direct and indirect effeds on the cornmunity, and suggest that it is probably an error to exclude waterfowl fiom studies of comrnunities in which they are present as predators. CHAPTER 1

Behaviour and habitat use of Common Eider (Somateria muUissima) ducklings

and females in a strongiy tidal environment

Introduction

Common Eiders (Somatena mollissrna) are large sea ducks, well known for rearing young in crèches (Ahlén and Andersson 1970, Gomand Milne 1972, Bédard and Munro 1976, Munro and Bédard 1977%b, and many others). These birds display an ontogenic shift in feeding behaviour as they mature (Gorman and Milne 1972, Cantin et al.

1974, Bustnes 1996). During surnmer, females and ducklings feed on invertebrates found in shallow intertidal waters, whereas during the rest of the year (and ail year for males), aduits feed primarily in deeper water on blue mussels (Mytilus edulis) and other prey.

Ducklings are most vulnerable to predation during the kst2 weeks of Life (Mendenhall and Milne 1985) and an abundant food nipply at this the is crucial (Swemen 1989).

Common Eiders are strictiy diunial feeders, and exhibit feeding peaks in the morning and evening (Campbell 1978). However, in strongly tidal environments, this rhythm can be superseded by a tidal cycle that dows feeding only at certain tide levels

(Campbell 1978, Minot 1980, Keller 199 1), particularly for young duckiings that have a

Limited ability to dive and therefore have access to food only when it is near the surface.

Passamaquoddy Bay, New Brunswick, Canada is strongly innuenced by tidal activity

(range in excess of 7.5 m during sp~gtides), with an abundance of food for ducklings available near the dacemainly during rishg and falling tides. Eider ducklings and accornpanying females fhquently feed on invertebrates found in association with rockweed (AscoplyZZurn nodom)(Cantin et al. 1974, Minot 1980, Bustnes 1996,

Appenàix 1).

In this study, 1 consider whether two aspects of eider breeding ecology and habitat use. First, 1 examine importance of the tidal cycle and rockweed to eiders by addressing the following questions: a) What is the relative importance of tidal and diudactivity cycles for this species, and does availability of rockweed for feeding correlate with tidal patterns? b) Do ducklings feed in ways which render them dependent on rockweed habitat, and does this change as they mature? Ifthey do require rockweed, we shouid expect the bulk of feeding activity to occur during peak periods of rockweed availability, on incoming and outgoing tides. Feeding by dabbling would indicate more dependence on rockweed than would diving.

Second, 1 examine whether behaviour of attending females is afEected by ducklings, and if so, whether this effect persists throughout duckling development. There may be

Little adaptive significance to crèching under normal conditions for eiders and other watenowl species (Eadie et al. 1988, Kehoe 1989, Seddon and Nudds 1994). Female eiders have been show to be not particularly affêcted by duckling rearing in terms of feeding site and behaviour (Bustnes 1W6), and duciciing age may be unimportant in determinhg habitat use mot1980). However, in systems where predation of ducklings is common, crèching may enhance duckhg suMval (Ahlén and Andersson 1970, Munro and Bédard 1977a, Minot 1980). If additional care of ducklings is required under these conditions, then femaies may behave differently than they wouid in a more benign environment. 1tested whether behaviour of female eiders in this system, where predation

of ducklings by Great Black-backed Gulls (Lmsmarinus) is intense, was iduenced by

duckling rearing.

Methods

Observatiomi &a collection

1 observed duckhg and adult female behaviour at Indian Point, St. Andrews, N.B.

(45" 4' N, 67" 2' W) from 5 June to 19 August, 1996. This area is part of Passamaquoddy

Bay, an inlet of the Bay of Fundy. Rockweed (perenniai macroalgae), primarily knotted

wrack (Ascophyllum ndstrm)but also Fucus vesimlosz1s, dominates the rnid-intertidal

zone and harbours invertebrates (periwinkles, amphipods, etc.) (Appendk 2) which are

prey for eider ducklings (Cantin et al. 1974, Minot 1980, Bustnes 1996, Appendix 1).

1 observed ducks and duckhgs for a total of 158 30-min penods, divided over four times of day, four tide levels, and two duckling classes. 1divided the of day into momulg (06:OO to 10:00), midday (10:OO to 14:00), aftemoon (14:OO to 18:OO), and evening (18:OO to 21:OO). Tide level was classifieci as low (1.5 h before to 1.5 h after low tide), high (the corresponding 3 h around high tide), incoming (the tirne between low and high as the tide is nsing), and outgoing (the corresponding period when the tide is falling).

Birds were divided into four groups - age class 1 ducklings, adults with age class 1 duckhgs, age class 2 and 3 duckiings, and adults with these birds. Duckhgs were aged according to a modified fom of the classifications of Gollop and Marshall (1 954). Class 1 in eiders stretches approximately to 3 weeks of age, class 2 from 3 to 6 weeks, and class 3 to fledghg (approximately 8 weeks) (K. Mawhinney, pers. comm.). Older classes had to

be wmbined because heavy duckling mortality limited the number of available groups

(hereafter they will be referred to as class 2 because they compnsed the majority of

observations). 1 obtained data f?om 5-7 observation periods for each tirne and tide for

class 1 ducklings and females. At ciass 2, high duckhg mortality forced me to lirnit data

collection to an average of four observation periods per time and tide combination (range

2-5).

For each observation penod, I selected a group of ducklings and accompanying

adults and observed them for several minutes to determine group composition. Groups

were selected randomly with the stipulation that they were easily visible and appeared to

be a cohesive unit (Le. not merging and separahg nom other crèches). Group sizes

ranged fiom 1 or 2 ducklings with 1 or 2 females to 34 females attending 76 ducklings.

Inevitably, because some of the same groups of birds were present in the study area on a

daiiy basis, some crèches were observed more than once through the study. However, to

limit possible bias this may create and to mùiimize lack of statistical independence, 1 never

observed the same crèche more than once during a day. Because this resarnphg was

rninimized, and that which did occur was random, it probably had Little effect on results.

Birds were observed with a Bushnell Spacemaster II (Bushnell Optical of Canada

Ltd., Vancouver B.C.) spotting scope (zoom lem 20-45x). At the beginning of each penod, 1 noted whether rockweed was available (floating at or just under the sufiace),

scarce, or absent. Because 1 worked extensively in these areas at low tide and knew the pattern of rockweed availability, I am confident of these classifications of availability. 1 scanned (Altmann 1974) and recorded on audio tape the behaviour of each duckling and addt every 2 min. 1 classined behavioun as swim (movement fiom place to place in which females were usually vigilant), dabble (icluding tipping up, swimming with the head submerged, and picking invertebrates off exposed rockweed), dive (includes birds in the process of diving or surfacing and those known to be under water), groom, rest (loafing, head tucked under wing or cleariy seen sleeping), feed (seen manipulating food at the

Wace but excluding dabbhg), waWsit (when out of water), display (wing flapping display), and aggressive behaviour (adults defending ducklings f?om predators). Between scans, 1 periodically recount ed birds t O determine group composition. When ducklings or adults Iefl the group or new ones arrived 1 adjusted group counts accordingly. Total number of ducks under water at each scan was determined by subtracting the number accounted for fiom the total hown to be present. Occasionaiiy trials had to be terminated eady because birds mixed completely with other groups or were disturbed by predators or human activity. When fewer than 10 of 16 scans could be completed, 1 deleted the trial.

Stutisticd analyses

Only the five most common behaviours (swim, dabble, dive, groom, rest), compnsing 95-9994 of all behaviour, were used in analyses. 1 averaged results from individual scans so each period provided a single estimate of behaviour for adults and for ducklings. Behaviours were converted to proportions of total activity so that ducklings and adults hmeach group received equal weight, regardless of the number of ducks involveci in the trial. This is important because there were typically more ducklings than adults in each group, and more duckhgs in class 1 groups than in class 2 (due to heavy mortality SUffered in class 1).

1 analysed ail data using multivariate and univariate analysis of variance in SAS version 6.1 1 (SAS Institute, Cary, NC). Before analysis, data were checked for adherence to statistical assumptions. Proportions were arc-sine square root transfomed (Zar 1996).

Normaiity was met for some behaviours but not others because of zero observations in some ceUs, which further transformation would not fïx. Homogeneity of variance (as assessed by Bartlett's test) was met for 3 of 5 behaviours (swim, dive, and groom). 1 assessed multivariate normality and homogeneity of variance (following Scheiner 1993) by examining correlation and covariance among behaviours for each analysis group. Signs of correlations were consistent acrou groups with few exceptions. Most violations of assumptions were minor, and MANOVA and ANOVA are relatively robust in such situations (Winer et al. 199 1). 1 evaluated significance of ail multivariate tests using Piliai's

Trace, the most consemative of the four common multivariate test statistics and the least affected by violation of assumptions (Tabachnick and Fidell 1989, Scheiner 1993).

1 performed a MANOVA using the five behaviours as dependent variables and tirne, tide level, and status of the group as classification variables. 1 used the canonical option in SAS (SAS Institute 1989) to generate canonical variates associated with each test. 1 ran a saturateci model MANOVA then sequentially (higher order first) deleted non- signtficant interactions and reanalysed the data. When sisnificant interactions occurred, 1 split data according to one of the interacting variables and reanalysed it using a reduced model. When significant main effects were detected, 1 carried out aposter?ori contrasts among specined levels of each dependent variable, using the Bonferroni correction to control type 1 error rate (Scheiner 1993). I then examined standardited canonical variates associated with signifiant effects. High positive or negative coefficients of particular dependent variables indicated which behaviours were wntributing most to clifferences among treatmentS.

1 also used a univariate ANOVA of total time spent foraging (diving + dabbling) to determine whether ducks maintained an overall constant feeding rate, independent of age or tide level. As before, 1 ran saturated rnodels and sequentiaiiy deleted non-significant interactions. When interactions were detected, 1 split data and reanalysed as described above. When sigrilficant main effects were detected, 1 made oposteriori cornparisons among mûuis using Tukey's HSD test (Zar 1996). I tested for a relationship between tide level and rockweed availability using mntingency X* analyses (Zar 1996).

Time budget &ses

Multivariate analyses of effects of tide, time of day, and duck age group (adult or duckling, class 1 or 2) on duck behaviour reveaied sigiuficant interactions (tide x group

P4.0273 and tide x tirne W.0004), so data were analysed separately for each tide level and again for each time.

At al tide levels except high, behaviour differed among times and ages. At high tide effects were detected ody among ages (Table 1.1). Aduit and duckling behaviour at each class always dBered (Table 1.1). Duckhgs spent substantidy more time feeding

(divhg and dabbling) than did adults (Fig. 1.1). Behaviour of class 1 duckiings dflered Table 1.1. Multivariate ANOVA of effects of duck age and status and time of day on duck behaviour for each tide level. Data were split as required by significant interactions (see text). The F statistic and probability are associated with Piliai's Trace. When signincant effects were detected, signincant aposterion' contrasts are indicated. Class 1 or 2 females refer to females accompanying class 1 or 2 duckhgs.

Tide level EEi df F P Signifiant Contrasts

w3h Group 15,213 6.4 0.0001 Female class 1 versus 2" Duckhg class 1 versus 2' Female versus duckhg class 1" Fernale versus duckling class 2"

High Time 15,213 1.4 0.14 Incoming Group 15,243 6.4 0.000 1 Female class 1 versus 2" Duckling class 1 versus 2" Female versus duckling class 1" Female versus duckling class 2" Incornhg Tirne 15,243 2.5 0.0024 Momir~gversus midday ' Moming versus aftemoon ' Moming versus evening ' Midday versus aftemoon ' Midday versus evening ' Low Group 15,171 3 -6 0.000 1 Femaie class 1 versus 2" Duckling class 1 vernis 2' Female versus duckling class 1 Fernale versus duckling class 2" Low Tirne 15,17 1 2.2 0.0076 Midday versus aftemoon ' Midday versus evening ' Aftemoon versus evening Tide level Effect df F P Sigdicant Contrasts Outgoing Group 1 S,2 13 6.0 0.0001 Fernale class 1 versus 2" Duckling class 1 versus 2" Femaie versus duckling class 1" Femaie versus duckling class 2"

Outgoing The 1 S,2 13 2.1 0.01 1 Moming versus evening " Midday versus evening O*

Note:

hdicates significance at the 0.05 level.

" indicates sigrilficance at the Bonferroni corrected level(0.0125 for group effects and

0.0083 for time effects). Fig. 1.1. Behaviour of ducklings and adults at the 4 tide levels. Data are pooled across

times of day. Bars indicate the proportion of total time spent in each activity for

each group. Signifîcance of merences among groups is indicated in Table 1.1. a) High tide b) Low tide

u Duckling cls 1 Adult ds 1 Duckling ds 2 Adult cls 2 Duckling cls 1 Adult cls 1 Duckling cls 2 Adult cls 2 Age and class Age and class c) Incoming tide d) Outgoing tide

- - Duckling cls l Adult cls 1 Duckling cls 2 Adult cls 2 Duckling cls 1 Adult cls 1 Duckling cls 2 Adult cls 2 Age and class Age and class

swim dabble dive groom rest other f?om that of class 2 ducklings at incoming and outgoing tides, but was similar at high and low tides (Table 1.1). Generally, effects were due to young ducklings dabbling more and divhg less than older ones (Fig. 1.1). Older ducklings tended to groom more and swim less than younger ones (Fig. 1.1). Adults accompanying these duckliogs also differed in behaviour at ail tide levels (Table 1.1). Females with class 1 ducklings dabbled and swam more, and dove and groorned less than did those with class 2 ducklings (Fig. 1.1).

Eider behaviour was less influenced by time of day than by age of ducklings or status (adult versus duckling) (Fig. 1.2). At high tide, behaviour was not influenced by theof day (Table 1.1). When tide was incoming, behaviour differed across thes, but pod hoc contrasts failed to detect Merences among them at the heightened significance level required by the Bonferroni correction (Table 1.1). At low tide, ducks spent less he feeding (dabbling and divhg), and more tirne resting and groomuig at midday than they did later in the day (Fig. 1.2). They also dove more in the afternoon and dabbled more in the evening. Finally, on the outgoing tide, overd behaviour dinered among times of day, with significant Merences detected between evening and rnidday and evening and morning (Table 1.1). There was more grooming and resting, and leu dabbhg in the evening than du~gthe rest of the day (Fig. 1-2).

When data were analyseci by time of day, at ali times both duck age and tide level significantly infiuenced behaviour (Table 1.2). In the morning, class 1 duckhgs and fernales attending them dabbled and swam more, and groomed and rested less than did class 2 poups (Fig. 1.3). Ducklings also tended to feed (especially dabble) more than did femdes (Fig. 1.3). At midday, behaviour of duckiings of the two ages did not daer (Table Fig. 1.2. Behaviour of eiders at different times of the day for the 4 tide levels. Data are

pooled across age groups. Bars indicate the proportion of total the spent in each

activity for each Mie. Significance of differences among groups is indicated in

Table 1.1.

Table 1.2. Multivariate ANOVA of effects of duck age and status and tide Ievel on duck behaviour for each the of day. Data were split as required by signifiant interactions. The F statistic and probability are associated with Piliai's Trace. When signincant effects were detected, significant a posteriori contrasts are indicated. Class 1 or 2 fernales refer to femdes accompanying class 1 or 2 ducklings. Significance is indicated as in Table 1.

Theof day Effect df F P Significant Contrasts Morning Group 15,174 6.7 0.000 1 Fernale class 1 versus 2" Duckling class 1 versus 2" Female versus duckhg class 1" Female versus duckhg class 2' -- Morning Tide 15,174 5.6 0.000 1 High versus incoming/outgoing '* High versus low '

Incoming versus low/outgoing *' Low versus outgoing '* Midday Group 15,204 6.2 0.0001 Female class 1 versus 2" Female versus duckling class 1" Female vernis duckling class 2" Midday Tide 15,204 5.7 0.0001 High versus incoming Ao w/out going" Incoming versus low ' Incoming versus outgoing "

Low versus outgoing " Afternoon Group 15,192 4.4 0.000 1 Female class 1 versus 2' hickling class 1 versus 2" Female versus duckling class 1" Female versus duckling class 2" Time of dav Effect Mernoon Tide 1 5,192 3.1 0 -0002 High versus incorning/outgoing " Incoming versus low ' Low versus outgoing ' Evening Group 15,162 4.6 0.0001 Fernale class 1 versus 2" Ducklùig class 1 versus 2"

Femde versus ducklhg class 1O* Female versus duckling class 2"

Evening Tide 15,162 2.6 0.0017 High versus low/outgoing " Incoming versus outgoing ' Low versus outeomrr Fig. 1.3. Behaviour of ducklings and adults during the 4 times of day . Data are pooled across tide leveis. Bars indicate the proportion of total the spent in each activity for each group. Si@cance of differences among groups is indicated in Table 1-2.

1.2), though females attending class 1 ducklings tended to rest more and groom less than did femaies with older ducklings. However, feeding behaviour was generdy consistent for the 2 groups, as observed with ducklings (Fig. 1.3). In both age categories, ducklings fed

(both dabbled and dove) more and swam less than did females with them (Fig. 1.3). In both afternoon and evening, behaviour of class 2 ducklings differed fiom that of class 1 birds (Table 1.2). Older ducklings tended to dive, groom, and rest more, and dabble and swim less than did class 1 ducklings (Fig. 1.3). Fernales attending them displayed the same pattern, but the ciifference between them was only marginally signi6cant (Table 1.2).

Again, ducklings of both age groups fed more and swam less than did females accompanying them (Table 1-2, Fig. 1.3).

Tide level had considerable effect on behaviour, with an overd trend to increased dabbling on the changing tides, and to somewhat increased diving at high and low tide

(Fig. 1.2), but the effect and specific differences among tide levels varied with time of day

(Table 1.2). During morning, behaviour difEered at all tide levels, though ciifferences between high and low tide were less than among other tide levels. Ducks dabbled most and swam least on outgoing tides, followed by incoming, low, and high tides (Fig. 1.2).

Diving was most common at hi@ tide, followed by low, outgoing, and incoming tides.

Grooming and resting time aiso diiered among the tides (Fig. 1.2). At midday, again behaviour differed arnong tide levels though the contrast of incoming versus outgoing tide was only marginaliy sigdicant (Table 1.2). Overaii feeding rate (especidy dabbling) was greatest on the outgoing and incoming tides, and lowest at low tide, where there was frequent resting and grooming (Fig. 1.2). In the afternoon, behaviour at high tide differed hmthat of incoming and outgoing tides, and low tide activities were marginally dinerent from those during changhg tides, but behaviour on incoming tides did not Wer from outgoing tides. Hîgh and low tides were also sirnilar (Table 1.2). Ducks dabbled more on the chaaging tides, and dove more at high and low (Fig. 1.2). In the evening, ali tide level contrasts showed sigaifiant effects except high and low versus incoming tide (Table 1.2).

Dabbling was common at al1 tides except high, where diving was more prevalent (Fig.

1.2). Grooming and resting were also much more cornmon on the outgoing tide than at the other three levels (Fig. 1.2).

Rockweed avmQTIabifity

Nearly 75% of aii observations were made in areas when and where rockweed was available (visible on or just under the dace). Rockweed covered only a srnail portion

(approximately 20%) of the local area in which observations were carried out, so it appears that ducklings were selecting this habitat. However, rockweed availability also varied with tide (contingency X-7.0, df4, P<0.0001).Rockweed was most available to ducklings (which feed only when in the water) at the daceof the water on the outgoing tide, followed by incorning, low, and high. Therefore, duckiings dabbled most when rockweed was most available, and dove more when it was less available. Class 2 ducklings switched nom dabbiing to diving more readily when rockweed was less available than did class 1 duckhgs.

Feeuïng raie

ANOVA of total foraging tirne (dabbhg + divhg) with respect to time of day, tide, and age group showed a significant interaction of group (class and age) and tide (W.0003) as well as thex tide (W.003). 1 therefore analysed the data split by the and then by group to examine variation in feeding rates on the different tides (Table 1.3).

When data were sorted by tirne, there was an interaction between group and tide for morning and aftemoon, but no interaction for midday and evening (Table 1.3). This necessitateci Mersplitting of the data with separate 1-way ANOVAs for each group in the morning and afternoon.

in the moming, class 1 duckhg feeding rate difred arnong tides, but the only a posteriori diffeience occurred between outgoing and incoming tides (Table 1.3).

Ducklings fed more on the outgoing than Uicoming tide (Fig. 1.4). Females acwmpanybg these ducklings showed no signincant diierences in feeding rates (Table 1.3, Fig. 1.4).

Moming feeding rate was only marginaiiy affected by tide for class 2 ducklings, and not affected for adults (Table 1.3). Class 2 ducklings tended to feed most at incorning tide and least at low tide (Fig. 1.4). At midday, both group and tide were important in deterdg feeding rate (Table 1.3). Ducks fed most on the outgoing tide, least at low tide, and ducklings fed more than adults, though class 1 and 2 ducklings did not differ, and adults with the two duckling age groups fed at sirnilx levels (Fig. 1.4). In the aftemoon, both class 1 duckhgs and females were bfluenced by tide level (Table 1.3), with feeding most on the outgoing tide and least at high tide (Fig. 1.4). There was no effect of tide level on feeding rate for class 2 ducklings (Table 1.3). In the evening, there was no effect of tide level on feeding rate, but ducklings fed more than adults at each class.

Finally, when data were sorted by group, there were no significant interactions between time and tide (Table 1.3). Class 1 ducklings tended to feed more in the moniing Table 1.3.3-way ANOVA of proportion of tirne spent feeding by ducks on group (age and class), tide level, and time of &y. Because of sipdicant interactions, data were split and analysed in smaiIer sub-groups. Mythe lowest level analysis is reported here. A posteriori cornparisons were made using Tukey's HSD test.

------Time Group Effect df F p Signifiant Differences Moming Adult class 1 Tide 3,20 2.06 0.14 Moming Adultclass2 Tide 3,10 2.61 0.11 Morning Duckling cls 1 Tide 3,20 3.91 0.024 out > in Morning Duckling cls 2 Tide 3,10 3.21 0.07 Midday ail Group 3,79 12.47 0.000 1 duckling > adult @oh classes)

- Midday al Tide 3,79 8.47 0.000 1 out > high, in, low Afternoon Adult class 1 Tide 3,20 5.61 0.0059 out > hi& in Afkemoon Adult class 2 Tide 3,13 0.96 0.44

-- Afternoon Ducklingclsl Tide 3,20 3.09 0.051 out>high Aftemoon hickhg cls 2 Tide 3,13 0.41 0.75

-- Evening aii Group 3'65 13.9 0.0001 duckling>adult(both classs) Evening ali Tide 3,65 1.13 0.34

- - - - al1 Adultclass 1 Tirne 3,91 2.37 0.075

- -- - - all Adult class 1 Tide 3,91 9.8 1 0.0001 out > in, low, high all Adult class 2 Time 333 2.36 0.082 all Adult class 2 Tide 333 1.26 0.30 all Ducklingclsl The 3'91 2.88 0.04 aU Ducklingclsl Tide 3,91 5.83 0.011 out~ui,low,high all DucklingcIs2 Time 3'53 0.47 0.71 d Duckling cls 2 Tide 3,53 2.42 0.076 Fig. 1.4. The spent feeding for each age group at each tune of day. Bars indicate the

amount of thespent in dabbling and diving combinecl, expressed as a proportion

of the total activity budget. Significance of merences among groups is indicated

in Table 1.3. a) Morning Feeding b) Midday Feeding

High Outgoing Low lncoming High Outgoing Low lncuming Tide level Tide level

c) Afternoon feeding d) Evening Feeding cn œ

0.7 r

High Outgoing Low lncoming High Outgoing Low lnwming Tide level Tide level Duckling Claçs 1 IIAdult Class 1 than at other times (though a posterion' differences were ody marginally significant), and more on the outgoing tide than at other tide levels (Table 1.3, Fig. 1.4). Females accompanying them were marginaily innuenced by time and strongly influenced by tide

(Table 1.3), displayhg the same pattern as did ducklings. CIass 2 ducklings were not influenced by the, and only marginally by tide (W.08), with a tendency for more feeding on the incoming tide than at high tide (Fig. 1.4). Females accompanying them were not

Muenceci by tide, and only margindy by time (P=û.08), with less feeding in the evening than at other times of the day (Fig. 1.4).

Discussion

Rackweed use and tidal actMly cycles

Common Eider ducklings relied heavily on rockweed habitat. Most of their feeding was done in rockweed, and they ate invertebrates found in association with rockweed

(Appendk 1). These redts are consistent with other studies (Gorman and Milne 1972,

Cantin et al. 1974, Minot, 1980, Bustnes 1996). However, 1 also specifically addressed the question of whether ducklings altered activity budgets based on tidal activity, if availabfity of rockweed correlated with this and whether this changed as ducklings matured.

Most observations involved crèches of eider ducklings found near available rockweed. They tended to follow floating rockweed in and out fiom shore as the tide rose and feu. 1 rarely observed crèches in areas where there was no rockweed, especidy if it was avaiiable elsewhere at the sarne tirne, suggesting that eiders preferentially used this habitat for food. Analyses of activity budgets also indicated that ducks changed behaviour with tide levei, and rockweed availability is correlated with tide. Variation in rockweed

avdability therefore appears to lead to changes in behaviour.

Tide level had the greatest effixt on feeding behaviour, though resting and

groorning activities were also affecteci. Total feeding was generally greatest on the

outgoing tide, and least at high or low tide, though this varied with age of ducks and thne

of day. In contrast, Cantin et a% (1974) found that fèeding was most common near low

tide (especidy on the ebb tide), because that was when food was most available.

Mendenhaii (1979) found that ducklings fed most during the low and outgoing tides, and

virnially not at all at high tide. Minot (1980) concluded that eiders fed most actively

during the rising tide, and that this was when prey items such as amphipods and

periwinkles were most abundant and active. He also found that, although dabbling should

be the preferred method of feeding for ducklings, they did this only on low and king

tides, and tended to dive on hi& and falling tides. I also found that there was Little

dabbling at high tide @rey items are too far under water to dabble effectively), but that

dabbling was common on the outgoing tide. These ciifferences may uidicate substantial site to site variability in behaviour of this species, even within a small geographicd area, or they may stem fiom different data collection methods. 1 calculated proportion of tirne spent in various activities for each crèche, whereas Minot (1980) classified entire crèches as either feeding or not based on observation of any feeding activity at all.

Effects of age on Ackling activity aradfeedng

Very young ducklings are poor divers (pers. obs.) and therefore dependent on dabbling to get their food. As they grow older they develop better diving skills (Cantin et ai. 1974), and can change their feeding behaviour. This was the case in my study.

Behaviour of class 1 ducklings differed f?om that of class 2 ducklings in that younger bkds tended to dabble more and dive less. Older ducklings also spent more time in cornfort activities (resting and grooming) and less time swimmiag than did class 1 ducklings. Total feeding rate did not meramong the two age groups, suggesting that class 2 bkds shply switched fkom dabbling to divhg when it was appropriate, whereas class 1 ducklings continued dabbling at a higher rate. These results are consistent with previous work done in thk area. Cox and Hamilton (unpublished data) found that arnong birds observed withh

2 h of high tide, young duckiings spent approximately equal time dabbling and ciiving, whereas older birds dove alrnost exclusively. Younger birds also spent less tirne in cornfort activities than did older ducklings.

These results suggest that class 2 ducklings were somewhat less dependent on rockweed fioating at the surface than were younger birds. It therefore rnay be during only the fïrst few weeks of life that Common Eiders depend on rockweed habitat. Effeas of tide level on duckling behaviour and feeding rate in particular were strong for class 1 ducklings, but rnostly marginal or not present at al1 for older birds. While class 1 ducklings fed disproportionately ofien when rockweed was most available, class 2 ducklings fed at similar rates during aii tide levels. Dabbling may stiil have been the preferred method of feeding by class 2 ducklings (as indicated by a continued high rate of dabbling when rockweed was available), but becarne less necessary. Swennen (1989) found that, by 3 weeks, eider ducklings were capable of swaliowing smali blue mussels, which at my study site are usually found independent of rockweed. Minot (1980) concluded uiat eider duckling habitat use did not change with age.

My results suggest that while actual habitat use may not have changed (though 1 did not test this speciiïcally), feeding method and dependence on certain habitats did. Class 2 ducklings continueci to feed in rockweed, but they began to dive more and dabble less, especially when rockweed was relatively unavailable. This suggests that they could feed more successfully than could younger duckhgs in a situation when food was mbmerged for most of the tidal cycle. Minot (1980) studied ducküngs over a period of only 11 days in rnid-July. By this tirne, many of them would have reached class 2 and would have been capable of diving efficiently. Perhaps if he had looked at ducklings over a wider time fhme, results would have been Merent.

DiumaI activity Me

Common Eiders, like many ducks, are thought to display a daily activity rhythm independent of changes in their environment (Campbell 1978, Minot 1980). However, this can be superceded when other factors (such as tidal fluctuation) limit availability of food to certain penods during the day (Campbell 1978). In my study, time of day signiflcantly infhenced behaviour, but effècts varied across tides, suggesting that birds altered theû

Ncadian rhythm and cornpensateci for a variable food supply. Feeding rate was not strongly influenced by the of day for either class 1 or 2 groups, though there was some tendency for increased feeding during moming, probably because birds had not fed during the night. In this strongly tidal environment, while diurnal patterns ex&, tidal variation

(and availability of rockweed in particular) is most important in driving foraging activity in

Common Eiders. EDcts of parental meon femaie ei&s

There is considerable debate about the identity of female eiders caring for duckling crèches, and the effixt of parental areactivities on these females. Many females associated with crèches are not mothers of ducklings in those groups (Bédard and Munro

1976, Munro and Bédard 1977a,b, Schmutz et al. 1982). Adult eiders and duckluigs have different diets, and their foods do uot always occur in the same areas. It has been suggested that créches in this species fonn only when foods for the two groups ocair separately, because females cannot stay long with duckiings and rapid turnover of females attending ducklings occurs (Goman and Milne 1972). mershave found that crèches form even when foods are completely interspersed, and that parental bonds form between adults and ducklings (Bédard and Munro 1976).

Schmutz et al. (1982) suggested that crèches are just a result of the gregarious nature of eiders. This implies that duckliag care does not impose much of a burden on females. Bustnes (1996) observed females with and without young and concluded that both groups fed in the intertidd zone by dabbling. He thought that parental care did not impose constraints on female eiders, because they would feed in these areas anyway, and adoption of ducklings did not lead to increased One spent in parental we(Bustnes and

Erikstad 199 1a). The presence of fernales without young in the group (failed nesters and brood abandoners), and the phenornenon of crèching in generai, may be to the benefit of adult eiders by providing them with increased vigilance against predators, and improved access to food (Schmutz et al. 1982, Bustnes and Erikstad 1991 b, Bustnes 1993).

At my study site, where eider ducklings were nodyreared in crèches, foods commonly eaten by ducklings and adults overlapped spatially to some degree, but much ncher feeding grounds for adults were present in nearby blue musse1 beds. Females accompanied duckhgs and dabbled in rockweed, probably taking the same prey that ducklings consumeci mecause their normal prey were not available by dabbling). While 1 made no attempt to mark individual ducks, it was clear that at lest some crèches remained intact for the duration of the shidy. It appears that females caring for ducklings consumed diEerent foods during the rearing penod than they did during the rest of the year, contrary to the observations of Gorrnan and Milne (1972), but consistent with those of Cantin et al.

(1974).

This switch may have occurred for 2 reasons - either duckling prey were also profitable for females, or females were in fact constrained by parental care and settled for a less profitable food source. Male eiders and females not associated with young also fed during summer near rny study area, but they were almost always in much deeper water diving on mussel beds. They made no attempt to feed near shore, suggesting that prey for duckhgs were unlikely to be more profitable than adult prey elsewhere. However, foraging rates of females associated with ducklings (Fig. 1.4) were sidar to those of adults foraging during the rest of the year in deeper water (1 8.8 % k 2 %, based on 30 min observations of 26 groups of eiders during fawinter and spring). Duckiing care may therefore alter female habitat use, but not reduce available feeding time and thus may not be especiaily costly to females. This is important because female eiders do not feed while incubating eggs, and therefore their nutrient reserves are severely depleted and they must feed afker duckIings hatch (Korschgen 1977). Behaviour of femaie eiders attendhg duckhgs of different ages differed, mer suggesting that parental care had an effect on females in crèches. Females with younger ducklings dabbled and swam more, and dove and groomed less than did those with class 2 ducklings. 1 was unable to compare behaviour of fexnaies with and without Young, because ducks not associateci with crèches usuaiiy fed in difrent areas, but it appears that in this system, fernale eiders were affecteci by duckling rearhg in tenns of both behaviour and habitat use, contrary to the conclusion of Bustnes (1996).

Young ducklings are most vulnerable to predation during their first 2 weeks of life

(Mendenhall and Milne 1985). Predation of duckhgs by Great Black-Backed Gulls was intense during my study (greater than 95% mortality based on duckling counts), and the match between adult and duckling behaviour may be a by-product of this. The observed increase in swimming and reduced comfort activities by femaies with class 1 ducklings relative to class 2 may be a resuit of heightened vigilance necessary to ensure duckling survival. During attacks by guIls, crèching helps to Limit duckling mortality (Ahlén and

Andersson 1970, Munro and Bédard 1977% Minot 1980). Under conditions of apparently high duckling survival relative to this study, Bustnes and Erikstad (1991a) found no significant benefit to this behaviour. It is possible that under the rea~gconditions faced by eiders at my study site, females were more affected by duckling care than they would be in a more ben@ environment. The eEect of duckling rearing on femaie Common

Eiders may therefore Vary fkom place to place depending on habitat, food supply, and threat of predation. Conclusions

This study hiwghts two aspects of Common Eider breeding ecology. First, eider

ducklings, especially in the first weeks of We, appear to depend on rockweed habitat for

food. This result is consistent with other studies. A commercial harvest of rockweed is being starîed in New Brunswick, and it may be detrimental to this species. Harvea

involves removing the top part of plants. Ducklings in this area are probably not food

Limiteci, because when feeding they do not reduce invertebrate biomass (Chapter 4), so even if harvest reduces invertebrate abundance somewhat (which it appears not to do, see

Chapter 4), it will not likely lead to a food shortage for these birds. However, cutting plants reduces the rockweed canopy height, meaning that it wouid be floating on the surface and available for dabbiing for a shorter time during each tidai cycle. This could reduce daily feeding time for class 1 duckiings because invertebrates would be inaccessible for longer periods. Combined with intense predation pressure fkom gulls, this might lead to increased duckling mortality (Swennen 1989). Disturbance of duckiings by harvesters may also be problematic, because mortaiïty due to predation increases following disturbance

(Keller 1991). Therefore, carefiil studies of effects of rockweed harvest on ducklings should be made before this practice becomes widespread.

1 also found that females in crèches switched foods (baseci on feeding locations and known availability of prey in the area) and feeding behaviour according to the ages of ducklings they were accompanying, suggesting that fedebehaviour was constrained by duckling rearing. There was also a diurnal pattern present at my study site, but it was

Iargely overwhelmed by tidal variation. My results Mered f?om those of other studies in terms of both feeding behaviour and activity rhythm. These patterns may be explallied by the very high mortality of ducklings at my study site. My results suggest that local conditions, including tidai fluctuation, predation pressure, and availability of foods, may be most important in determinhg Common Eider behaviour, and that spatial variation in these factors may limit our ability to discem general patterns. CHAITER 2

Size-seleetive predation by Common Eiders (Somatda niofhsima) on blue mussels

(Mytilsedulis) under controiied field conditions

Introduction

Many optimal diet models assume that anirnals sample a variety of prey environments such that energy intake is maximized relative to costs associated with feeding (Stephens and Krebs 1986). However, the performance of such models under complex natual situations is questionable (Ball 1994), and it is seldom possible to determine how animals perceive coas, benefits, and risks associated with foraging choices.

Their abiïty to assess their environment and distinguish among prey may influence how they perceive these factors, and in tuni, rnay determine the degree to which they are selective, and the prey they choose (e.g. Elner and Hughes 1978, Hughes 1979).

It might be inferred, fiom the observation that prey are not always selected according to mode1 predictions, that animals feed "suboptimally" (eg Draulans 1984).

However, animals may actually feed in a manner that still maximizes net reward, based on constraints they face and their ability to distinguish prey. For example, Bail (1994) argued that animals that forage in highly variable environments where prey availability and profitability change unpredictably, or where they are faced with a wide array of choices, may be unable to disthguish srnail dserences in profitability, and instead use "rules of thumb", possibly feeding in a manner which we wouid consider suboptimal. On the other hand, if profitability changes predictably (e.g. seasonally), animais may be better able to assess the environment and feed more selectively.

There has been considerable research hto prey selection by aquatic birds,

especially watenowl and oystercatchers (Haematopus o.stra1egu.s) (e.g. Draulans 1982,

1984, 1987, Meire and Ervynck 1986, Bustnes and Erikstad 1990, Ward 1991, DeLeeuw

and Van Eerden 1992, BaU 1994, Barras et al. 1996). Birds often eat prey that appear to

be of low quality compared to other available food. Explmations for this include

hypotheses that 1) individuals rnay minimize shel ingestion (Bustnes and Erikstad 1990)

andior salt intake (Nystrom et al. 1991) as opposed to ma>timizingshort-term energy

intake, 2) large prey are more costly in tems of handIing the and crushg resistance and

therefore should be avoided, even if energeticdy nch (DeLeeuw and Van Eerden 1WZ),

and 3) predators are unable to discriminate prey of different sizes (Ward 1991). Draulans

(1982, 1984) suggested that ducks may select mussels of smailer than optimal size because

this enables them to avoid the risk of ingesting one which is too large to handle, or

because larger musseis have highly variable profitabiiities. This assumes that ducks are

imperfect at disthguishg prey of ditferent sizes; the better their selection skilis, the less

they shouid have to compensate in higher risk situations (Draulans 1987).

I examined predation by Common Eiders (Somateria molIissima) on blue mussels

(MytiZus eaulis) in two areas of Passarnaquoddy Bay, New Brunswick, Canada. Eiders are

present year-round in the area and feed primady on mussels. Using a senes of prey

selection experirnents, I tested two hypotheses: 1) The sheil mass minimization hypothesis

(Bustnes and Erikstad 1990) argues that eiders select prey items which minllnize sheii intake, rather than muhize short-term energy gain. Relative tissue mass is greatest in smd mussels (Bustnes and Erikstad 1990), but differences among sizes may vary with

season. When relative tissue mass is greatest in smalI mussels, birds should choose them

over larger prey, but when Merences in tissue content are smaller, other selection factors

may corne into play. 2) The risk-averse foraging hypothesis (Draulans 1982, 1984) argues that ducks select srnaII prey which reduces the risk of taking prey too large to be handled

profitably. Assuming that ducks are imperfect at disfinguishing prey of different sizes, in

areas and seasom where large mussels are more abundant and less profitable, ducks

should switch to feeding on smder prey than they would when or where large prey are less cornmon. Finally, I used results to account for variation in results among several previous -dies (MaeUi et al. 1990, Nystrom et al. 199 1, Guillemette et al. 1996).

Methods

1 perforrned prey selection experirnents at two locations (Ban Road and Indian

Point) approximately 3 km apart near St. Andrews, N.B.,Canada (45 O 4' N, 67 O 2' W) during spring, summer, and fd 1995, and winter and spring, 1996. Blue mussels were collected nom these areas and stored in aquaria with a flow-through water system in a lab at Huntsman Marine Science Centre, St. Andrews, N.B.

Square ceramic floor tiles (900 cm3 served as substrate for musse1 atiachment in the lab. Mussels were dMded into four size classes: (1) 10-18.9 mm (referred to hereafter as 10-19 mm), (2) 19-27.9 mm (19-28), (3) 28-36.9 mm (28-37), and (4) 37-50 mm. Ail classes were within the size range which ducks were physiologically capable of eating

(Appendix 3). 1 divideci tiles into four equal sections using corrugated plastic dividers aîîached with silicone. Mussels were placed on tiles, one size class per section. 1

randornized positions of Werent size classes across des. Two types of tiles were set up:

those in which prey availability was equal across size classes (referred to as regular tiles),

and those which had large mussels added to each section (manipulated tiles). The latter

aliowed a cornparison of prey selection under conditions of equal availability of all size

classes with results when large prey were more abundant. This provides a direct test of the

nsk-averse foraging hypothesis. I equalized dacearea covered by mussels in each class

by placing the foilowing number of mussels on each regular tile- class one - 145; class two

- 66; class three - 40; class four - 2 1. Each rnanipulated tile had- class one - 110; class two

- 52; class three - 30; class four - 21 mussels, and six 37-50 mm mussels added to sections

containing size classes one through three. The experimental setup on tiles is shown in Fig.

2.1.

1placed tiles in aquaria with aeration and a flow-through water system for 3-7 d, until mussels attached by means of byssal threads. Aquaria were drained and left empty for approximately 2 h ddy to simdate a tidal cycle and to accelerate attachent (VanWinkle

1970). Musse1 attachent was checked by tuming tiles upside down. Mer attachent was complete, 1 recounted mussels in each tile section (because sometimes a few mussels died or failed to attach).

1 placed tiles (in groups of three to six at a tirne) in the intertidal zone in areas where ducks were known to feed. All were at approxhately the sarne depth (6 to 7 m under water at high tide, depending on the moon phase) to reduce potential effects of diving depth on prey selection @radans 1982, Beauchamp et a!. 1992, DeLeeuw and Van Fig. 2.1. Diagram of a typical a) regular and b) manipulated tile with mussels of the 4 size

classes. Number of mussels in each section does not reflect the actual nurnber used

in experiments (see text). Positions of the four classes were randornized for each

tile.

Eerden 1992), and near the low tide line. This minimized possible predation by gulls and crows, because they do not feed under water and tiles were exposed for a very short the each day. However, 1 also placed four des higher in the intertidai zone and observed them ushg a spothg sape to determine whether gdsattempted to rernove mussels f?om them. In ail cases gulls avoided tiles, so they were excluded as possible predators.

1 placed a single tile firom each group under a predator exclosure designed to prevent ducks fiom feeding under it. These cages were 1.5 x 1.5 m, 30 cm high, with a roof made of 3 cm plastic mesh, and had no sides. This design has been used in the past to effectively exclude ducks (Hamilton et al. 1994); cages do not attract or exclude other predators, and offer no protection f?om wave action (Chapter 3). Cages therefore acted as controls for mussels lost nom tiles due to wave action or other predators.

1 counted ducks feeding in the area daily around high tide using a sponing scope

(Bushnell Spacemaster II, Vancouver. B.C.). 1 checked tiles daily at low tide for evidence of predation. When it became clear that mussels were rnissing from tiles in at least one size class (or within 3 d if nothing had happened), 1 recovered tiles and counted the remaining mussels. Number of missing mussels which could be attnbuted to duck predation was detennined by subtracting the number of mussels in each size class missing fiom control

(protected) des fiom those missing fiom experimental tiles which were exposed to predation during the same tirne period.

I estimated availability of mussels of different size classes at both sites by coilecting

20 100 cm2samples fiom each area during swnmer 1995. AU mussels fiom each sarnple were counted and divided into size classes. 1 periodically coliected approximately 40 mussels (10-50 mm long) from the study area to assess relative tissue and shell mass at

different times of the year. Mussels were steamed open, and tissue and sheil were dried

separately for 20 h at 90" C, then weighed. 1 also measured shell thickness and cmshing

resistance during one collection period. Force (N) required to crack mussels of different

sizes was assessed using a Hodeld Temorneter (Micrometallurgid Ltd., Thornhill,

Ontario). Mussels were placed width wise in the tensometer with one valve in contact with

each of the cnishg daces. Tension was increased slowly and the force at which

mussels £irst cracked was rewrded as crushing resistance.

1 assessed prey value for ducks using several means. Energy (0gained by eating

mussels of each size, and the amount of sheii which had to be consurned at the same time

was estimated for all seasons using estimates of energy content of musse1 tissue (1 g dry

tissue = 4900 cal = 20 5 1 1 J, Bustnes and Erikstad 1990) and predicted average sheii mass

for each mussel size class. 1 calculated work (N) (force x distance compressed) required to

crack mussels of each sue, and detennined force required to pdmussels off tiles using a

spring loaded Pesola @ de.From literature estimates of daily food requirements for

eiders (Bédard et al. 1980, Bustnes and Erikstad 1990, Egerrup and Laursen 1992,

Hilgerloh 1997), 1estimated an average dry tissue biomass requirement of 130 g 6'.1 then

calculated for each season the mass of shell which would be consumed per day from each

size class if an eider were to obtain the 130 g requirement completely from mussels of that

size. The estimate of 130 g 6' is likely to be an overestirnate during summer and an

underestimate in whter (Hilgerloh 1997), but that does not matter because relevant cornparisons of sheii ingestion are among size classes within seasons, not across seasons (see results).

StutisticaJ analyses

1 used a series of x2 analyses to test for size selectivity at different times,

locations, and arnong tile types (regular versus manipulated). Groups were analysed

separately, though when Uisufncient replication was available, some had to be pooled. For

each tile, corrected (for losses other than to ducks, as describeci above) nurnbers of

mussels eaten fiom each size class were taken as observed values, and expected numbers

eaten were caiculated based on a nul hypothesis of random removal. 1 surnmed observed

and expected nurnbers across aU tiles in an experimentai group to generate single observed

and expected values for each size class, and an overall x2. I then tested dinérences among

times and locations in prey selection by eiders using a senes of heterogeneity x2 analyses

(Zar 1996).

This non-parametric analysis is a conservative approach, because it resulted in

reduced degrees of eeedom and potentially rninimized effects in the system by combining

observations taken at different times (though seasons and locations were kept separate)

under slightly different conditions. It also had the advantage of giving greater weight to

tiles on which more predation had occurred. This approach was necessary because on

many tiles, too few mussels were removed, resulting in violation of x2 assumptions ifeach was considered separately (Zar 1996).

Ail parametric analyses were performed using SAS Version 6.1 1 (SAS Institute,

Cary, NC). Data were examined for conformity to assumptions and, when necessary, were transfomeà. 1determineci the relationship between musse1 length, total mass and dry tissue mass using anaiysis of covariance (ANCOVA), with season as the classification variable. Average tissue mass and relative tissue content (dry tissue masshotal mass) of mussels in each size class were cddated using predicted (from regressions of biomass on length) total and tissue biomass. 1 used predicted as opposed to raw values to ensure that the true mean for each size class was achieved. This likely introduced very Little bias into the analysis, because ? values for each regsession exceeded 0.94.1 then compared relative tissue content (arcsine transformeci) across size classes and seasons using 2-way anaiysis of variance (ANOVA) and the ci posteriori Tukey's HSD test (Zar 1996). I also calculated means and variances of relative tissue content for each sue class using raw data. They were examined for a correlation between variance and mean, and variances were compared using ANOVA to test the assertion that larger mussels tend to have more variable profitabilities @radans 1982). 1 regressed mussel volume, sheli thickness, and crushing resistance on shell length, and compared attachment strength of mussels to tiles across size classes using ANOVA and Tukey's test.

Results

Prey selection experiments

Cornmon Eiders were size-selective predators on both regular and manipulated des during aii seasons at both locations (Table 2.1). Preferred size classes differed among treatment groups. At Indian Point, preferences differed among tiie types (spring: heterogeneity ~113.6,df%, W.004; summer: heterogeneity ~122.5,&3, P

W,PC0.000 1). Ducks generally preferred the srnalier size classes, especially 19-28 mm, Table 2.1. Results of x2 analpis of prey selection experiments. Reps refen to the number of tiles pooled to obtain the finai result Symbols associated with mussel size classes are as follows: - strongly avoided, - avoided, O eaten randody, + selected, * strongly selected, and were determined based on ce11 x2 values. If the ceIl x2 2 6.63 (x~~.,,,at 1 df), the class was strongly selected or avoided, and if 6.63 r x2 23.84 at 1 df). the class was selected or avoided. These divisions are not intended as a posteriori statistical tests, but rather as a means of standardking levels of preference/avoidance. For overall x2 compariso~~s,de3 and critical value x2 0.05=7.815. Values under symbols indicate percentage of the total x2 attributable to that ceIl; high values indicate strong selection or avoidance.

Place Season Type Reps x 10-19 19-28 28-37 37-50 mm mm mm mm

Ind. Pt spring195 regular 17 19.8 0 + - O- 3% 30% 23% 44% ------Ind. Pt. summerI95 regular 23 106.5 u ++ -- -- 9% 17% 45% 29%

Ind. Pt. spr and sum/ rnanipulated 12 15.1 + - 0 O 95 and 96 33% 28% 2% 37%

5% 14% 81% 0% Ind. Pt. winted96 regular 16 182.5 - ++ ++ ++ 35% 7% 7% 51% Ind. Pt. wllited96 rnanipulated 6 16.2 0 - 0 0

Barr Rd. spr and sud regular 20 22.4 -t+ - 0 -- 95 34% 22% 0% 44%

Barr Rd. spring/96 regular 9 84.3 0 ++ -- _O 4% 65% 15% 16% Barr Rd. spr and sud manipulated 6 29.6 -- tt 0 0 95 and 96 34% 65% 1% 0% and avoided larger classes during sp~g,summer, and fd (Table 2.1). However, in winter, large mussels were select4 (Table 2.1). Ducks tended to take smaiier mussels fiom dpulated tiies than they did fiom wrrespoading regular tiles (Table 2.1).

Preferences at Barr Road dso dEFered among seasons (heterogeneity ~~73.7, de3, P

Mussel chacteristics

Mussels in areas where ducks fed at Indian Point tended to be smaller than those at

Barr Road, though sizes also varied within sites (Fig. 2.2). Musse1 tissue biomass increased exponentidy with length, and varied with collection period (ANCOVA length x date -,240, F=7.5, P<0.0001), though trends appeared consistent across seasons (Fig.

2.3a). Generally, tissue content was greatest in July oust prior to spawning) and lowest in

March and August (Fig. 2.3a); however, when predicted size class means were considered, Fig. 2.2. Sue fiequeacy distributions of mussels colleaed £?oma) Indian Point and b) Barr

Road in summer 1995. Within each site, mussels were collectai f?om two areas

(Indian Point left and right, Barr Road rock and weir). Different bars represent the

different areas at each site. Size dass (mm)

50 Size class (mm) BarrRd rock BarrRd weir Fig. 2.3. Tissue content of mussels coilected during each sampling period. Values for a)

average dry tissue of mussels were calculated as an average of predicted (fiom

regression) dry tissue content for mussefs in each size class. b) proportion tissue

biomass was calculateci using values fiom part a) above and similar ones predicting

total dry musse1 biomass. Error bars represent 1 standard error. oniy July and March difEered signiIicantly @posteriori Tukey's HSD test). Relative tissue biomass also vatied among mussel size classes, and the merence differed arnong seasons

(ANOVA season x size class df=12,185, F=68.4, P<0.000 1) (Fig. 2.3b). Durllig sp~g and summer, smd mussels had substantiaily higher relative tissue than did Iarger ones

(Fig. 2.3b). However, in December and March, dinerences, though significant, were slight

(Fig. 2.3b). AU prey size classes had low variability in relative tissue mass (standard errors of raw data (n=40 in each case) ranged from 2.5 to 9% of relative tissue mass), and there was no relationship between variance and musse1 size class (ANOVA df=3,16,F=û. 5 1,

W.68).

Shell thickness was positively correlated with length (de1,4 1, W.75, P<0 .O00 1)

(Fig.2.4a), and longer mussels were harder to crush (& l,46, *. 86, PCO. 000 1) (Fig.

2.4b). When crushed, shells usually failed fkst in the rniddle or toward the rear (away fiom the umbo) of one valve or the other. Attachent of mussels to the substrate differed among size classes (ANOVA df-3,556, F= 140.1, P

Costs and benefitF

In any season, eiders that met their energetic requirements by feeding on large mussels would have to consume more sheii biomass than ducks that fed on smaii ones

(Fig. 2.5a). However, as descrïbed above, ciifferences among size classes in shell mass consumed while eating a set arnount of mussel tissue varied among seasons. In Decernber and March, there were relatively smail dinerences (7.8 % and 4.9%, respectively) in shell masses which would be consumed by eating quivalent amounts of the srnailest versus Fig. 2.4. a) Regression of sheli thickness on mussel Iength. b) Regression of force

(measued in newtons) required to cmsh mussels on mussel length. Results of

significance tests are provided in text. Thickness = 0.0625 x length0-65-

20 30 40 50 Musse1 length (mm)

.Force =2.754 x length' ''

Mussel length (mm) Fig. 2.5. a) Estimated sheii mass consumeci per day by eiders based on average

consumption of 130 g dry tissue. Values were calculated based on prediaions of

regression of tissue and shell on musse1 length. b) Ratio of energy gained to work

done by eiders eating mussels of each size class during a day. Work is based on

cost associated with crushing 130 g dry tissue of mussels of each size class. Season largest size classes of mussels (Fig. 2.5a). hiring other months, ciifferences were pater, ranging fkom 18.5% in May and July to 52.5% in August (Fig. 2.5a). Relative benefit

(energy intake relative to work) was always greatest when large mussels were taken (Fig.

2.5b). The ratio of energy gain relative to cost (estimated as the work required to crack a mussel) increased steadily across size classes, and trends appeared simiilar for all seasons, though the increase in benefit was somewhat less during May and August than it was at other times of the year (Fig. 2.5b). These dineremes in relative value mean that to obtain the same amount of mussel tissue, ducks feeding on the srnallest mussels would have to expend in total more energy to crush them than would birds feeding on large prey.

Discussion

Common Eiders feeding in this experirnent were size-selective predators. This has been shown before, though preferred sizes Vary arnong studies (RafEaelli et al. 1990,

Nystrom et ai. 199 1, Guillemette et al. 1996). However, unlike in previous work on eiders, 1 quantified and controUed availability of prey of different sizes. This is important, because according to foraging theory, prey selection should be inauenceci by avdabiiity of the most profitable size classes (Stephens and Krebs 1986). Further, Eher and Hughes

(1978) and Ward (1 99 1) found that availabiïty of non-preferred prey also affected selection of preferred size classes. 1 standardized availability of each size class, thereby eliminating differential encounter rates fiom consideration. Search tirne was also standardized, because when birds encountered a tiie, ail size classes were equally available. Seasonal vatllatllatratron

Eiders selected mussels of different sizes at Merent Mes of the year. For most of the year at Indian Point, ducks feeding fkom regular des preferred the two srnallest size classes, 10- 19 mm and 19-28 mm, and avoided large mussels. However, in winter this trend was reversed; ducks strongly avoided 10-19 mm mussels and selected others (Table

2.1). The largest prey (35-50 mm) were most preferred, though others were also selected, probably after d large mussels were removed fiom des. This switch was probably related to changes in coas and benefits of feeding on prey of different sizes at different times of the year (see below).

If eiders selected prey that maxhized short-tenn energy gain, they should always have fed on large prey. The ratio of energy gained to work done (Fig. 2.5b) varied little across seasons. Notwithstanding the increase in sheil thickness and force required to crush large mussels, energy intake appeared to be maximized by taking the largest prey. This was a consistent trend across seasons, because although crushing resistance was measured only once during the experiment, shell mass relative to length (and therefore thickness) was relatively constant through the year. Search thewas controiled by equabhg availabiiity of aiI size classes, but 1 did not attempt to quant@ handling tirne, which is considered to be greater for large mussels (Draulans 1982, DeLeeuw and VanEerden

1992), so the relative benefits of consuming large prey may be somewhat less than indicated. However, based on relative tissue biomass (Fig. 2.3a) and costs associated with crushing mussels (Fig. 2Sb), unless ducks codd consume 15 to 20 small mussels in less tirne than it takes thern to eat one large mussel large prey would remain the most energetidy profitable.

However, foraging ducks aiso need to consider costs associated with ingestion of shell. The presence of food in the digestive tract limits consurnption of other food (Bd

1990, 1994). so if eiders have to consume a high proportion of shell in a feeding bout, there will be less room for mussel tissue. Mussels had lower proportionai tissue biornass

(more shell per tissue) in winter than in other seasons, but variation among size classes in winter was much lower than during the rest of the year (Fig 2.3b). Dhgmost of the year, eiders could minimize shell ingestion by feeding on small size classes. However, during winter, there was little difference among size classes of mussels in the amount of shefl that wouid be consumeci by ducks while acquiring the daily requirement of musse1 tissue. Under these circumstances, it may have been more profitable to feed on larger prey than during the rest of the year.

My results are consistent with the shell minimization hypothesis of Bustnes and

Erikstad (1990). When sheii mass ceased to be a factor, eiders switched tactics and attempted to maximize energy intake by taking large mussels. Further support for this hypothesis was provided by Barras et al. (1996) in a study of acom selection by Wood

Duck (Aix spmsa), and by Zwarts and Blomert (1992) who studied preferences of knot

(Caiidi?'~camrhrs) for Macorna baifhica.

It is possible that during part of the year, ducks also used a mixed foraging strategy as proposed by Ydenberg (1988). Ducks fed preferentialiy on one class, but also took prey fiom other classes. Occasionally, one of the smd classes was favoured, the other avoided, and one of the larger groups taken randomly. Because large mussels were most favourable in temof energy gain per crushing force, it may have been beneficial for birds to include

some large prey in their diets, even when that meant ingesting more shell. Ydenberg

(1988) suggested that the gizzard worked most effectively when there were mixed prey

sizes present. Ifeiders exerted sufncient force to crush large mussels smaller ones would

be crushed as weil with little or no added effort. By feeding preferentially on srnall

mussels but including a few large ones, ducks codd therefore maromize energy intake per

effort and continue to rninimize shell ingestion.

Attachent of mussels to the substrate appeared not to be a factor in prey selection by eiders, because prey selection that wouid mlliimize the force required to remove mussels should favour only small mussels. Draulans (1982) found that the force required by Tufteci Ducks (Aythyafdiigula) to remove zebra mussels (Drezssenu polymo~pha)fiom the substrate did not alter profitability of dflerent size classes. Eiders have a nail on the upper bill, and a strong grasping action, aliowing them to remove even large mussels from the substrate (Meire 1993).

Geogrqhic variation

Selection of prey by eiders varied among locations. At Indian Point in 1995, ducks generally preferred mussels 19-28 mm. At Barr Road during the sarne tirne, they strongly preferred the smaller 10-19 mm size class. Tiles had the same size composition at each site, but naturdy ocairring mussels at Indian Point were smder than those at Barr Road

(Fig. 2.2). This result lends support to the risk-averse foraging hypothesis. In an environment (Barr Road) where mussels were generally large and therefore unprofitable

(in terms of sheil ingestion), ducks selected smd prey that minimized the nsk of inadvertedly taking large mussels.

Draulans (1984) found that as the proportion of large, unprofitable prey in the population increased, ducks took sdermussels, either to reduce the risk of taking one too large to hande (and therefore unprofitable), or because large mussels were more higbly variable in profitability, and therefore presented a greater risk. Draulans' suggestion that birds took smd prey to avoid those which were roo large is supportai by my data. hicks in my study wuld eat the larger mussels, but they appeared to be less beneficid during most of the year because of large shell masses. However, contrary to Draulans

(1984), large prey were not more variable in profitability and therefore a riskier investment. 1 found no increase in variability of relative tissue biomass as prey size increased, and only a slight tendency toward increased variation in crushing resistance among the largest mussels (Fig. 2.4b).

During spring 1996,I found that at Barr Road ducks preferred 19-28 mm mussels, contrary to redts fiom the previous year. During 1996, ail tiles were placed in the rock area of Barr Road, where mussels were somewhat srnalier (Fig. 2.2b). The previous year, tiles were mixed eveniy among the two areas. Ducks in 1996 were therefore not feeding in an environment as dominated by very large mussels as were those in 1995; accordingly they selected rnid-sued prey.

Vmation in prey avaiIubzlity

In a Mertest of the nsk-averse foraging hypothesis, 1 compared regular and manipulated tiles at each location At Indian Point, ducks feeding on manipulated tiies

(with mussels of the largest size class added to other tile sections) during spring and summer preferred mussels 1O- 19 mm, as opposed to prey 19-28 mm preferred by ducks

feeding on regular tiles. When the proportion of unprofitable prey increased in the

population, ducks responded by selecting smaller mussels.

Surprisingly, selection of mussels also varied with tile type during winter. Given

that large mussels were preferred on regular tiles during winter, ducks shodd have

contlliued to select them on manipulated tiles. However, in this situation ail size classes

were taken randody, except 19-28 mm, which was strongiy avoided. Possibly, 10- 19 mm

mussels were taken incidentdy because ducks were feeding on large ones located in the

same tile section as srnail prey. The srnallest mussels (which are more mobile than Iarger

individuals) sometirnes attached on top of large mussels on manipulated tiles. Ifducks

selected these sdprey, they could easily remove them without dislodging the larger

mussels, which were attached more firmly to the underlying tile. However, if ducks were

selecting the larger prey (only during winter), small mussels on top of them may have been

removed at the same tirne. This phenornenon may have contributed to the result described

here. It is also possible that the very sdsample (n=5) of manipulated des in winter

provided unclear results due to indcient replication.

The nsk averse foraging hypothesis was not supported at Barr Road when

manipufated and regular tiles were compareci. Birds appeared to select somewhat larger

mussels from manipulated tiles than they did fkom regular ones, especially in 19%. I

carmot offer an explanation for this, except to suggest that again results rnay be suspect due to a very smali sample at this location (n=6 manipulated tiles). Conclusions

Common Eiders are six-selective predators. 1 found support for both the shell

mass mhimkation hypothesis and the risk-averse foraging hypothesis, though the latter

should probably be investigated Merdue to questionable results in one area where 1 had

Iimited replication. 1 examineci only a few of rnany decisions confkonting predators each

time they dive. Although other factors (e.g. depth, prey avaiiability) were controlled, 1 did

not deal with things such as handling tirne, dive duration, and when during dives birds fed

on mussels fiom the tiles. AU of these factors could influence results (Beauchamp et al.

1992, DeLeeuw and VanEerden 1992, Guillemette et al. 1992). Prey selection under

natural conditions is complicated and often ditFcult to explain using simple models (Bal1

1994). The fact that 1 obtained significant, interpretable patterns suggests that the factors examineci are important to foraging eiders.

Some of the variability in previous estirnates of prey selection by Common Eiders may be explained by these fïndings. RaEaelli et al. (1 990) found that eiders preferred mussels 10-25 mm, and that these were large relative to the available population. They coilected ducks in December and January, when, acwrding to my results, large prey should be preferred. Sirnilarly, GuiUemette et al. (1996) reported that eiders fed in winter on a modal mussel size of 8 mm, when the modal availability was 3-4 mm (though they ascribed part of the Merence to different collection times for mussels and ducks).

Nystrom et al. (199 1) found that eiders selected mussels 17- 18 mm, but that these were smaller than the average of those avaiiable. They attributed this to attempts by eiders to minùnize salt intake by eating small mussels. However, they collected their data in September and October, when, accordhg to my findings, selection of smailer mussels rnay also be favoured to minimize shell ingestion

My results highlight the importance of wnside~gfactors such as prey avaiiability? local background conditions, and season in prey selection studies. Foraging choices of

Common Eiders are innuenced by both season and reIative abundance of unprofitable prey. Eiders are capable of adjusting feeding pattern relative to seasonal profitability changes. This is a regular, repeatable pattern. They are less good at handling uopredictable variation in prey profitability within seasons, and tend to respond to increased abundance of unprofitable prey by taking smaller mussels. CHAPTER 3

Direct and indirect efkts of predation by Cornmon Eiders in an intertidal

community under disturbed and undisturbed conditions

Introduction

Of interest to ecologists is the relative importance of biotic interactions (predation,

competition) and abiotic factors such as disturbance in stnicturing communities (e-g.

Menge and Sutherland 1976, 1987, Menge and Farrell 1989). Much of the fundamental

research on competition (Conneil 196l), predation (Paine 1966, 1974, 1980, Menge 1983,

Menge et al. 1994), and disturbance (Lubchenco and Menge 1978, Sousa 1979%b) has

been conducted using the marine intertidal zone as a model. Recently, the infiuence of

indirect effects of species interactions on cornrnunity dynamics has received attention

(Strauss 199 1, Wootton 1992, 1993% 1994%b, Menge 1995, 1997, Navarrete 1996,

Navarrete and Menge 1996, Strong 1997), though it had long been recognized that

interactions among species affect more than just the direct participants (Paine 1966, 1980,

Carpenter et al. 1985, Sih et al. 1985).

Predation (including herbivory) by invertebrates is important in stnicturing

intertidal cornmunities (e.g. Paine 1966, 1974, 1980, Lubchenco 1978, Suchanek 1978,

Menge and Lubchenco 198 1, Menge 1983, Petraitis 1987, Farrell 1988, Robles and Robb

1993, Navarrete 1996, Navarrete and Menge 1996, and many others). However, until recently, most intertidal studies have not included vertebrate predators (Pimrn et al. 199 l), and the sigdicance of excluding them is not weil understood (Yodzis 1993). Birds (primarily gulls, oystercatchers, and other shorebirds) can signincantly reduce abundance

of their inverâebrate prey, and sometimes iafiuence cornu* structure (e-g. Schneider

1978, Quammen 1984, Bazely and Jefferies 1986, Mamh 1986a,b, Hahn and Demy 1989,

Good 1992, Székely and Bamburger 1992, Wootton 1992, 1993a,b, 1994% 1997, Dumas and Witman 1993, Meese 1993), so it may be important to Uiclude them intertidal community studies. Bronmark (1988) concluded that bird predation was the major force stnict-uringbenthic fieshwater gastropod communities, and Wootton (1997) stated that even though birds may be present in low numbers, their high metabolic rate compared to invertebrate and fish predators makes it possible that they are significant intertidal predators.

Waterfiowl (Anatidae), in particuiar, have received Little attention in htertidal systems, possibly because they are fiequently transient rnembers of communities and, as such, may exert little lasting influence on the system (sensu Marsh 1986%Hamilton et al.

1994). However, Navarrete (1996) concluded that variable predation, such as that by a seasonal predator, can have signifïcant effkcts on cornmunities. There is lirnited and contradictory evidence wnceming the ability of waterfowl to act as keystone predators

(sema Paine 1966, 1969) and thus to affect community structure (Smith et al. 1986,

RaEaeIli et al. 1990, Nudds 1992, RafEaeUî and Hall 1992). Bazely and JeEeries (1986) found that removal of snow geese (Chcaerdescens) fiom a salt-marsh ecosystem led to marked vegetation changes and increased species richness, suggesting that they were important predators in that system. Direct effects of predation by waterfowl on their hvertebrate prey can be assessed relatively quickly. However, cascading or indirect effects, which would si- importance in the cornmunity for these birds, take more time to develop (Yodzis 1988, Wootton 1992) (but see Menge 1997 for an altemate view).

Therefore, studies assessiag community-wide effects need to be of adequate duration.

The role of abiotic disturbance and resuiting succession in srniauring comrnunities has been weil studied in intertidal systems (Dayton 1971, Sousa 197941, Paine and Levin

1981, Menge and Sutherland 1987, Menge and Farrell 1989). Disturbance is thought to affect species diversity by limiting the dominant cornpetitor, and thereby increase the ability of inferior cornpetitors to coexist. Intermediate levels of disturbance are considered to provide opportunity for highest levels of diversity in the system (Conne11 1978, Sousa

1979%Paine and LeWi 1981). Mer a disturbance occurs, diversity kstincreases, then declines as the system returns to initia conditions (Sousa 1979b, Collins et al. 1995).

Effects of disturbance on intertidal invertebrate predators have been studied using mensurative and manipulative field experiments, primarily by looking at effects of predators in difEerent locations which are exposed to dioering levels of disturbance or environmental harshness (e-g. Lubchenco and Menge 1978, Menge 1978a,b, 1983).

Effkcts of predators on recolonization and succession in disturbed areas have dso been investigated (e-g. Marsh 1986%McCook and Chapman 1993, Wootton 1993b). However, rarely has the effect of predation under both disturbed and undistubed conditions been considered in the same location using a combination of pulse (simulated catastrophic disturbance) and press (predator exclusion) perturbations (but see Lubchenco and Menge

1978). This interaction of predation and disturbance is important to consider, because the effects of predators (including herbivores) are known to change with disturbance (Menge 1978a, b, 1983, Lubchenco 1986, Menge and FarreIl 1989). As weu, while hidies that

compare predation in locations subject to differing levels of disturbance provide

information on possible effects of disturbance on predaton, it is often difficult to separate

these effects fiom other ciifferences among sites (Underwood and Petraitis 1993).

Birds are highly mobile predators and their response to disturbance may Mer from

that of less mobile invertebrates. Waterfowl tend to be gregarious on feeding grounds (e-g.

Nisson 1972, Stott and Olson 1973. McCuilough 1981, Guillemette et al. 1993),

increasing the possibility that they will have detectable eEison the system (sensu

Wootton 1997). Their ability to quickly lave unfavourable feeding areas when better sites

are avdable suggests that they may respond more rapidly to a catastrophic disturbance

than would a less mobile predator. Similady, they may respond to recovery in the system

quickly and resume feeding in a previously disturbed area sooner than would less mobile

predators.

tndirect effects and cornpetition among predators have recently been discussed in

intertidd coxnmunity ecology (Strauss 1991, Wootton 1992, 1993a, 1994% b, Menge

1995, 1997, Navarrete 1996, Navarrete and Menge 1996). Menge (1 999,in a synthesis

and reanalysis of previous data, concludeci that about 40% of the change in community structure resulting &om manipulations came f?om indirect effects. Sometimes, removal of keystone or other important predators (as in an exclusion experiment) allows other

(formerly "redundant'*) species to increase and effectively take over ail predation in the system (temed compensatory predation) (e.g. Robles and Robb 1993, Navarrete and

Menge 1996). This indirect effect results in iittle change in the community except for an increase in the compensating predator, and may be a stabilizing factor. Such an interaction

has been demonstrated in an intertidal system involhg seastars (Pimer)(the dominant

predator) and wheks (the cornpensating predator) (Navarrete and Menge 1996). As well,

Marsh (1986a) provided evidence that when both avian and other predators reduce prey

density, removal of birds from the system may have linle effect on the community, because

other predators compensate.

I investigated predation by Common Eiders (Somateria mollissima) in an intertidal invertebrate community under disturbed (simdated effects of strong wave action or ice

scour) and undisturbed conditions. Eiders are predaton of blue mussels (Mytiusehlzs)

(Milne and l3un.net 1972, Baird and Milne 198 1, Goudie and Ankney 1986, Raffaelii et al.

1990, Wemette et al. 1992, 1993, 1996, Gorman and RaEaelli 1993, Kilgerloh 1997), the dominant invertebrate in my study area. These ducks are present and feed year-round in the ara (though they are more abundant during winter), making it possible that they may exert strong effects on the community (sem Marsh 1986a).

Using predator exclusion cages and sKnulated abiotic disturbance, 1 examineci whether predation by eiders influenced comrnunity structure and had indirect effects on other species. 1assessed whether disturbance, such as that caused by ice scour or unusually strong wave action, altered the effect of eiders on the system., and whether predation by eiders changed the rate of recovery by the comrnunity after disturbance.

Findy, 1 examineci whether exclusion of eiders Ied to an increase in effectiveness of other predators in the system; i.e. was predation by the different species in the cornmunity additive or cornpensatory? Methods

1 wnducted experiments between August 1994 and December 1996 in the

intertidal zone at Indian Point, in Passarnaquoddy Bay, St. Andrews, N.B.,Canada (45"

4' N, 67O 2' W). This area experiences extreme tidal fluctuation twice daily (vertical range exceeds 7.5 m during spring tides), generathg an intertidd zone extending more than 500 m f?om shore at low tide. The site is moderately protected, but during fd and winter is fiequently subject to strong wave action, causing some disturbance to the substrate. Experiments were conducted in the lower intertidal near the low tide line. The area was covered prirnarily by rocks and srnall boulders. Blue mussels were the dominant primary space occupier, but barnacles (SemzbuIms baIanoiCres), and limpets (Collisella testudinais) were also found there. Common Eiders were the most abundant vertebrate predator in the area. Invertebrate predators such as dogwhelks (Nucella lapiIIus) and seastars (Asferias forbesii and A. vuigms) were dso present. Periwinkles (primarily cornmon periwinkles Lzttorim linorea), moonsnails (Lumtiu spp.), amphipods (mostly

Gammarus spp. and M~nogcanrnmsspp.), and other smail gastropods (notably Onoba adeus) were common. Ephemeral algae (e.g. IBva, Porphyra) were present at the site only occasionaliy in summer, and rockweed (primarily AscophyIZurn nwlosum but also

Fucus vesi~tlIosis),while abundant in the mid-intertidal zone, was rare in the experimental area.

Predaor exclusion experiments

During summer 1994, I positioned 12 experimental sites of 7 m x 3.5 m at Indian

Point. Sites were chosen randomly with the stipulation that each had a visualiy uniform cover of blue mussels and was near the low tide he. This ensured that all sites wodd be

subject to approximately the same environmental conditions. At high tide, sites were 5.5 to

7 m under water, depending upon the moon phase. The vertid Merence in tidal height

between the deepest and shallowest site was 40 cm. Mer sites were selected, six were

randomly assigned to the cüshûbed treatment. Disturbance consisted of manually removing

approximately 80% ofall biomass in the area using rakes, shovels, and trowels to scrape

rocks. 1 lefi residual matenai on rocks because this disturbance was designed to mimic the

effects of strong wave action or ice scour associateci with an unusually bad stonn or cold

winter, and not to restart primary succession. There is an annual pattern of fa11 and winter disturbance in the area which reduces invertebrate biomass fiom fàU to spring @ers.obs.), but this experimental disturbance was of a more catastrophic nature, and represents one which occurs only rarely.

After disturbance was complete, I placed a predator exclusion cage and paûed control area in each site. Positions of treatments within sites were randomly assigned.

Cages were designed to prevent ducks fiom feeding under them but permitted access to ali other potential predators. Cages were square, 1.5 m x 1.5 m and 30 cm high, constructed of 3.8 cm ABS pipe and covered on the top surface by green construction fencing (mesh size approximately 3 cm). Cage sides were left uncovered to minimize effects associated with changes in water flow inside versus outside a cage, and to ailow free access to the area under the cage to other predators, such as seastars, whelks, and fish. This design has been used effectively by others with no evidence of cage effects (RafEaeUi and Hall 1992,

Hamilton et al. 1994) (see discussion for Merconsideration of cage effects). There was a 2-rn gap between cage and wntrol, and a 1-m border around all outside edges of cages and controls (Fig. 3.1). This avoided potential problems in disturbed sites of encroachment of mussels fiom the area immediately outside the site (Paine and Levin 198 1). Cages were anchored by roping them to angle iron poas pounded into the substrate at the four corners. I marked control areas at the corners with four wooden pegs.

Cages were maintaineci in place throughout the 28 mo experimental penod, and sites were monitored regularly. Durhg sp~gand swnmer, 1 checked cages and cleaned them of algal buildup biweekly. During fdand winter, and whenever storms occurred during the rest of the year, monitoring was more fiequent (daily during severe weather) to check that cages remaineci intact and to replace damaged or missing ones. Several cages were lost over the course of the experiment, but these were always replaced within 48 h.

Shortly fier the experiment was initiated, and eveiy 4 months thereafter, 1 collected samples £?om cage and wntrol areas. Sampling consisted of removing four randomly selected 100 cm2 cores of material fiom each cage and control area. Only the centre 1 m2 under each cage or in each control area was sampled to avoid possible edge effkcts associated with ducks reaching under the edges of cages. Mer material was removed, I replaced it with an quivalent amount from outside the cage to avoid reductions in density of invertebrates in experimental areas. Areas which were sampled were noted using a grid system and never resarnpled on subsequent visits. In August 1996, dercollection of the ha1sample, 1 reversed the position of cages and control areas. This was done to observe the response of ducks to a superabundant food source (mussels which had been protected by cages for 2 yrs) and to ver@ that effects observed were due Fig. 3.1. Example of a site, including the cage and control area. The disturbed area was 7

m x 3.5 m.

to duck predation and not cage position. 1 mooitored these sites until December 1996, at

which time another sample was collected and the experiment termihateci.

1 visuaily estimated percent cover of blue mussels in cages and control areas every

two months fiom August 1995 to December 1996. Using a 1 m2quadrat divided into 100

squares, 1 determineci how much of each site was covered by mussels and, among sections

not covered by rnussels, the substrate type. I also estimated percent cover of rnussels and

the various substrate types annuaily in the area around the experimentd sites. Using a

quadrat as described above, 1 examined every metre dong three 40 m transects in the

musse1 bed during summer in 1995 and 1996. This was done to check for generai changes

in the musse1 bed due to abiotic disturbance and natural population fluctuations.

Mer collection, samples were rinsed in an 850 um sieve and preserved in 95% ethanol. Invertebrates (with the exception of polychaetes, other worms, and severai juveniie or damaged specimens) were identifid to species, and shelled prey were measured. I made identifications foliowing Bousefield (1 960, 1973), Gosner (1 97 1), and

Davis (1976). As weU, 1 veriiied specimens by cornparison with the reference collection at the Atlantic Reference Centre, Huntsman Marine Science Centre, St. Andrews, N.B.. Blue mussels were measured and sorted into size celasses. Samples were then dried to constant mass (90" C for approximately 20 h), each species was weighed, ashed for 2 h at 550" C, and reweighed. 1 estimated dry tissue biomass by subtracting predicted sheil loss due to ashing (eom a reference set of empty shelts ashed under the same conditions) fiom observed ash free dry mas. This biomass was then used as a measure of abundance in analyses. Tests of cage effects

To check for unanticipated cage effects, 1 conducted a series of obsewations and analyses of supplemental data. I observeci sites at hi& tide by SCUBA diving to determine whether other potential predators were aîtracted to or avoided cages. I positioned settlement plates in cage and control areas to compare rates of settlement of juvenile mussels. Blue mussel veligers are carriexi by water currents (Koehn et al. 1984), so changes in water flow under cages could lead to changes in settlement patterns. 1 aiso examineci the population of nadyoccurrîng srnaIl(< 5mm) mussels in cages and control areas, again to look at settlement patterns.

Duck counts, feecing behaviour, and diet

I determined whether ducks in the study area were feeding on mussels and other prey by examinhg gizzards and esophagi of eiders shot during the fdhunting season.

Shell fragments found in ducks are usuaily readily identifiable into broad taxonomie groups. AU ducks feeding in the study area were counted at Ieast twice weekiy throughout the experiment. Scan counts were conducted from shore at approximately 1 to 2 hrs after high tide using a 60 mm spotting scope (Bushneii Spacemaster II, Vancouver, B.C.).

Statisfrcal anafyses

AU analyses were dedout using SAS version 6.1 1 (SAS Institute, Cary, NC).

Pnor to analyses, data were tested for violation of assumptions. Normality was tested using the D-statistic in SAS proc Univariate. Hornogeneity of variance was tested using

Bartlett's test, and Taylor's Power Law (Elliot 1977) was used to test for a linkage of variance to mean, and, when necessary, to determine the appropnate transformation. 1 evaluated significance of results using an a level of 0.05 for miin effects, and 0.1 5 for

interactions. 1 chose to relax the a level for interactions following Wmer et ai. (1991),

who recommended examining borderline interactions, because they may obscure main effects.

The basic model used for analyses was a split-plot, mked model analysis of variance. Variables under consideration included treatment (cage versus control area), type (disturbance level), site (a random variable) and sample penod (the split-plot factor).

The main model included all data except the December 1996 sample in which cages and controls had been reversed. The split-plot model is analogous to univariate repeated measures analysis when the temporal variable (within subjects factor) is assigned as the split-plot factor (Wmer et al. 1991). In this case it was a more useful approach than the standard repeated masures analysis because it allowed a straightforward conversion to multivariate analysis of individual species biomasses (see below).

Dependent variables anaiysed were total dry tissue biomass, species nchness

(number of species per sample), and species diversity (Shannon-Wiener index) (Begon et ai. 1986). 1 used a saturated model, then reran it seguentiaily delethg non-signifiant interactions. When interactions were detected, data were split and reanalysed separately for each level of the least interesthg interacting variable (e.g. in an interaction of sample and predation, data were split by sample). 1 mried out only pre-planned comparisons in these reduced models; however when main effects were detected elsewhere and a posfenori comparisons among means were aecessaryy1 used Tukey's HSD test (Zar

1996) to identify ciifferences. 1ran a similar model using only data collectai in Augua and December 1996 to examine redts of the cage/mntrol reversal experiment. Effects and

appropnate error terrns for these models are listed in Table 3.1. When 1 found non-

significant results in situations where differences among means appeared relatively large, 1

assessed statisticai power following Zar (1996).

The Shannon-Wiener diversity index combines measures of species richness and

evenneu (Begon et al. 1986), and will therefore detect shifts in dominance by the moa

abundant species in the community, but does not consider changes among individual

species. Therefore, 1 also performed a multivariate analysis of variance using the four most

abundant species as dependent variables, and the same model as described above for the split-plot ANOVA Significance of results was evaluated ushg Pillai's trace, the most conservative of the available multivariate tests (Tabachnick and Fidell 1989, Scheiner

1993). 1 used standardized canonical coefficients generated by SAS to examine interactions among species (Scheiner 1993). As before, 1 used a saturated model and sequentidy deleted non-signincant interactions. When interactions ocnirred, 1split the data set and ran the reduced model again. Mer determining the strongest set of interacting species in the community, 1analysed their abundances separately using a mixed model split-plot MVAas described above. Sues of blue mussels (exclucihg those smaller than 5 mm long) were also compared among treatments, disturbance levels, and samples using the same model.

Percent cover of mussels in cage and control areas was analysed using angular

(arc-sine) transformed values (Zar 1996) and the split-plot mixed model ANOVA (Table

3.1). Musse1 cover in transects nom the area around my experimental sites was compared Table 3.1. FoUowing are the effects tested in ANOVA and MANOVA models used

throughout the paper. Only effects of interest are listed. When signifiant

interactions occufed, data were split and analysed separately for each level of one

of the interacting variables. Error terms for each effkct are indicated. A term in

parentheses indicates that it is nested within the term to the lefi of it. Type refers to

disturbance Ievel, and treatment to predator exclusion (cage versus control).

Effect Sorted by Error term type sitettype) treatment treatment x site(type) type x treatment treatment x site(type) sample sample x treatment(site x type) sample x type sample x treatment(site x type) sample x treatment sample x treatment(site x type) sample x type x treatment sample x treatment(site x type) type sample site(type) treatment sarnple treatment x site(type) type x treatment sample treatment x site(type)

treatment type treatment x site sample type sample x treatment x site treatrnent x sample type sample x treatment x site treatment wunple and type treatment x site among years using the non-parametric Kniskal-Wallis test (Zar 1996) because data

violated assumptions of parametric tests. 1 analysai data from settlement plates using a

paired t-test which compared numbers of srnail mussels settled on plates in cages and

wntrol areas. Number of srnail mussels (CS mm) in samples was anaiysed using the split-

plot mixeci mode1 ANOVA (Table 3.1).

Results

Statistical power of most tests involving species richness, diversity, and total

biomass was relatively high. Unless otherwise indicated below, ciifferences of 20 to 30%

among means couid be detected about 80% of the tirne. Power of tests on individual

species was highly variable, and for the most part lower. It ranged fiom 80% power to

detect a 20% Merence to 50% power to detect a 75% dinerence. Power of tests which showed a large difference among means but a non-sigiilncant result is indicated below.

S~CISOMIdymmics

There were seasonal effects on dry tissue biomass, species richness, and diversity independent of duck predation, but which varied in some cases with disturbance level.

Biomass in undisturbed (referred to throughout the text as "naturai") sites varied with sample (df-6,66, F=9.87, P<0.0001). Surnmer biomass tended to be highest, followed by declines in fa11 and winter (Fig. 3.2). 1 observed a greater decline in biomass and less rewvery in surnmer 1996 than in the previous years (Tukey's test) (Fig. 3 -2). Seasonal trends in biomass of harvested sites could not be assessed statistically because of sigdicant interactions, but after biomass had recovered, the same pattern appeared (Fig. Fig. 3.2. Dry tissue biomass in cage and control areas of disturbed and naiural sites during

each sampling penod. Error bars are 1 standard error. Resdts of statistical

cornparisons are provided in Table 3.2. (LI I 3.2). Thai corresponds with changes in overail musse1 abundance in the area. The mussel

bed near my shidy site was more dense during 1995 than 1996 (Kniskal-Wallis x2 =29.1,

df-1 , P<0.0001). Average cover of mussels (combining all transects) in 1995 was 59 %

and in 1996 38 %, reflecting severe stomwhich ocairred during fdl 1995.

Species richness in both natural and disturbed sites was iduenced primarily by

sample (df4,126, F=17.75, P

progressed (Fig. 3.3). Richness was lower during summers of 1995 and 1996 than during

the first year of the experiment (Tukey's test). There was an interaction between sample

and disturbance for species diversity (P<0.0001), but for both types of sites, diversity

varied with season (df-6,66, F=9.13 and 16-25,P<0.000 1 in both areas). In disturbed

sites, diversity was highest in December 1995 and April 1996, and lowest during summers

of 1995 and 1996 (Tukey's Test, Fig. 3 -4). In natural sites, diversity generdy increased throughout the study (Fig. 3.4)' and was significantly higher fiom December 1995 to

August 1996 than during the rest of the experiment (Tukey's test).

Effects of predaiun und disfrrbmce on biomms

Effects of predation on total dry tissue biomass differed among both disturbance levels and samples (3-way interaction, Table 3.2). During August 1994, biomass at disturbed sites was less than that in natural areas, but there was no ciifference between cages and controls (Table 3.2, Fig. 3.2). Effects of disturbance on biomass persisted through August, 1995, but disappeared by December (Table 3 -2,Fig. 3.2) (though this test had only 50% power to detect a 60% Merence arnong means). Predation had a marginal effect on the system by December 1994; there was more biomass under cages Fig. 3 -3. Species richness in cage and control areas of disturbed and natural sites durhg

each sampling period. Emor bars are * 1 standard error. Results of statistical

cornparisons are provideci in the text.

Fig. 3.4. Species diversity in cage and control arG of disturbed and natural sites during

each sampling period. Error bars are 1 standard error. Results of statistical

cornparisons are provided in Table 3.2.

Table 3.2. Results of ANOVAs for biomass, species diversity, and species nchness. Only

signincant effects, or those approaching sigdicance are listeci. When interactions

occurred, data were split and reanaiysed separately for each level of one of the

interacting variables. Type refers to disturbance and treat to predator exclusion.

Sample Dist. Level Effect df F P ciifferences Biomass

all samp*type*treat 6,120 2.5 ail type 1,lO 192.2 nat > dlst all type 1,lO 30.0 nat > dist all treat 1,11 3 -2 cage > control au VI"= 1,10 56.6 nat > dist all treat 1,11 13.5 cage > control

all type 1,10 15.1 nat > dist au treat 1,11 9.3 cage > control au typeftreat 1,10 4.5 disturbed treat 1,10 8.1 cage > control all type*treat 1, IO 7.7 disturbed treat 1,IO 15.4 Species diversity

-- - al1 all w~*tYPe 6,126 6.4 <0.0001 Aug/94 aii type 1,10 37.8 <0.0001 dist>nat Dec/94 al1 type 1,10 15.8 0.0026 dist > nat Apr/95 d tYPe 1,10 14.0 0.0039 dist>nat Aug/95 ail type 1,lO 4.3 0.064 dist > nat Au@6 ail type 1,lO 4.0 0.074 nat > dist (Table 3.2), especially in oatural sites (Fig. 3 -2). By April 1995, there was a highly signiscant differeace between cages and control areas (Table 3.2), and the effect was consistent in both disturbed and naturai sites (Fig. 3.2). The overali effect persiaed in

Augua 1995 (Table 3.2), but was mostly in distuhed sites; the effect in naturd sites was declining by this time (Fig. 3.2). The fdof 1995 was unusually harsh, with severe wind stomdisturbing the experiments. By December 1995, though the trend was consistent in both types of sites, there was no longer an effkct of predation (Fig. 3 -2). However, power was low; a 40% ciifference was detectable only 50% of the tirne.

In April and August 1996, there were interactions between disturbance and predation, requiring separate analyses of disturbed and natural sites (Table 3.2). In both samples, there was no effect of predaîion in naturai sites, but more biomass under cages than in controls in disturbed areas (Table 3 -2, Fig. 3.2). Disturbance delayed the effect of predation, but ultimately dowed it to persist longer in the system.

Effects of predatim and disturbance on species richness and divers@

There was an interaction between disturbance and predation for species richness

11). Predation had no effect on nchness in natural sites, and only a marginal effect in disturbed areas (P=û. 11); richness was greater under cages than in controls. For species diversity, an interaction of sample and disturbance required reanalysis separately for each sample (Table 3.2). Diversity was greater in dishirbed than natural areas through August

1995 (though the effect was marginal then), after which the ditference disappeared (Table

3.2, Fig. 3.4). This loss of dEerence was coincident with the disappearance of biomass merences. The effect reversed by August 1996, when diversity was marginaUy higher in natural than disturbed areas (Table 3.2, Fig. 3.4). There was no eEiof predation on species diversity (Fig. 3.4). There were marginal interactions between distuhance and predation in December 1994 (P=û. 14) and December 1995 0.12). In both cases, when disturbed and nadsites were dysedseparately, there were no predation effkcts on diversity in dishirbed sites, and non-significant trends toward increased diversiq in wntrol areas of naturai sites (Fig. 3.4). This trend was also evident in April 1995 (Fig. 3.4).

S@es composition

The four rnost abundant species in samples were blue mussels, limpets, common periwinkles, and whelks. Barnacles were abundant by the end of the experiment, but present at only a few sites during most of the study, so they were not included in the analysis. A complete List of species identifiecl during this experirnent, together with their abundance, is provided in Appendix 2.

Mussels are stationary primary space ocnipiers, hpets and periwinkles are mobile grazers, and whelks are predators of other invertebrates. When dry tissue biomasses of these four species were compared in a MANOVA, there was a 3-way interaction of season, disturbance, and predation (&24,480, F=1.47, P==û.07),necessitating andysis separately for each sarnple. As before, there was an effect of disturbance on the biomass of invertebrates fiom the beguuiing of the experirnent through August, 1995 (Table 3.3). Ln most of these samples, blue mussels contributed most to the observed variation. Whelks and mussels varied similady across treatrnents, but wbeiks did not contribute much to the redt until August, 1995 (Table 3.3). Limpets and penwinkies were only occasiondy influentid in the muitivariate result (Table 3.3). Predation had no efkt on biomass of Table 3 -3. Results of MANOVA for dry tissue biomass of the four most common species

present. Coefficients are of standardized canonid variates. Similar signs of

coefficients indicate a positive correlation between species. Magnitude of values

indicates the relative contribution of each species to the result . Type refers to

disturbance level and treat to predator exclusion. OnIy signincant or margindy

significant results are indicated. There were no significant redts for December

1995 or April 1996.

Sample Effect df F P limpets rnussels periwinkles wheiks Aug/94 type 4,7 47.3 <0.0001 -0.18 3.60 -0.26 0.59

Ded94 type 4,7 12.3 0,003 1.O6 2.07 -0.02 0.29 Apd95 type 4,7 13.9 0.002 0.11 2.20 0.26 0.3 1 Apr/95 treat 4,8 9.2 0.004 -0.40 2.25 1.30 -1.48 Aug/95 type 4,7 7.5 0.011 -0.11 1.66 O. 19 1.22 Aug/95 treat 4,8 3.0 0.088 -0.58 1.53 0.73 0.94

Aug/96 type* 4,7 4.2 0.048 treat AL@% treat 4,7 4.6 0.04 0.06 1.51 -.O32 0.74 disturbed these species until April 1995. At that the, mussels again contributed most to the result,

followed by whelks and periwinkles (Table 3 -3). Whelks correlated negatively with

mussels and perbhkies (Table 3.3). A sdpredation effect was observed in August

1995 involving the same three species, but then all were positively correlateci. The only

other predation effect observed in the system was in August 1996 in disturbed sites.

There, blue mussels and whelks were also positively correlated and contributed most to

the redt (Table 3 -3).

1also assessed multivariate data for a thetrend by analysing disturbed and natural

sites separately. In disturbed sites, there was an interaction of sample and predation

(@24,240, F=2.4, W.0004). In natural sites, there was no interaction, and a strong temporal effect (df=24,264, F=5.7, P<0.0001).This generated four eigenvectors, of which the first 3 were significant. The first two accounted for 87% of the variability explained by the model. 1Uiterpreted standardized canonid variates following Scheiner (1993). The first variate had strongest loadings for mussels, followed by periwllikles, whelks, and limpets. Mussels and whelks were positive, and the others negative, meaning that rnussels and wheUcs were positively correlated across samples, i.e. when musse1 abundance increased, so did whelks. The second variate had positive loadings for whelks and periwinkles, and negative for mussels and limpets. This means that, withui at least sorne samples, mussels and whelks were negatively correlated, Le. an increase in wheks meant a relative decrease in mussels.

To clarifjr the relationship between mussels and whellcs in this system, 1 ran separate ANOVAs for each species. For blue mussels, results were very sirnilar to the result for total biomass. There was a sample x disturbance x predation interaction

(W.0 15). requiring redysis separately for each sample. There was an effect of

disturbance, with significantly more biomass in natural thao disturbed sites, that persisted

through August 1995. There was sipnincantly more musse1 biomass under cages than in

wntrol areas in December 1994 (P+.058), April 1995 (P4.017), and August 1995

0.034).Effxts were strongest in aatural sites early in the experiment, but becarne

more apparent in disturbed areas by August 1995 (Fig. 3 3.There were no Merences in

December 1995, but in both April (W.007) and August 1996 (P=O.002), there were

more mussels under cages in disturbed sites that in controls (Fig. 3.5).

There was more whelk biomass in natural sites (where mussels were also more

abundant) than in disturbed areas in December 1994 (P=û.04), and marginally more in

April 1995 (W.1 1) (Fig. 3.6). By August 1995, there was an interaction of predation and

disturbance (P=û.05)for wheik dry tissue biomass. When analysed separately, neither

dishirbed nor natural sites showed a significant result, but there was a strong trend in

naturd areas for there to be more whelk biomass in cages than in control areas (Fig. 3.6).

The power of this test was very low, with an 80% difference among means detectable only

50% of the tirne, so the lack of signiEicance should not be interpreted as indicating that a

ciifference did not exkt From December 1995 through the end of the experiment, there

was a growing trend for increased whelk biomass in cages relative to controls (Fig. 3 -6)

(December 1995 W.072, April 1996 W.068, August 1996 W.038). The last result was especiaily pronounced in disturf,ed sites, where blue mussels were also more dense under cages than in controls (Fig. 3.5). There was also a clear (though difficult to assess Fig. 3 S. Dry tissue biornass of blue mussels in cage and control areas of disturbed and

natural sites during each samphg period. Error bars are * 1 standard error.

Results of statistical cornparisons are provided in the text.

Fig. 3.6. Dry tissue biomass of whelks in cage and control areas of dimirbed and naturd

sites during each samphg penod. Error bars are * 1 standard error. Results of

statistical cornparisons are provided in the text. Disturbed sites

Ilml cage control Natural sites cage control

Aprl95 Augl95 Decl95 Sample statistidy because of interactions) thetrend in whek biomass, with increases during the summer months in 1995 and 1996. These seasons corresponded with hcreases in mussel biomass (Fig. 3 -5).

Over the course of the experiment, there was general correspondence between musse1 and whelk biomass, generating the positively correlated loadings in canonical variate 1. However, within a given sample (eg. August 1995), mussel and whelk abundance was negatively correlated, generating the negative correlation for canonical variate 2. In this example, there was a large increase in whelk abundance in cages relative to controls of natural site, but the reverse was tme for mussels.

Mussel Zength

Mean lengths of blue mussels varied over tirne and with predation. There was a strong temporal trend, with overall increasing size as the experiment progressed

(df%,13 5, F=26 -4, P<0.0001) (Fig. 3.7). Mussels under cages were larger than those in control areas (W.027); effects were rnost sigdcant in disturbed sites and began in April

1995 (Tukey's test). The largest Merences were observed in April and August 1995 and

1996 (Fig. 3 -7). Size fiequency distributions of mussels in samples illustrate the effkct of duck predation on the mussel population (Fig. 3.8). In August 1994, shortly afler the experiment was started, there was very little merence in the size distribution of mussels in cages versus controls for either disturbed or naturai sites (Fig. 3.8a). By April 1995, in natural sites there was salittle Merence in the actual distribution of mussels, but a

Merence between the two treatments in abundance of mussels was evident primarily in the 11-25 mm size ranges (Fig. 3.8b). These sizes correspond to preferred size classes of Fig. 3.7. Average mussd length in cage and control areas of disturbed and natural sites

during each sampling penod. Error bars are * 1 standard error. Results of

statistical cornparisons are provided in the text.

Fig. 3.8. Size fkquency distributions of mussels collected fiorn cages and controls in

disturbed and naturd sites in a) August 1994, and b) Apd 1995. Il Cage O Control r Natural sites

Q; 160 çt Disturbed sites E 120 z 80 40 n

Size class (mm)

Natural sites 500 2 400 300 g 200 E 100 aQ) E 801 Disturbed sites

- 57 7-9 9-1 1 1 1-13 13-15 15-17 17-20 20-25 2530 30-35 >35 Size class (mm) 107 mussels selected by ducks during most of the year in a prey seledon experiment

conducted in the area (Chapter 2). In disturbed sites, by April 1995 the trend for increased

mean mussel size in cages relative to controls (Fig. 3.7) was evidenced by the increase in

mussels in the largest ske class in cages (Fig. 3.8b).

Reversal of cage and control areus

When cages and controls were reversed after Augua 1996, there was an immediate switch in biomass in both disturbed and natural sites (Fig. 3.9). Interactions of disturbance x predation (P=0.038), sarnple x predation (P=0.003),and sample x disturbance m.025) indicate that effects of predation and disturbance on the system ditfered fiom before to afler cages were moved, and that predation had a different effect, depending on disturbance. Before the switch, there was significantly more biomass under cages than in controls of disturbed sites, but no difference in natural areas (see results for

August 1996 sample, Table 3.2). In December 1996, there was no difference between cages and controls in either disrurbed or natural sites, though the Merence approached significance in naturd areas (Fig. 3.9) (and power was low, with a 75% dserence detectable only 50% of the tirne). By moving cages, formerly proteaed mussels in disturbed sites were subject again to predation by ducks (as well as whelks), leading to reductions in their abundance (Fig. 3.9). In natural areas, mussels which used to be exposed were protected, dowing them to recover somewhat (Fig. 3.9).

Cover of mussels in cages and conirols

When 1 compared percentage cover by blue mussels among samples, predation, and disturbance levels for August 1995 through December 1996 (the reversed sample), Fig. 3.9. Dry tissue biomass in cage and control areas of disturbed and naturd sites both

before (August 1996) and afker @ecember 1996) exclusion areas were switched.

Numbers under bars indicate original positions (e-g. number 1 indicates disturbed

cages before the switch, and bernes disturbed controls afterward). Error bars are

* 1 standard error. Results of statisticai cornparisons are provided in the text. cage O control

12 3 4 Disturbed Natural Disturbed Natural Pre-switch Post-switch Sample significant interactions throughout (sample x predation W.001, sample x distuhance

W.067,disturbed sites sample x predation W.004, natural sites sample x predation

W -062)required separate analyses for each observation penod. In dishirbed sites, musse1

cover under cages exceeded that under controls until the naal sample, when cages and

controls were reversed (probabilities ranged fiom 0.009 to 0.03 5) (Fig. 3.10). In natural

areas, the only merence was in August 1995, when cover was rnarginally higher in cages

than controls w.071)(Fig. 3.10). These results correspond well with results fiom

samples coliected at the sites.

Tests for cage effects

1 observed cages at high tide by SCUBA during the sumrners of 1994, 1995, and

1996. There was no evidence that fish [e-g. winter flounder (Pleuronectes americms)

and sdpins (Myoxocephaius spp.)], which could potentially feed on invertebrates in the

study W. Litvak, pers. comrn.), were either attracted to or avoided cages. 1 observed

abundant seastars under cages during sumrner 1995 (but not the other y-), but this was

in response to the high density of mussels under cages, and was an indirect effect of duck

exclusion (see discussion). No other potentiai predators which might have confounded

effects ~f duck exclusion were noted.

There was no tendency for srnail mussels to settle on plates at a dEerent rate under cages than in control areas (paireci t-test W.28), indicating that water fiow was not altered under cages. Analysis of number of smaü (CS mm) mussels coliected in samples revealed an interaction between sample and disturbance level (P=0.003),but when data were analysed separately for each sample, there were never ciifferences between cages and Fig. 3.10. Percentage cover by blue mussels in cage and control areas of dishirbed and

naturai sites nom August 1995 to Demnber 1996. Obsemations were made every

two rnonths. The dashed vertical line indicates where age and control areas were

reversed after August 1996. Results of statistical cornparisons are provided in the

text. controis (probabilities ranged fkom O. 11 to O. 88).

Eider a~~ceand food habits

Birds were most abundant at my study site during the fd penod (September through December), 4th an average count of 295 per day. There were about 2 13 ducks per day nom January to Apd, and 168 per day during spring and summer (most of these were during May and lune) (Appendix 3). Birds were frequently observed diving over mussel beds and dacing with clurnps of mussels in their bills. Eiders retrieved fiom hunters had fed primarily on blue mussels. Of 43 eiders 1 exarnined which contained any fragments of food, 33 of them contained blue mussels, 22 contained periwùikles, and oniy five contained whelks. Other prey were taken incidentdy (Appendix 1). Arnong birds which had fed recently, d wntained primarily blue mussels.

Discussion

Results of this study Uidicate that, by feedmg on blue mussels, Common Eiders have indirect eEects on other invertebrate specieq that disturbance alters the effect of predation by eiders on the community, and that duck predation influences recovery of the syst em after disturbance.

EMct of disturbance

The artifïciai disturbance that I irnposed on this systern had an imrnediate effecton the community in ternis of both total biomass and species diversity. Disturbance led to an increase in diversity, which later retumed to pre-disturbance levels as the community recovered. Ditferences in diversity between disturbed and undisturbed sites disappeared at the same time as the cornmunity recovered in terms of biormss. Most of the diversity changes were due to shifts in evenness, because disturbance had no effkct on species richness in this system Generdy, the more abundant were mussels, the lower the species diversity.

This pattern is similar to others in intertidal systems, and is consistent with the idea that abiotic disturbance results in maintenance of community diversity by limiting abundance of the dominant cornpetitor (Conne1 1978, Sousa 1979a,b, Paine and LeWi

198 1). On the west coast of North America, the musse1 Myrius califmianus is the dominant space occupier. When it is reduced through disturbance or predation, species diversity increases (Paine 1966, 1974, Paine and Levin 1981). Sousa (1979b) found support for the intermediate disturbance hypothesis (Comell 1978); after boulders were overtwned, opening up new space, species diversity first increased then declined as succession proceeded and dominant species emerged. In my experiment, disturbance led to an immediate increase in diversity and slow decline, rather than the gradua1 increase then decline. That may be because the disturbance did not remove all individuals fkom the system, so species richness was relatively unafkted, and the dominance of blue mussels was reduced, leading to an elevated diversity.

Effect of prechtion by duch

Common Eiders were signifiant predators in this system. Analyses of gizzard and esophageal contents indicated that blue mussels were the main food of eiders in this area.

Within 8 months of initiation of the experiment ducks had reduced invertebrate biomass in control areas relative to cages by nearly 50%. In undisturbed areas during this time biomass declined under cages (presumably due to storrn action) by about 30%, but in controls by 6P!.At the end of the experiment, when previously protected areas were exposed to predation for 4 rnonths, ducks reduced biomass in disturbed sites by two thirds. However, eiders had little effect on species richness or diversity throughout the experiment (see Effectivenese of eiders ar intertr-&llpreabttos below).

Eiders were responsible for virtudy all predation of blue mussels in controls relative to cages. There is no evidence that other predators caused this result, and sizes of mussels missing fiom wntrol areas relative to cages correspond with preferred sizes of mussels selected by ducks feeding in the area during most of the year (Chapter 2). 1used a combination of iiterature estimates of daily food requirements for eiders and duck counts to estirnate the amount of blue musse1 tissue eiders would be expected to consume during set intervals in the experiment. 1 then measured the area of the musse1 bed and combined that with the observed net change in musse1 biornass in controls relative to cages during the same intends to detennine the amount of biomass missing fiom the system. I compared estimated consumption by ducks with biornass missing fkom the musse1 bed for fd 1994 and fall 1996 in undisturbed sites. 1 did not attempt a similar calculation in disturbed sites, because they did not resemble the surroundhg musse1 bed. 1 used only data from the beginning and end (after cages and controls had been reversed) of the experiment, because during most of the rest of the time, confounding indirect effects and compensatory growth of uncrowded mussels wouid have made it difncult to accurately estimate how much biornass had been removed (see Indirect efleccts below).

Estimates of musse1 consumption by ducks matched the clifference between cage and control areas. In fd 1994, there was an average of 226 ducks present per day.

Assumllig an average consurnption of 130 g dry tissue biomass of mussels per day [an

estimate based on published results of Bédard et aï. (198O), Bustnes and Erikstad (1 WO),

Egerrup and LaUrsen (1992), and Hilgerloh (1 997)], over the fd they should have eaten

approximately 3 584 kg (dry tissue) of mussels in the research area. The musse1 bed was

approximately 59 700 m2, and there was an average of 49.75 g me2dq tissue biomass

missing fkom controis relative to cages (corrected for seasonal losses unrelated to duck

predation). Ifthis pattern was consistent across the musse1 bed, there would be

approximately 2790 kg of mussel tissue missing fiom the area due to duck predation.

These figures dser by leu than 30%. Hiigerloh (1997) estimateci that 80% of food for an

eider was blue mussels. If this figure is incorporated into my estirnates, the daily musse!

requirement drops to 104 g d-l dry tissue mas, and eiders would have eaten approximately

2867 kg of mussels during the period in question, ody about 3% different from the

amount calculated to be missing based on csiging studies. In fd 1996 (after cages and

controls were reversed), 1 made the same calculations, but had to build in a correction

factor for the dinerence between percent cover by mussels in the newly exposed areas in

which ducks were feeding (53.6% cover) and the rest of the musse1 bed (38.1% cover).

Assuming ducks fed in proportion to mussel availability and that 80% of their diet was

mussels, during fd 1996 they should have eaten 5083 kg of mussels, and there were 5453

kg missing f?om the system, a ciifference of about 7%. This provides good evidence that eiders were responsible for the obse~edpredation effécts in this study.

Other birds present in the area also eat mussels. Gulls (mainiy Lmus mgentatus and hsrnarims) and crows (Corvus brachyrhynchos) ffeed on intertidai invertebrates during low tides, and other ducks (primarily scoters Melaniita uèglandi, M. perpicilk, and nigra) dive for rnussels at high tide in the same areas where eiders were feeding.

However, gulls and crows were not a factor in my experiment because they feed on invertebrates ody when the substrate is exposed, and my sites near the low tide line were exposed for only a short the each day. Eiders made up about 95% of the total number of ducks in the area (AppendDc 3). Scoters accounted for most of the remainllig 5% and were present only in fd and for a short time during spring of each year (Appendix 3). They are much smder than eiders and require less food so, altbough they do feed on blue mussels, their efect on the systern was probably negligible.

It is also unlikeIy that cage effkcts caused any of the observed results in this experiment. 1 ciid not use cage controls (the cage structure with partial or no mesh) commonly associated with exclosure studies (e-g. Viein 1977, Lubchenco 19 83, 1986,

Marsh 1986a, Wootton 1993a) for several reasons. First, the iikelihood of a cage effect with this design is substantialy lower than in the traditional cage, which has both a top and four sides, because horizontal water flow was not restricted and other predators were f?ee to move in and out of the cage. Second, this design has been used effectiveiy by others with no evidence of cage eEects (RafEaeili and Hall 1992, Hamilton et al. 1994). Third, there is evidence that birds in other studies may avoid the partial cage structure, Limiting its effectiveness as a wntroI (Marsh 1986% b). However, results of several tests of mussel settlement patterns indicate that caging had no effa on this system beyond the exclusion of ducks. The conclusion tbat eiders feed heavily on blue mussels is not unique. Mussels are known to be an important part of their diet in several locations in both Europe (Milne and huinet 1972, Nilsson 1972, Raffaelli et al. 1990, Nystr6m et al. 1991, Hilgerloh 1997) and North America (Cantin et al. 1974, Goudie and Anlaiey 1986, Guillemette et al. 1992,

1996). However, the effect of predation on musse1 biomass varies greatly fiom study to study. Egerrup and Launen (1992) found that eiders and other avian predators feeding in a musse1 bed in the Wadden Sea (Denmark) consumed approximately 17% of the standing mussel biomass present in a year, and Ndsson (1980) wncluded that only 6% of mussels were eaten in the Baltic Sea by al1 duck species combuied. However, in the Gulfof St.

Lawrence during winter, eiders were estimated to have eaten nom 48 to 69% of available musse1 biomass (Memette et al. 1996).

Variation in observed effects of eiders on different systems is partially due to merences in feediig rates and duck and mussel abundance. Avian predators do not necessarily respond to prey in a hear fashion, resulting in a possible disproportionate effect of birds in areas with high prey density (e.g. Marsh l986b). Swennen et al. (1 989) estimateci a predation pressure of 1.19 g m" y-' ash fiee dry weight (AFDW) and an average intertidal benthic biomass of 38 g m-2 AFDW, indicating that ducks consumed just over 3% of musse1 biomass. By contrast, at the begllining of this study there was approxhately 240 g m-* chy tissue biomass (equivalent to about 255 g m-2AFDW), and ducks ate nearly halfof it within 4 month. The mussel bed at my study site was much srnder than that of Swemen et al. (1989), and 1 had many more ducks per unit area, so although they were estimated to eat about the sarne amount of food per day, the effect was much greater.

Variation in results among studies may also be due to different methods of

estimation. Authon fkquentiy deveiop estimates of biomass consumption by eiders based

on feeding requirernents and counts during a defined penod, and combine these with a

knowledge ofavailable biomass to estimate the exploitation of the food stock by ducks

(e.g. Cantin et al. 1974, Swemen et al. 1989, Guillemette et al. 1996). Others combine

estimates of exploitation with estimates of available biomass both before and after

predation (e.g. Nilsson 1969, 1972, Milne and Dumet 1972, Hilgerloh 1997). This allows

a more interesthg cornparison, because it then becomes possible to estimate what is

rnissing and what proportion of that was removed by birds. However, these estimates

hinge on accurate assessments of food habits and number of birds present. Wootton

(1997) reviewed this observational approach to determinhg effects of avian predators on

their prey. He concluded that it was a relatively quick and usefùl method, but that further

testing was necessary to detennine its accuracy.

Use of predator exclusion cages which exclude only ducks is the only way to

directly measure effects of eiders on a system. If results fiom exclosure experiments

wrreIate weIi with estimates of what ducks could eat based on numbers and food

requirements, researchers can be confident both that ducks are responsible for changes

seen in the system, and, for fùture reference, that estimates of what ducks eat in the area

are reasonably accurate. That was the case in this experiment, but estimates are not always this close to each other. Egemp and Laursen (1 992) found that mual predation in control areas of an exclosure study was about 3 1 tonnes AFDW per year. Based on duck counts and feeding requirements, they estïmated that if eiders and oyaercatchers had eaten only mussels they would have consumeci 116 tonnes per year. By using both methods, it became clear that either ducks must have fed on other prey as weU, or that estimates of removai fiom the system based on exclosures were inaccurate. Therefore, wMe observational methods may give accurate results, they should be interpreted with caution.

Idireci effects

Predatioo by Common Eiders on blue mussels had sigrilficant indirect effects on another species in the system. Exclusion of eiders led to dense patches of mussels forming under cages. Within one year of initiation of the experiment, these attracted whelks, which in tum reduced musse1 abundance bide cages relative to outside. This reduced and eventually eliminated the Merence between cage and control areas in natural sites. This may be an exarnple of exploitation competition (Menge 1995), though in this case it is highly asymmetric, with eiders affectkg wheiks by limiting their prey, but with no apparent effect of wheiks on eiders.

Based on ingestion rates of whelks feeding on blue mussels (Bayne and Scullard

1978, Brdar and Hamilton unpublished manuscript), and abundance of whelks in natural sites in August 1995, Nucella could easily have been responsible for the perceived loss of effect of duck predation. However, whek were probably not solely responsible for changes observed under cages. 1 observed large aggregations of seastars feeding under cages during late mer1995. These were impossible to include in samples because they tended to retreat to subtidal areas during low tides, but they may have contributed to the result in some sites. Such an aggregative response of wheks and seastars to an abundant food source has been demonstrated before (Fairweather 1988, Robles and Robb 1993, Robles et al.

1995), and these predators are capable of dramatic reduaions in prey density when present in high numbers (Seed 1993). Mussels are a major component of the diet of whelks and seastars on the Atlantic coast (Lubchenco and Menge 1978). Predation by whelks led to a perceiveci loss of effect by ducks in the system. However ducks did not stop feeding in naturd sites, because when cages and controls were reversed at the end of the experiment and duck predation was no longer compensated by whelks, merences reappeared. The indirect effect of duck predation on whellcs obscured the main effect in this system (semBender et al. 1984), and possibly prevented eiders nom having a wider effect on the community.

This response by other species to removal of the dominant predator has been documented before. Marsh (1986a) suggested that effects of bird predation on a community rnay be obsmed by other predators &er birds are excluded. Navarrete and

Menge (1 996) found that whelks (Nucella emmgiinata and hl canaIicuIatu) were major predators of mussels (MFIus @ossuhis)oniy when the keystone species, the seastar

Pisarir ochraceus, was excluded. They suggested that predation by these so called

"redundant" species may be important in stabibg the cornmunity. That may also be the case in this study. Other than the increase in whelk abundance, there was no large indirect effect of duck exclusion in this system.

As in the study by Navarrete and Menge (1996), the observed increase in whelks tder cages was probably the result of an aggregative response to an increased food supply. However, Menge et ai. (1994) found that ifthe main predator (in their case, seastars) continueci to be excluded, the increased population of whelks could persist for several years. This could lead to a numeric response in the local feeding are4 because whelks attach egg capsules to hard substrats and are direct developers, so there is no mandatory dispersal phase for young.

Wheks were probably not completely responsible for the wntinued lack of

Merence between cages and control areas in naturai sites (though they did persist at higher density under cages than in control areas). Unusually severe storms in fdand winter 1995-96 damaged both cage and control areas of mussel beds, and probably obscured effects of ducks in the system. However, whelks were again abundant the foliowing surnmer and continueci to reduce mussel biornass under cages. A study in which wheiks and ducks were excluded separately and together could help to clam this situation.

Most research about eider predation on blue mussels has considered the potential effect of predation on the musse1 bed, but not the rest of the community (e.g. Egermp and

Laursen 1992, GuiUemette et al. 1996, Hilgerloh 1997). Only RaffaelIi et al. (1990) specifically considered whether eider predation would have comrnUIùty wide effects. They found that while eiders ate approximately 36% of musse1 biomass, they had no effect on other invertebrates in the community. However, their study lasted 60 days, and may not have been long enough to allow indirect effects to develop (Wootton 1992). As well, their research was conducted during winter, when invertebrates do not reproduce and are typically less active. The whelks which became signifiant predators in my study increased in abundance only &er a fidl year of exclusion. They were abundant and active only during sumer, as their feeding rate is linked to temperature (Bayne and Sdard 1978,

Brdar and Hamilton unpublished data), and they are wnsidered to be highly seasonal predators (Menge 1983).

This highlights the importance of long term and year round studies in comrnunity ecology. If this study had been conducted during only fd and winter, when eiders were most abundant, 1 would have concluded in the first year that ducks sigdicantly reduced musse1 biomass, but had linle effect on other species. Menge (1 997), in a review of studies of indirect effects, found that in moa cases indirect effects were detected at the same time as, or shortly after, direct effects. My study is an exception to this, and supports suggestions by Yodzis (1988) and Wootton (1992) that indirect effects can be slower to develop in systems and may not be detected in short-term experiments. This may be especially important in seasonal environments where different predators are active at

Werent tirnes of the year. Navarrete (1996) found that variable predation had a diierent effect on the intertidal comrnunity than did continuous predation pressure. Although eiders are present year-round, they are less abundant during summer than at other times of the year. Conversely, whellcs are most abundant and active during summer. Therefore, to get a complete picture of the effects of different predators in this cornmunity, it was important to study the system tbroughout the year.

While few researchers have studied community effects of predation by eiders, several have exsunined this question for other waterfowl. Bames and Nudds (unpublished data in Nudds 1992), working in the prairie pothole regioa of Manitoba, found no evidence that predation by ducks influenced invertebrate abundance, and concluded that waterfowl were not keystone predators. Similady, Smith et al. (1986) found that winterhg diving ducks in South Carolina had littie effkct on the benthic community. However, again, neither of these studies was long term. In one study which did suggest important community-wide effkcts for waterfowl, Bazley and Jeffenes (1986) found that a salt marsh community changed in ternis oftotal biornass and species composition when geese were excluded. In this experirnent birds were excluded for 6 years. It is clearly possible for waterfowl to have effects on the communities in which they feed, but they may not be detected unless the system is studied for a period of several years.

Other birds waterfowl have rreceived more attention in intertidal systerns, especially in recent years. Wootton (1992, 1993%b, 1994a) has demonstrated that predation by guils, oystercatchers, and other shorebirds on the west Coast of North America has both direct and indirect effiects on the intertidal cornmunity. Quamrnen (1984) studied the effect of shorebirds and fish on invertebrates in intertidal mudflats. She concluded that fish were of minor importance, but that birds could have a sigtilficant effect on the system under some conditions. Beukema (1993) found that when the main prey of oystercatchers were elirninated, they switched to altemate prey rather than move elsewhere, eventualiy depleting them too.

Marsh (1986a) found that predation by shorebirds reduced recruitment of Mytils califmi~ll?tlsand increased patchiness of sedentary prey in heterogeneous habitats. He also suggested that, as demonstrated in this study, invertebrate predators were capable of compensatory predation in the absence of birds, and that bird exclusion may have a Ming effêct on the system only ifpredatory invertebrates are not common. In another experirnent (Marsh 1986b), he concluded that birds had signincant predation effects on limpets, and suggested that this may have community wide consequences. His results indicated that predation effécts of birds were highiy variable and, whiie they shodd be considered in intertidal communities, they should not be extrapolated from one area to another. Other researchers, including Schneider (1978), Hahn and Demy (1989), Meese

(1993), and Mercier and McNeil(1994) have examuied predation by birds on intertidal invertebrates. Conclusions Vary, but all discuss the possible importance of birds in these systems. Clearly, based on results fiom my study and many others, birds should be considered as potentidy significant predators which can produce indirect, cornmunity- wide effects in intertidai systems in which they occur.

Interaction of predation ami disturbance

On many levels, predation and abiotic disturbance have similar effects on communities. If both reduce the abundance of the dominant cornpetitor, space is opened and other species Uicrease in relative abundance. This lads to increased species richness and diversity in the system (e.g. Paine 1966, 1974, 1980, Connell 1978, Lubchenco 1978, many others). However, when both predation and disturbance occur at the same tirne, effects may be less straightfoward. If disturbance is of a regular seasonal type, such as losses due to winter wave action, and is not severe, it may not have much effect on predation. In fact, if both predators and disturbance reduce the abundance of the dominant species, they may act in an additive fashion to structure the co~nmunity.However, ifthe disturbance is of a rare and catastrophic nature, it may alter or obscure the effect of predation on the community, and predation may influence the process of recovery after disturbance.

In this study, predation and disturbance clearly interacteci. Effects of predation on community biomass were delayed in appearance in disturbed relative to natural sites, but dtimately persisted longer in the system. Predation slowed the remof disturbed sites to their original state because predators began feeding before recovery was complete. The initial delay in predation on disturbed sites is not surprising. Biomass was substantidy reduced, and with more musseIs avaiiable ali around them, there was no reason for ducks to feed in these disturbed areas. However, as available biomass in other areas was depleted, and there was recovery in disturbed sites, ducks began to feed there.

The expianation for persistence of effects in disturbed sites after they stopped in natural areas is more complicated. As described above, whelks and seastars were probably responsible for the absence of differences berween cages and controls in natural sites in mer1995. This merence did not reappear through the course of the shidy because of a combination of invertebrate predation under cages and unusudy severe storms (effects in disturbed sites were also temporarily eliminated during this time). However, whelks did not respond to elevated musse1 biomass in cages of dishirbed areas untii the final sample in

Augua 1996. This result is probably partially due to lower mussel biomass in disturbed sites, and partidy to the larger size of mussels in these areas.

Mussels under cages in disturbed areas were larger than others by summer 1995.

This is a common redt of disturbance andior predation, in that blue mussels (and other bivalves) tend to exhibit compensatory growth when their density is reduced (Petraitis 1995). Mussels which remaineci under cages derdisturbance were fkee of predation and uncrowded, so they grew quickly. Compensatory growth results in invertebrates rnaintaining dominance in the system, and, in cases where size ofers a refuge f?om predation, provides a means of protection (Petraitis 1995). Mussels in wntrol areas of disturbed sites were not larger than those in other areas. Although they were also disturbed and should exhibit compensatory growth, they were subject to predation which removed many of them before they attained large size.

Whelks are size selective predators; adults (25-30 mm) prefer rnussels of 20 to 25 mm in length, though they are capable of consuming prey up to 50 mm long (pers. obs.), and juveniles select srnalier prey (Hughes and Dunkin 1984). Therefore, akhough mussel biomass in disturbed areas had recovered by December 1995 (Fig. 3 S), because the recovery was due, in part, to an increase in musse1 size (Fig. 3.7), whelks may have been less aîtracted to them. It was not until August 1996, when there was more biomass under disturbed cages than anywhere else, that whelks aggregated strongly there.

In sumrnary, these results indicate that predation by eiders and abiotic disturbance interacted in this system uidirectly through an aggregative response by whelks and compensatory growth by rnussels. In naturd areas, eider predation reduced musse1 biomass, but the effect was short-lived because whelks responded to the increased food supply in areas fiom which ducks had been excluded, and obscured effects of eider predation. In dishirbed sites, eiders resumed feeding after the system began to recover, and their exclusion continuai to have an effect because mussels grew quickly and became less attractive prey for whelks. Conciu.s-ïons- Effectivenes of eiders as interti&I prectrtors

Paine (1966, 1969) coined the term "keystone predator" with reference to a top predator that fed on a competitively dominant prey species, limiting its abundance and dowing other species to wexist. Such predation should therefore maintain elevated species richness and diversity. Keystone predators are found in systems where total predation is strong and they are responsible for moa of it (Menge et al. 1994).

Altematively, if totai predation is strong, but severai species share responsibility for it,

Wsepredation occurs (Menge et al. 1986, Robles and Robb 1993, Navarrete and

Menge 1996).

My results demonstrate that eiders had a significant effect on the intertidai community. However, it is difncult to know whether or not they were keystone predators

(following Paine's (1969) definition). Blue mussels were the dominant member of the inverîebrate community, total predation was strong, and eiders were responsible for most of it. Predation by other species was compensatory, not additive, and was uisignificant in the presence of ducks. Based on estimates of wheik abundance and feeding rates (Bayne and ScuiIard 1978, Brdar and Hamilton, unpublished manuscript), when ducks were not excluded whelks could consume only 2-1 00/0 of musse1 biomass. There were no other predators of blue rnussels present regdarly throughout the experiment.

However, predation had littie effeçt on species diversity or nchness. This may be because predation effects were not strong enough, but is more Mybecause of the indirect effect that exclusion of eiders had on whelks. The compensatory predation exhibited by whelks in the absence of eiders does not preclude the latter fkom being considered as keystone predators, as long as they were responsible for most of the predation in the system (Navarrete and Menge 1996). However, to determine whether eiders are a keystone species in the MITOW definition of the term, experiments, as described in Menge et al. (1994), in which ducks and whelks are excluded separately and t ogether, are required.

The keystone species concept, while usefbl, should not be the only approach to examining interactions among species (Mills et al. 1993). Non-keystone species also exert significant predatory effkcts within communities (e-g. Menge and Lubchenco 198 1, Menge et al. 1986, Robles and Robb 1993). Regardless of whether eiders can be designated as a keystone species in this comrnunity, they signincantly reduced invertebrate biomass within

8 mo of initiation of the experiment. Exclusion of eiders led to indirect effects on whelks, which in tum obscureci the effects of ducks through compensatory predation. Abiotic di&ance changed the outcome of these interactions and alîowed the detectable effect of eiders to persist. Eiders delayed the biomass recovery of disturbed sites because they began feeding there before recovery was complete. Results of this study suggest that, in fuhire, waterfowl (and other vertebrate predators) should be considered as possibly integral members of interticid systems, and that they should not be disrnissed on the basis of short term studies which fail to detect community level effects of predation. CHAPTER 4

Predation by Common Eiders on invertebrates in harvested and unharvested

rockweed beds

Introduction

Predation and cornpetition are important processes in rocky intertidal communities

(e.g. Connell 196 1, Paine 1966, 1969,1974, 1980, Dayton 1971, Lubchenco 1978, 1980,

1983, 1986, Menge and Sutherland 1976, Menge 1978a,b, 1983, Lively et al. 1993,

Menge et al. 1994, Navarrete 1996, Navarrete and Menge 1996). Although traditionally

birds and other vertebrate predators have received little attention in intertidal wmmunity

studies (Menge and Farrell 1989), they may be significant predators in many of these

systerns, Hiith both direct and indirect effkts on the invertebrate community (Quammen

1984, Marsh l986a,b, Hahn and Demy 1989, Good 1992, Székely and Barnburger 1992,

Wooîton 1992, 1993a,b, 1994% 1997, Dumas and Witman 1993, Meese 1993). There has been Little study of ducks in intertidal communities, though waterfowl predation significantly aiters other systems (e-g. Bazely and Jeffenes 1986), and 1 demonstrated an effect of duck predation on the structure of an invertebrate community in an intertidal mussel bed in Chapter 3.

In the rocky intertidal zone of eastem North Amenca, knotted wrack

(AscophyIIm nmiosum), or rockweed, is a dominant mid-zone macroalga of Iate successional stages in sheltered locations (Lubchenco 1983, 1986, Mathieson et al. 1986,

Rangeley 1994). In the Bay of Fundy, it dominates in exposed areas as well (Thomas et al. 1983). It is long-lived, grows to a length of 2 m or more, and generally has poor dispersal and recruitment (Lubchenco 1986). It generates considerable three dimensional structure in the system, providing habitat for several species of mobile and sessile invertebrates

(Lubchenco l983), and juvenile fish (Rangeley 1994).

Rockweed cornpetes with other species such as blue mussels (Mwhsedulis L.) and barnacles (Semibalamrs balmioids) for primary space (Lubchenco 1980). The mature

Ascophyllum canopy prevents colonkation of the substrate by ephemeral algae

(Lubchenco 1986), but settiing blue mussel veligers attach to rockweed and grow there, in some cases weighting it down and killing it. Predaton, by eating mussels, may prevent this from happening. On the New England Coast, rockweed and other algae are thought to dominate in sheltered areas where other primary space cornpetiton are reduced by predators, but are scarce in wave-exposed regions where predators are less effective

(Lubchenco and Menge 1978, Menge 1978%b, 1991, Lubchenco 198O), but see Petraitis

(1987) for a Werent opinion. Alternately, Vadas et ai. (1990) concluded that exposure physicaiIy limited algai distribution because rockweed zygotes could not settle at wave exposed sites.

Rockweed is commercially harvested in Nova Scotia (Sharp 1981, hget al.

1993) and the industry is now being developed in New Brunswick. Harvest involves removal of the top part of plants, and results in considerable loss of biomass, which may lead to significant habitat loss (see Rangeley 1994). Common Eider (Somateria mollissrna) duccklings and females feed predominantly on invertebrates found in association with rockweed (Canth et al. 1974, Minot 1980, Bustnes 1996, Chapter 1). Ducklings use this habitat heavily until they are near fledging, and are dependent on it for food during the first weeks of life (Chapter 1). Adult eiders feed predominantly in musse1 beds, but also on blue mussels and cornmon periwinkies (Linorina litîorea) in rockweed dominated areas.

1 studied the effects of predation by Comrnon Eiders in harvested and unhawested rockweed in Passamaquoddy Bay, New Brunswick. The rockweed 5ed at the research site is adjacent to a large musse1 Bat. Blue mussel veligers therefore have ample opportunity to senle on and around rockweed, leading to the potential for cornpetition between mussels and rockweed for space. Eiders feed heavily year-round on mussels in the adjacent bed

(Chapter 3), but aiso regularly, though less fkequently, over rockweed areas (pers. obs.).

So, while their effects may not be as great as in mussel beds, predation by eiders may still have an effect on invertebrates in rockweed, and this may be altered by harvest. Ifharvest results in a reduction in invertebrates available as prey for eiders, they may stop feeding there. Alternatively, harvest might make invertebrates more accessible to ducks by reducing the rockweed canopy.

Using predator exclosures and simulated rockweed harvest, 1 investigated whether a) predation by Common Eiders and harvest of rockweed affected invertebrate biomass or species diversity, b) predation differed in harvested versus unharvested areas, c) eider predation led to indirect effects on other invertebrates, and d) persistence of rockweed was influenced by duck predation. Findy, 1 compared results to a sirnilar study performed in the adjacent mussel bed (Chapter 3), and drew inferences about the effect of dserences in habitat heterogeneity on predation by eiders in both communities. Methods

I conducted experiments in a rockweed bed in the mid-intertidal zone at Indian

Point, St. Andrews, N.B., Canada ( 45" 4' N, 67" 2' W) between June 1994 and

August 1996. This site expenences twice daily tidal fluctuation in excess of 7.5 m (vertical range during spring tides). The rockweed bed is approximately 300 m long, fiom 20 to 50 m wide, and is located dong a ndge extending out from shore. The area is dominated by

Ascophylïum nodosum, but another rockweed, Fmsvesicuiosis, is also present in smd quantities. The region above the rockweed bed is rocky and relatively barren with some barnacles (Semibaianus baimoides) md rough periwinkles (Littorina smatiis), and below is dominated mainly by blue mussels. Cornmon invertebrates found in rockweed include blue mussels, periwinkles (LiItorina littorea and L. ohmta),dogwheiks (Nucella iqilus), and iimpets (Collisella lestudindis). A complete List of all species 1found in the study area is provideci in Appendix 2.

Site selecrion and sampïing

In June 1994,I randomly selected 12 sites in the rockweed bed7 each measuring 7 x 3.5 m, with the restriction that they had to have even and abundant rockweed cover throughout. Halfof the sites were then subjected to a simulated rockweed harvest. Using garden shears71 removed approximately 50% of rockweed biornass in a 7 x 3.5 m area by either cutting off haifthe plant, or cutting to a minimum length of 30 cm, whichever was higher. 1 assessed rockweed biomass using volumetric estimates both before and after harvest to ensure that 1 removed close to 50%. 1chose this value based on the anticipated maximum dowable removal of biomass during commercial harvest (G. Sharp, Department of Fisheries and Oceans, Halifax, N.S., pers. comm.).

I installed a predator exclusion cage (Fig . 3.1) and a paired control in each site.

Cages were wnstmcted of 3.8 cm ABS pipe, were 1.5 x 1.5 m and 30 cm high, and

covered on the top daceby green construction fencing (mesh size approximately 3 cm).

Cage sides were lef't unwvered to minirnize cage effects associated with changes in water

flow due to caging material, and to allow access by predators other than ducks. 1

randomly assigned cages and controls to opposite ends of sites, with 2 m between them

and a 1 m border which had also been manipulateci (in harvested sites) around the outside

(Fig. 3.1). Cages were anchored with four angle iron corner posts, and controls marked by

wooden pegs. 1 monitored sites regularly throughout the experiment. When a cage was

damaged or lost, it was replaced within 48 h.

1 started samphg immediately after cages were positioned. I collected sarnples in

June and August 1994, then every 4 months thereafier until August 1996. Sampling

uivolved removing all invertebrates fiom four randomly selected 100 areas in each

cage and control. 1 removed invertebrates from both the rockweed lying in the area to be

sampled, and underlying substrate. Only the centre 1 m2 under each cage or in each control

area was sampled to avoid possible edge effects associated with ducks reaching under the

sides of cages. Areas which were sarnpled were noted using a grid system and never resampled on subsequent visits.

After collection, samples were fmed in an 850 u sieve and preserved in 95% ethanol. Invertebrates (with the exception of polychaetes, other Worms, and several juvenite or damaged specimens) were identified to species, and those with shelis were measured. 1 made identifications following Bousefield (1960, 1973), Gosner (197 1), and

Davis (1976). As weil, 1 verified specimens by cornparison with the reference collection at the Atlantic Reference Centre, Huntsrnan Marine Science Centre, St. Andrews, N.B.Blue mussels were measured and sorted into size classes. Samples were then dried to constant mass (90' C for approximately 20 h), each species was weighed, ashed for 2 h at 550" C, and reweighed. 1used dry tissue biomass, correcteci for shell mass loss due to asbg (see

Chapter 3), as a measure of abundance in analyses.

Predator exclosures used here did not produce unwanted cage effects when used in musse1 beds (Chapter 3). However, because cage effects are fiequently considered a problem in exclosure studies (Vimstein 1977, Hulberg and Oliver 1980), and habitat in this system differed from the mussel bed, 1 also tested for artefacts of exclosures during the experiment. In January 1995,I rnarked four groups ofperiwuikles with nail polish and placed them in two cages and cuntroi areas which were located in the rockweed bed, but not part of the ongoing experiment. 1 searched the areas 2 and 4 d later, counting all remaining periwinkles to detemiine whether cages idiuenced dispersal rates. As weIl, during the experiment, 1 isolated the smaliest blue mussels (those < 5 mm long) fiom each sample and compared their abundances inside and outside cages. This served as an index of settlement rates, which rnight ciiffer if cages were iduencing water currents. 1also observed cage and control areas at high tide by SCUBA diving to check for a tendency of mobile invertebrates and fish to either aggregate under or avoid cages.

Rockweed and habitat heterogeneity dma

To assess temporal changes in the system independent of the exclosure study, 1 collectecl data on rockweed height and density, and habitat heterogeneity in the general study area throughout the experiment. Before the caging experhnent was initiateci in 1994,

1 established ten 40 m tnuisects through a 1600 m2 area of the rockweed bed. Five transects ran parailel with 10 m between them, and the other five perpendicular to them, aiso 10 m apart. At each metre dong the transects 1 visually estimated percentage wver by AscophyZIum and Fucus, as weii as habitat tyZe (boulder, medium rock, or small rock/pebble/mud/sand) using a quadrat rnarked off in 100 cm2 segments. 1 repeated this on a srnalier sale in 1995 and 1996, using three 40 m transects each year. In 1995 and 1996,

1 wilected sidar data in the adjacent musse1 bed in order to compare habitat heterogeneity in the two areas. I assessed rockweed height and density during spring and fall of each year by measuring aU plants which feil within 20 cm of two or three 50 m transects. Finally, every two months f?om August 1995 to August lW6,I visually assessed percentage substrate cover of rockweed and blue mussels in cages and control areas at low tide using the sampling quadrat.

StatistîcaZ analyses

AU statistical analyses were carrieci out using SAS statistical software (SAS version 6.11, Cary, NC). 1 tested data for norrnality using the D-statistic in SAS proc

Univariate and for homogeneity of variance using Bartlett's Test (Zar 1996). 1 used

Taylor's Power Law (EIliot 1977) to test for a linkage of variance to mean and, when one exiaed, to determine the appropnate transformation. 1 evaluated significance of main effects in al tests at a4.05 and interactions at 0.1 5 (see Chapter 3). In aiI analyses, saturateci models were first used, then rem sequentidy deleting non-signifiant interactions. When signifiant interactions were detected, 1 analyseci data separateIy for each level of one of the interacting variables. 1 used only pre-planned comparisons with these reduced models, but when a posteriori comparisons were necessary, 1 used Tukey's

HSD test (Zar 1996). 1 assessed statistical power (1-P) of tests foilowing Zar (1 996).

I analysed the predator exclusion cage experiment using a split-plot mixed model analysis of variance (ANOVA) with main effects of predation (cage versus control), harvest, and sample. Sample was the split-plot factor, and site was a random variable nested within harvest level. Details of the model have been described previously (Chapter

3, Table 3.1). Variables analysed included total dry tissue biomass, species richness, and species diversity (Shannon-Wiener index) (Begon et ai. 1986). 1 also carried out a multivariate analysis of variance using biomass of the five most abundant species in the system and the same ANOVA model described above (see Chapter 3 for rationale). AU multivariate tests were evaluated ushg Pillai's trace, the test least affected by violation of assumptions (Tabachnick and FideU 1989, Scheiner 1993). 1 used standardized canonical coefficients to examine relationships between species when the overd MANOVA was si@cant (Scheiner 1993). Biomass of the strongest interacting species was aiso analysed separately by ANOVA to clarify MANOVA results. Semimonthly observations of percentage wver of blue mussels and rockweed in cages and controls were arcsine transformed (Zar 1996) and analysed using split-plot mixed model ANOVA as described above.

To test for cage effects, I compared the number of smaii (< 5 mm) mussels in cages and controls using the split-plot rnixed mode1 ANOVA To test whether cages innuenceci movement of periwinkles, 1used a x2 goodness of fit test with observai values

the number of marked periwinkles found in cages and controls, and an expectation of

equal proportions in each. 1 compared rockweed cover in the area (expressed as

percentage of substrate covered at low tide by rockweed) among years using the Kruskai-

Wallis test (Zar 1996), because assumptions of parametric tests were violated. Rockweed

height and density data were compareci among years using 1-way ANOVA with the a posteriori Tukey's HSD test (Zar 1996). 1 assessed ciifferences in substrate type arnoag

years, and between rockweed and mussel beds, using the Kruskal-Wallis test on

proportions of boulder, medium, and small rock in the systern. Finally, I compared

variances (nom transect means) of proponions of the Merent substrate types among

locations using ANOVA to determine whether the two systems differed in heterogeneity

of substrate.

Results

Statistical power varied fiom high to very low for non-sigdicant results reported below. Overall tests (main effects when interactions were non-sigdicant) had at Ieast 80% power at d.OS to detect a 15-25% Herence among means, so a type 2 error is unlikely .

As expected, power declined when data were analysed in smaller subgroups. However, unless otherwise indicated below, tests of data divided on the basis of significant interactions had 70-80% power to detect 20-35% merences among means.

Effct ofharvest

Rockweed harvest had no effect on dqtissue biomass, species richness (del, 10, F?). 93, W .36), or diversity (ci+ 1, 1 O, F=O.63, W -45) of invertebrates. The effect of harvest on biomass merdarnong samples (Table 4. l), but analysis of each sample separately revealed no signifiant trends (probabilities ranged from 0.1 1 to 0.99).

Similarly, MANOVA indicated no effkct of harvest on the relative abundance (dry tissue biomass) of common species in the systern (&5,6, F4.17, W.98).

Eflect of season

hvertebrate biomass, species diversity, and richness varied over the course of the experiment (biomass: Table 4.1, diversity: @7,140, F4.47,P<0.000 1, richness: df=7,140, F=11.0, P<0.0001).Biomass declined fiom 1994 to 1996 (Fig. 4.1). Diversity increased slightly through the experirnent (Fig. 4.2), and richness varied without a clear pattern (Fig. 4.3). Multivariate analysis also showed a strong effect of sample (de35,805,

F4.9, P<0.000 1). Standardized canonical coefficients for variate 1 indicate that biomass of blue mussels contributed the moa variation across samples (coefficient of 1.66); it correlated positively with L. linorea (0.43) and negatively with whelks (-0.24). L. obtusutla and limpets varied relatively Iittle, as indicated by their small coefficients (-0.1 1 and 0.0 1, respectively). Coefficients for variate 2 indicated that, within samples, whelks

(0.82) and blue mussels (0.46) correlated positively, and that they varied inversely with periwinkles (-0.9 1). These relationships are clarified by examination of species biomass plots. Mussel biomass declined through the experiment (Fig. 4.4), and this accounted for most of the reduction in overd biomass. L. littorea showed some decline fiom December

1994 to the end of the experiment (Fig. 4.5). Conversely, whelks increased in biomass, peaking in late 1995 and declining slightly by the end of the experiment (Fig. 4.6). This Table 4.1. Results of ANOVAs for total dry tissue biomass, and biomass of periwùikles and whelks. Only signincant results and those approachuig significance are listed. When interactions occurred, data were split and reanalysed separately for each level of one of the interacting variables. Error temu associated with each test are Iisted in Table 3.1. Sample refers to season, type to harvest level, and treatment to predation effect (duck exclusion). Unharvested sites are referred to as "natural".

Sarnple Harv. Level EEect df F P merences Dry Tissue Biomass

Apr/95 d treat 1,11 4.26 O. 064 cage>control d harvested sample 7,77 8.13

L. Iinorea dry tissue biomass

all ail sample*treat 7,147 1.93 0.069 Augl94 ail type*treat 1,lO 13.2 0.0046 Aug/94 barvested treat 1,lO 3.63 0.086 controPcage Aug/94 naturd treat 1,lO 5.53 0.041 cagQcontroI Dec/94 d treat 1,11 6.64 0.026 cagQcontr01 Apr/9S aii treat 1,11 4.43 0.059 cage=zontrol N 2qiiIu.s dry tissue biomass

-- - aii ail type*treat 1,lO 2.76 O. 127 alf harvested sample*treat 7,70 2.1 1 0.053 Aug/95 harvested treat 1,lO 4.32 0.064 cage>control Dec/95 harvested treat 2,lO 5.15 0.047 cage>control Apr/96 harvested treat 410 5.26 0.045 cag€%ontrol Fig. 4.1. Total dry tissue biomass in cage and control areas of harvested and unharvested

(natural) sites during each sampling penod. Error bars are k 1 standard error. Results of statistical cornparisons are provided in Table 4.1.

Fig. 4.2. Species divers* in cage and control areas of hamested and unhmested (natural)

sites during each sampling period. Error bars are * 1 standard error. Results of

statistical cornparisons are provided in the text.

Fig. 4.3. Species richness in cage and control areas of harvested and unharvested (natural)

sites during each sampling period. Error bars are * 1 standard error. Resdts of

statistical cornparisons are provided in the text. Sample

+- harv cage --W.harv ctrl -4- nat cage -=-nat ctrl Fig. 4.4. Dry tissue biomass of blue mussels in cage and control areas of harvested and

unharvested (nahuai) sites during each samphg period. Error bars are * 1

standard error. Results of statisticai cornparisons are provided in the text. Harvested sites cage CI control Natural sites 4 cage control

Junl94 Augl94 Deci94 Aprl95 AugI95 DecJ95 Aprl96. Augl96 Sample Fig. 4.5. Dry tissue biornass of periwinkles L. Zinorea in cage and control areas of

hawested and unharvested (natural) sites during each sampling period. Error bars

are 1 standard error. Results of statisticd cornparisons are provided in Table 4.1. Harvested sites cage control Natural sites icage BI cont rol

9/94 Decl94 Aprl95 Augl95 Decl95 Aprl96 Aug196 Sample Fig. 4.6. Dry tissue biomass of whellcs in cage and control areas of harvested and

unharvested (natural) sites during each sarnphg period. Error bars are * 1

standard error. Results of statistical cornparisons are provided in Table 4.1. Harvested sites cage O control Natural sites cage control

Junl94 Aug194 Dec/94 Aprl95 Aug195 Decl95 Apr196 Aug196 Sample generated the negative correlation between whelks and the other species.

Effect of predah'on

There was no effect of predation on either species nchness (df=l, 11, F4.0,

W -98) or diversity (df= l,ll, F=û.05, W.84), and only a limited effect on total biornass

(Table 4.1). Biomass of each sample was considered separately due to an interaction

between sample and harvest. Beginnllig in December 1994, there was a non-significant

trend toward increased biomass in cages relative to wntrol areas 0.13) (Fig. 4.1)

which tended to be stronger in harvested sites. This effkct was marginaily significant in

April 1995, but merences decreased after that time (Table 4.1).

Multivariate analysis revealed no interactions, but a weak main effect of predation

(df%,7, F=3 -4, W.07). Standardized canonical variates indicate that the abundances of

whelks (1 .02), periwllikles (0.97), and mussels (0.72) were positively correlated across

treatments, and that the abundances of L obtmata (-0.72) and hpets (-0.67) were

negatively correlated. Generally, species with positive canonicai coefficients had more

biomass in cages than in controls and the reverse was tme for those with negative values.

When blue mussel biomass was considered separately, there was no effect of duck

predation (de1 , 1 1, F=û.58, P=0.46). Though there was a consistent trend to more

biomass under cages than in controls, Merences were small except during December

1994 in harvested sites (Fig. 4.4).

Variation in whek and periwinkle abundance drove the overd response of biomass to predation. Effects of predation on pendebiomass differed among seasons

(Table 4.1). so each sample was analysed separately. In August 1994, there was a fûrther interaction between West and predation (Table 4. l), with marginally signiticantly more biomass in wntrol areas than in cages of harvested sites, and the reverse in whvested areas (Table 4.1 ,Fig. 4.5). In both Deamber 1994 and April 1995, cages had significantly more biomass than did controls, and the trend was consistent in both harvested and unhaxvested sites (Table 4.1, Fig. 4.5). After that time predation effects disappeared.

Power of these tests was low (a Merence of 40 to 50% wuid be detected only halfof the time), but in most cases Werences among means were sdand results probably do not represent a type 2 error (Fig. 4.5).

Effects of duck exclusion on whelk biomass were influenced by both harvest and season (Table 4.1). In unharvested sites, whelk abundance in cages did not mer significantly fiom that in controls (Fig. 4.6), but power was extremely low (a 100% ciifference would be detected only 50% of the time), so results shodd be interpreted with caution. However, in harvested areas, there was an interaction between sample and predation, requiring separate analysis for each sample (Table 4.1). By April 1995, there was substantially more biomass in cages than in controls, though the merence was not signifiant @ut power was extremely low) (Fig. 4.6). This trend persisted throughout the experiment, and was significant fkom August 1995 through April 1996 (Table 4.1, Fig.

4.6).

Subsirute cover

There was a 3-way interaction between observation period, harvest, and predation for proporiional cover of the substrate by both mussels 120, F=3.3, W.005) and rockweed (df-6,120, F=3.4, W.004). When data for musse1 cover were andysed separately for each observation period, there was an interaction between harvest level and

predation in August 1995 (Table 4.2), but when harvest levels were considered separately

neither showed a.effect of predation. Predation effects were consistent with respect to

West at ail other times. An increase in mussel cover in cages relative to controls was

fkst detected in FebruaIy 1996, and persisted through the end of the experiment (Table

4.2, Fig. 4.7a). Effects of eider predation on rockweed cover were Iess clear. Increasing

mussel cover tended to correlate with deciining rockweed (Fig. 4-71, but there were only marginal effects of harvest in February and June 1996, with more rockweed in unharvested areas, and of predation in Apd 1996, with more rockweed in controls (Table 4.2, Fig

4.7b). However, as a result of high variability among sites, power for many of these non- signifiant tests was very Iow (in some cases even a 100% merence could be detected less than 50% of the time). Therefore, lack of sigdicance here indicates only that I failed to detect a difference, not that one did not exist.

Assesment of cage effects

Exclosures had no effect on periwinkle movement. Numbers of periwinkles remvered two and four days after placement did not differ between cages and controls (x' analyses, W .98 and 0.75 for two separate exclosure sites). Similarly, there was no effect of exclosures on settlement of smaii bfue mussels on rockweed or the substrate. ANOVA of the nurnber of mussels <5 mm in cages and controls revealed no effect of cage on mussel settlement (df=l, 11, F4.08, W.78). During both day and night SCUBA dives, 1 observed no tendency for fish or mobile invertebrates to be either attracted to or avoid cages. Table 4.2. Results of analysis of transformai percent cover by blue mussels and rockweed in cage and control areas fiom August 1995 to August 1996. Oniy significant and borderline results are listed. When interactions occurred, data were spü and reanalysed separately for each level of one of the interacting variables. Error te= associated with each test are listeci in Table 3.1. Type refers to harvest level and treatrnent to predation effect (cage versus control). Unhamesteci sites are referred to as "natural".

Season Harvest Effect df F P Merences Level Blue mussel cover

all au sampiettype*treat 6,120 3-3 0.0048 Aug/95 ail type*treat 1,lO 3.54 0.089 Feb/96 aii treat 1,11 4.57 0.056 cagtPcontr01 Apr/96 all treat 1,11 8.78 0.013 cage>control Jun/96 all treat 1,11 3.68 0.081 cage>control Aug/96 aii treat 1,11 7.45 0.02 cage%ontrol Rockweed cover

d d sampIe*type*treat 6,120 3.4 0.003 9

Feb/96 aii VPe 1,10 4.05 0.072 naturabhamested Apd96 al1 treat 1,11 3 -8 0.077 controPcage Jun/96 al1 type 1,10 4.75 0.054 natural>harvested Fig. 4.7. Proportional a) musse1 and b) rockweed cuver in cages and controls of harvested

and unhaIvested (naturai) sites fiom August 1995 to August 1996. Statistical

compa.rkons are provideci in Table 4.2. Augf95 Ded95 Aprt96 Aug/96 Oct/95 Febt96 JunM Season

+-hanr cage ---=---harv ctrl -*- nat cage =-nat ctrl

- harv cage .----.- harv ctrl -A- nat cage ---- nat ctrl Rockweed chacten'stics ami habitat heterogenezty

Mean height of rockweed in transects differed among seasons (W,ll89, F=29.6,

P

P-0.995).

Substrate heterogeneity in the rockweed bed was higher than in the adjacent mussel bed. There were no differences among years in proportions of boulder, medium rock, and small rock/pebble/sand at either location (Kruskal-Wallis tests, probabilities ranged fiom 0.09 to 0.99). Years were therefore pooled for cornparison among habitats.

There was a significantly higher proportion of boulder and medium rock in the rockweed bed than in the adjacent musse1 area (Table 4.3). For ail substrate types, mean variance

(estimated for each transect) of estimated proportions was higher in rockweed than in the musse1 bed (Table 4.3). This was not an artefact of a linkage of variance to mean, because the same redt was obtained using both untransformeci and ranked data. Fig. 4.8. Size-fiequency distributions of rockweed fkom spring 1994 to spring 1996. The

spring 1994 sample included di plants from a total of 30 m2 (three 50 m transects

measuring plants in a 20 cm wide sîrip). In aii other samples, two transects were

done, for a total of 20 m2. Cornparisons among seasors of plant height are

discussed in the text. a) Spring 1994 .c)- Spring 1995

Sne class (cm) Sie dass (cm)

b) Fall 1994 30 r 30 25 - VI u S C 5" mm-e. h FmO 15- L Q) 15 n $ E 5 10- 5 10 Z 5

Sie dass (cm) Sie dass (cm)

e) Spring 1996 50 r

Size dass (cm) 162 Table 4.3. Comparison of rockweed and mussel bed habitat. Variance refers to variance among transects within locations. A superscript * beside values indicates a sigdïcant difference between habitat types (Kniskai-Wallis test). Average biomass refers to unhamesteci sites in mussel beds and all sites (because harvest had no effect on biomass) in rockweed beds.

Cornparison Rockweed bed Musse1 bed

structure 3-dimensional 2 -dimensional proportion boulder 7.6 0.94 %' 1.4 * 0.24 %' proportion medium rock 20.3 It 0.85 %' 8.0 * 0.50 %' proportion pebble/sand/mud 72.1 k 1.10 %' 90.6 * 0.54 %' variance - boulder 445.1 * 69.9 ' 14.0 * 102.3 ' variance - medium rock 355.3 46.8 ' 30.5 =t 68.9 ' variance - pebble/sand/mud 561.4 53.7' 44.1 79.1 ' average starting ~IYtissue biornass 5.55 g 9.43 g - proportion M. eedulis 75.5 % (4.19 g) 90.8 % (8.56 g) - proportion L. linorea 21.4 % (1.19 g) 8.5 % (0.80 g) - proportion N. !qillus 0.8 % (0.045 g) 0.3 % (0.029 g) average finishing dry tissue biornass 2.52 g 5.34 g - proportion M. edulis 44.4 % (1.12 g) 85.0 % (4.54 g) - proportion L. Ziîîorea 41.1 % (1.04 g) 10.4 % (0.553 g) - proportion N I'iZZus 6.5 % (0.164 g) 3.1 % (0.164 g) use by ducks moderate year-round, heavy year-round heaviest in summer effect of duck predation limited sigmfïcant interaction of predation and some, but unclear strong Discussion

Cage effects

Cage controls (cage fiames with some or aii mesh removed) are fiequently used in exclosure studies to separate effects of predator exclusion from those of the caging material itself (e.g. Viein 1977, Lubchenco 1983, 1986, Marsh 1986%Wootton

1993a). 1 did not use such controls here because birds may avoid partial cages, limiting their effectiveness (Marsh 1986qb). As well, my cages had no sides, and therefore dowed free flow of water through them, greatly reducing potential for confounding effects.

Finally, others have used this design effectively without evidence of unwanted cage effects

(RaEaelli and Hàii 1992, Hamilton et al. 1994). AU tests 1 did through the course of this experiment, and in the adjacent musse1 bed (Chapter 3), suggested that cages did not modii the environment or abundance or behaviour of invertebrates in the community.

Effect of harvest

Rockweed harvest had no detectable eEion the invertebrate community. There was no Merence in total invertebrate biomass, species richness, or diversity between harvested and unharvested sites. Neither did the relative abundance of individual species

Vary between harvested and unharvested areas. The substrate was not aEected by harvest, so substnue dwelling species would not necessarily be expected to show an effect, but this is a surprishg result with respect to species such as the smooth periwinkie (L. obfusatu), which is found almost exclusively on rockweed.

Rockweed branches out after cutting, resulting in shorter but bushier plants (G.

Sharp, pers. comm.). 1 assesseci the volume of rockweed (using length and maximum cirderence, and assumuig a conid shape) in a site immediately &er harvest and one year later. While length in the harvested area was stiU significantly less than it was before hawest, volume had retumed to pre-harvest levels suggesting that the plants chaoged shape signifïcantly. Perhaps, then, there was the same amount of algal surface area available for periwinkles and other species.

That may explain the lack of harvest eEects in the second halfof the experiment, but ca~otaccount for why there were no Merences in the first year, before the rockweed began to recover. Possibly, either periwinkles and other hvertebrates were not crowded, so removal of part of their habitat had no effect on them, or the bestwas not on a sufnciently large scale, and invertebrates fiorn surrounding areas moved into harvested sites shortly after rockweed was removed. To assess the second possibility, invertebrates would have to be sampled fiom large harvested areas and abundances compared to adjacent undishirbed sites. Although rockweed harvest may not be detrimental to invertebrates, because harvest reduces plant height and therefore limits tirne during each tidal cycle when rockweed floats at the dace, it may stiil alter prey availability to young (class 1) duckhgs which have a limited ability to dive (Chapter 1).

Effect of pre&on in hawesîed und unharvested meas

Overall, although I observed ducks feeding regularly in this system, they had little effect on it. Biomass generdy deciined as the experirnent progressed, but was only marginaIly signtficantly affecteci by predation in April 1995. Predation had no effect on either species diversity or nchness. Biomass in harvested cages exceeded that in controls fiom December 1994 through the end of the experiment. The Merence was greatest in December 1994, but declhed as the experiment progressed (Fig. 4. l), suggesting that eider predation may have afEected total biomass early in the experiment. The same trend was evident in mharvested sites, but it is diflicuit to draw inferences about the effect of duck predation there, because the first samples, coilected immediately after cages were positioned, also had somewhat more biomass in cage areas (Fig. 4.1).

Blue mussels, the main prey for eiders in the area (Chapter 3) and the dominant invertebrate in rockweed, were Myunaffkcted by predation. This is in sharp contrast to other studies, in which eiders signiflcantly reduced blue mussel biomass (RaEaelli et al.

1990, Egerrup and Laursen 1992, Guillemette et ai. 1996, Chapter 3). However, ail of these studies dealt with ducks feeding in blue mussel beds. Mussels in rockweed areas can be found either on the plant itself or under its canopy aîtached to the substrate. Ducks have access to mussels attached to the floating canopy, but it may be harder for them to reach prey located undemeath, so their effect may have been limited to mussels in the canopy. This may explain why trends toward a predatory effect of eiders were stronger in harvested sites; because harvest reduced the canopy height, ducks may have had greater access to the substrate.

Common periwinkles (L. linorea) and whelks were the species responsible for most of the merences between cages and controls. In both December 1994 and April

1995, pendeswere more abundant in cages than they were in controls of both types of sites. The same was true for Augua 1994 in unharvested sites only. The effect disappeared completely by August 1995, though. Common Eiders feeding in the area were probably responsible for these Merences, as cages do not attract periwinkles or Ettheir movements. The August 1994 result in unharvested sites may have been related to female and duckling predation in rockweed. AIthough duckhgs prefer the smaller pende species (L.obiusata and L mmtilis) over L. linorea, adult eiders take larger prey (see below). Cantin et al. (1974) found that predation by ducküogs and females removed 10-

30% of the standing crop of Litlorina spp. The effects 1 observed in August 1994 may have occurred only in unhawested sites because, as described above, harvest reduced plant height and lirnited accessibility of prey, especidy for young duckhgs and associated femaies which feed primarily by dabbiing at the dace(Minot 1980, Bustnes 1996,

Chapter 1).

Ducklings could not have been responsible for differences in periwinkle biomass observed during fall and winter. However, although periwinkles are not eaten as regularly by adult eiders as are mussels, 22 of 43 adult eiders 1 obtained fiom hunters and which containeci food had eaten perides. Because periwinkles chbon rockweed, they are probably more accessible to eiders than are mussels on the substrate. Diving is energetically costly, so if ducks dive hto rockweed and no mussels are available, they should take other available prey (Beauchamp et al. 1992).

Effects of predation by eiders on periwinkles in rockweed were short-lived; by

August 1995 all effects on biornass had disappeared. Survivorship of ducklings in 1995 was low relative to the previous year (Appendk 3), so females and ducklings may have fed less in rockweed and predatory effects may have declined. Further, there was a series of severe storms during fall 1995 and winter 1996 wbich may have obscured effects of ducks on periwinkles. As mobile invertebrates, periwinkies were probably moved around sigdicantly by waves.

Predation by wheks may have limited the perceived effect of ducks in this system.

From August 1995 through April 1996, whelks were more abund=t in cages than in controls in harvested sites. There was a similar, but non-signïficant, trend in unharvested areas. However, the low power of tests in unharvested sites suggests the strong possibility of a type 2 error here. Whelks are not a cornmon prey for eiders (5 of 43 ducks examined had eaten whelks), so the increase in their abundance may be attributed to aggregation by whek under cages, not to duck predation. Whelks are capable of an aggregative response to high prey density (Fakeather 1988, Robles et al. 1995), so they may have been atîracted to these cages, which had somewhat elevated invertebrate biomass under them, and over the become signifiwitly more abundant than in control areas. Blue mussels are a preferred food for whelks (Bayne and Scullard 1978), but whelks dso feed on periwinkles (pers. obs.). By increasing predation in cages, wheks may therefore have obswed the srnall effect of ducks on the rockweed invertebrate cornmunity. Such compensatory predation by whelks has been docwnented in the literature (Robles and

Robb 1993, Navarrete and Menge 1996) and was observed in the mussel bed adjacent to this study ara (Chapter 3). To assess this more clearly, experiments wouid have to be performed in which ducks and whelks were excluded separately and together.

EMct ofckuch on rockweed

Eiders had linle effect on total blue musse1 biomass. Nevertheless, fiom February

1996 through the end of the experiment, there was signincantly more musse1 cover in cages than in controls (Fig. 4.7a). When rockweed cover was assessed in the same way, there was a tendency toward the end of the experiment for there to be more rockweed in

controls tban in cages* and somewhat more in unharvested sites than in harvested areas

(Fig. 4.7b), but effects were significant in oniy some samples (however power was very

low). ûveral17 mussel and rockweed cover tended to be negatively correlateci across

observation periods (Fig. 4.7). These data suggest that mussels may have starteci to

replace rockweed by late in the experiment in areas where ducks were excluded (especially

in harvested sites), so ducks may mediate competition between two species which use

pNnary space.

Although ducks had no effect on overall mussel biomass, by removing mussels

fiom rockweed, they may have contributeci to its persistence. The rockweed decline

appeared strongest in hamesteci cages (Fig. 4.7b), possibly because it was less abundant

initialiy there and mussels senling on it were able to weight it down more quickly. Results

were most clear in several sites located closest to the mussel bed. By the end of the

experiment there was virtualiy no rockweed in those cages, but it was unchangeci in the

paired controls. In other places where mussels were not as close to rockweed, there was

no change in rockweed cover. Rockweed cover assessed throughout the experiment on

transects in the general study area (outside experimental sites) indicated no decline, fùrther

suggesting that changes in cover in the experimental sites were the result of predator

exclusion. These results suggest that ducks affect community structure in a manner similar

to that of invertebrate predators in other studies, and support the contention of Lubchenco and Menge (1978) and Lubchenco (1980) that predators rnay enable perennid aigae to persia in sheltered areas. Ducks dso consume periwinkles, which are grazers of new shoots of rockweed

(Keser et ai. 1981, Keser and Larson 1984). Keser and Larson (1984) reported large

Merences in rates of establishment and recovery of rockweed after its removal fiom two

estuaries; recovery was slower in the area having the larger periwinkle population. They

concludeci that periwinkle predation limited recniitment of rockweed. Petraitis (1987)

reported a simiiar result with periwinkles feeding on Fmsin the Gulfof Maine. While

predation by periwinkles may therefore have some effect on rockweed recruitment in this

system, their effect on established plants is unlikely to be great, because mature rockweed

is not prefemed prey (Lubchenw 1978).

cornparison among sites

Results fiom this experiment Mer substantially from those obtained when ducks

were excluded fiom the adjacent musse1 bed (Chapter 3). There, ducks had signifiant

direct and indirect effects on biomass of cornmon species, but in rockweed beds effects

were subtle, and difncuit to separate from underlying naturai variability and effects of

other predators. In both kinds of habitat, indirect effects probably obscured the direct

effkt of duck predation (se- Bender et ai. 1984). In the musse1 bed, direct effects were

strong and appeared weii before indirect changes occurred, making it possible to separate them. However, in this study, where only weak effects were noted, and were probably quickly compensated for, it was Wtually impossible to determine the actual effect of ducks in the system.

DifFerences between the two sites may be anributed to a combination of dserences in habitat heterogeneity and dimensionality, invertebrate abundances, and behavioural characteristics of ducks and invertebrate predators. The rockweed bed was

three-dimensional, signincantly more heterogeneous, and had a higher proportion of

boulders and rnid-shed rocks than did the mussel bed (Table 4.3). Variance in proportions

of rocks of d sizes was higher in rockweed areas (Table 4.3). meaning that different

substrate types were less unifomly distributed in rockweed than in mussels beds. The two

areas were separateci by only 50 to 100 m, and therefore experienced the same

enWonmental conditions throughout the experiment. The decline in total biomass,

probably a combined result of storm action and predation, was similar in both areas.

However, in rockweed beds, virtualiy all the decline was due to reduction in blue musse1

biornass, whereas in musse1 beds both mussels and periwinkles declined (Table 4.3).

Ducks may have helped to maintain rockweed by grhgpendes and

removing mussels fiom the canopy, but they nevertheless had no effect on musse1 biomass

and little effect on total invertebrate biornass. This was probably because the increased

heterogeneity of the rockweed system provided protection for blue mussels fiom duck

predation. Marsh (1986a) stated that when predators are substantialiy larger than their

prey, heterogeneous surfaces provide refuges and limit the ability of predators to feed.

However, this heterogeneity appeared to be beneficiai to whelks, which can access

mussels in crevices. As weU, feeding rates of whelks are greater in protected areas with

algal canopies (Menge 1978a,b, 1983), where they are able to feed longer duMg the tidal cycle without risk of dessication. The abundance of wheiks relative to mussels in rockweed was approximately four times that in musse1 beds. Whelk abundance in the mussel bed peaked in sumrner (Chapter 3, Fig. 3.6), but they were more wnsistently present throughout the year in rockweed (Fig. 4.6). This pattern has been documented

elsewhere (Seed 1993 and references therein), and may contribute to the increased effect

of whek predation in rockweed.

Whelks have strong predatory effects in intertidal communities (Lubchenco and

Menge 1978, Menge 1978a,b, 1983, Seed 1993), and several studies have concluded that

they ktmusse1 biomass and allow algae to dominate (Lubchenco and Menge 1978,

Lubchenco 1980). WheIk abundance increased four fold during the course of the

rockweed experiment, and mussel abundance deciined four fold. Based on estimates of

whek abundance in the rockweed bed and feeding rates (Bayne and Scullard 1978, Brdar

and Hamilton unpublished manuscript), it is possible that whelks were responsible for

much of the overd decline in musse1 biomass in cage and control areas. Whelks may

therefore have been the more effective predator in rockweed, having strong effects in the

systern beyond their compensation for duck exclusion. The opposite result was observed

in musse1 beds. There, in a relatively homogeneous environment, ducks were the dominant

predator and wtieks hcreased in abundance and acted as compensating predators only

after ducks were excluded. Thus, the dominant predator in the intertidal area as a whole

rnay have been determinecl by local habitat conditions.

To Merinvestigate the relative predatory effects of both these species, experiments in which ducks and whelks are individually and both excluded from each habitat should be done. Results might cl* the effect of predation on species richness and diversity; my results indicate little effect on either. That is not surprishg in the rockweed bed, because ducks had Little effect there. In musse1 areas, whelks took over predation inside cages, and ducks fed outside, so agah it was impossible to determuie whether predation had a major eEkt on community structure beyond eEects on the main prey species.

Combined results of this study, and the same experiment in the adjacent mussel bed

(Chapter f), offer possibilities for fùture work on the effects of predation by "redundant" species after removal of the main predator. This type of interaction is a form of exploitation cornpetition (Menge 1995), albet highly asymmetric. It has been documented previously in an intertidal system for two or more invertebrate predators by Robles and

Robb (1993) and Navarrete and Menge (1996), and was suggested by Marsh (1986a) in connection with bird and invertebrate predation. I have already demonstrated this effect in the musse1 bed system (Chapter 3). In the rockweed bed, wheiks may have compensated to some degree for duck predation, but they were probably also the main predator in the system. Possibly, ifwhelks were excluded fiom the area and mussels became super abundant, ducks may have become the compensating predator. Such a result would suggest a system in which two predators, the relative importance of which is controlied by the local habitat, were capable of limiting the dominant prey species, but in which ody one was effective at a tirne. GENERAL DISCUSSION

Cornmon Eiders both depend on and have a signifiant effect on the intertidd uivertebrate community in Passarnaquoddy Bay, New Brunswick. Early in life, eider ducklings use rockweed habitat for foraging. Later, they feed heavily and selectively on blue mussels, and in doing so indirdy affect whelks by limiting the availability of mussels. Predation by eider ducklings and adults appears to have littie effect on biomass, species nchness, or diversity of invertebrates found in association with rockweed, but by removing mussels from floating rockweed, addt eiders may mediate cornpetition between mussels and rockweed for primary space. This, in tum would dowthe persistence of feeding habitat for ducklings in rockweed.

In Chapter 1,1 demonstrated that eider ducklings and associated females fed regularly in rockweed. This in itselfis not new information (Cantin el al. 1974, Minot

1980, Bustnes 1996), but 1 iinked changes in behaviour of ducklings to changes in rockweed availability throughout the tidal cycle. As well, 1 showed that duckling behaviour changed as birds matured and that, in my study area, the intrinsic diunial activity pattern of eiders (Campbell 1978) was superseded by the tidai cycle. Weyoung ducklings spent most of their feeding time dabbling, and fed disproportionately often on the changing tides (when rockweed was most available), older birds were unafected by tide level and spent more tirne diving, though total feeding the did not change. Females changed their behaviour according to the age of duckhgs in their are, suggesting, contrary to the conclusion of Bustnes (1 W6), that eiders were afEected by duckling rearing. These resuits may be explained by the high duckling mortality rate at rny study site. Under such conditions it was advantageous for females to rernain vigilant when caring

for young ducklings, creating the close match between female and duckling behaviour.

Thus eider ducklings require rockweed habitat during their fist weeks of Me.

These results lay part of the groundwork for Chapter 4, and LUik directly to the portion of

that chapter that dealt with the effm of rockweed harvest on invertebrate abundance.

Harvest of 50% of rockweed biomass had no effect on hvertebrate biornass, species

nchness, or diversity. That result may be iduenced by the size of my harvested plots, but

current pilot harvest practices involve only patchy removal of about 17% of total

rockweed biomass ( Rad Ugarte, pen. comrn., Habitat Subcommittee Meeting M,

Regional Advisory Process, Department of Fisheries and Oceans, St. Andrews, N.B.). At that Ievel, it is unlikely that Westwould reduce invertebrate biomass, so rockweed harvest is probably not presentiy a threat to duckling food supply. However, harvest does reduce canopy height. in the fùture, harvest is increased to the maximum aiiowable

50%, it may be detrimental to young duckhgs by limiting feeding tirne during each tidal cycle.

In Chapter 2,1 focussed on predation by adult eiders on blue mussels. Eiders were size-selective predators, and preferred prey sues changed depending on background prey availability and season. Mers have aiso concluded that eiders are seleaive predators of blue mussels (Bustnes and Erikstad 1990, RafFaeili et al. 1990, Nystr6m et al. 1991,

Guillemette et al. 1996), but 1 controlled and quantifïed availability of prey to examine seasonal variation in prey selection. 1found that prey selection by eiders was consistent with the sheli mass minirnization hypothesis (Bustnes and Erikstad 1990). Large mussels were preferred during winter, when a0 size classes had approxhately the same ratio of

sheU to tissue biomass, but srnaiIer prey were selected during the rest of the year when

they had more tissue relative to shell. 1 also found support for the risk-averse foraging

hypothesis (Draulans 1984), though results were leu clear. When the proportion of large

mussels was ïncreased, ducks selected sderprey than when all size classes were equally

available, possibly to avoid the risk of taking one which was large and unprofitable.

These results helped to explain mechanisms behind prey selection by eiders, and

provided a possible explanation for merences in results among previous selection shidies.

As well, they provided background information for predator exclusion experiments

descnbed in Chapters 3 and 4. Sizes of mussels missing f?om controls relative to cages in

Chapter 3 matched sizes of mussels eaten by ducks during most of the year. This indicated

that ducks were responsible for observed predation effects, and highlighted the importance

of understanding predator feeding preferences in wmmunity studies (Lubchenco 1978).

As weli, because ducks preferred relatively smd mussels during most of the year, mussels may have had a partial size refbge fiom predation during the$ reproductive periods. This rnay have helped them to maintain their dominant position in the comrnunity, even under intense predation pressure (Petraitis 1995).

In Chapters 3 and 4, I focussed on the effkct of predation by eiders on the structure of the intertidal mussel bed and rockweed communities. Birds have recently received attention as predators in intertidal systems (Good 1992, Székely and Bamburger

1992, Wootton 1992, 1993a,b, 1994% 1997, Dumas and Witman 1993, Meese 1993, and others), and the effects of eider predation on blue mussels have been documented (Ratfaeu et al. 1990, Egemp and Laursen 1992, Guillemette et ai. 1993, 1996, Hilgerloh

1997), but mine is the longest experimental study of direct and indirect eEects of eider predation in an intertidal wmrnunity.

In mussel beds, eiders reduced dry tissue biomass of invertebrates in controls relative to cages by nearly halfwithin 8 months. However, effects did not persist beyond the fht year because of compensatory predation by whelks, which aggregated under cages and fed on the hi& density of mussels found there. This indirect effect is an example of an interaction chah (Wootton 1994b), and the relationship between ducks and whelks may be a fom of exploitation cornpetition (Menge 1995). A relationship such as this was su~estedby Marsh (1986a), and Navarrete and Menge (1996), in intertidal experiments involving two invertebrate predators, donimented compensatory predation by a formerly

"redundant" species after removal of the dominant predator.

Effects of duck predaion in disturbed sites appeared later than in undisturbed areas, but persisted throughout the experiment, even after mussel biomass returned to pre- disturbance levels. WheUcs were less abundant in cages in disturbed than undisturbed sites, because, as a result of compensatory growth of uncrowded mussels (Petraitis 1995), mussels were larger than the preferred prey size for whelks. These results highlight an interaction of predation and disturbance which was rnediated by a growth response of mussels to reduced density, and size-selective predation by whelks.

In rockweed, eider predation had much less effect on the system. There was somewhat more biomass in cages than control areas early in the experiment, caused mainly by clifferences in periwinkle biomass, but again this effect disappeared within the first year. As in the mussel bed, whelk biomass increased under cages relative to controls at the same time as Merences in total biomaçs disappeared, mggesting that predation by whelks may have obsmred the effect of ducks in the system.

DiEerences in results between the two sites may be explained by a combination of habitat variability and predator behaviour. The higher heterogeneity of the rockweed bed, combined with the protective effiof the rockweed canopy at low tide, provided a better habitat for wheiks than did the mussel bed. Tfieir relative abundance in rockweed was higher than in mussel beds, so they should have had a greater effect on mussels under the rockweed canopy. Conversely, ducks feedhg in rockweed would have limited access to mussels under the canopy, so they would be less efficient predators there (semMarsh

1986a). Their effectiveness may have been limited to those mussels attached to ff oating rockweed, whereas in the musse1 bed they would have had access to all prey. Given these constraints on predators, ducks should be the more effective predator in the mussel bed, and whelks more effective in rockweed areas. Therefore, whiIe the Limited effect of duck predation in rockweed rnay have been obscured by a compensatoq effect of whelks, it seems unlikely that eiders were the primary predator there anyway.

Although ducks may not have been the main predator in rockweed, they may nevertheless have removed mussels, which otherwise weight down and eventudy kill it.

Rockweed is thought to dorninate only in areas where predators keep musse1 populations low (Lubchenco and Menge 1978, Lubchenco 1980). 1observed an increase in musse1 cover under cages relative to control areas, and a corresponding decline in rockweed, as the experiment progressed. While this phenornenon needs fiuther study, my results suggest that predation by eiders in rockweed may achially contribute to persistence of the algal

-0PY-

Predation by eiders had direct and indirect effects on the invertebrate commmity,

but there was iittle effect on species nchness or diversity. That is not surprising in the

rockweed bed, because there was little overall effect of duck predation on the

hvertebrates themselves (though there was an effect on rockweed as described above). In

the mussel bed, whelks wmpensated for duck predation in naturd sites, so there were no

overall effects on community composition. It is therefore not possible to say whether

ducks (or whelks in rockweed) qualify as keystone predators according to Paine's (1966,

1969) original definition. More studies, in which ducks and wheks were excluded

separately and together, would need to be done to cl* this.

Conclusions

This thesis is a comprehensive study of the relationship between Common Eiders and the intertidai comrnunity in Passarnaquoddy Bay. Results of chapters 1 and 2 contribute to our knowledge of eider behaviour and habitat use, provide tests of mechanisms goveming prey selection by ducks, and help to determine possible effects of commercial rockweed harvest on eiders and other species. Chapters 3 and 4 provide new insights into the effects of predation by ducks in intertidai systems, demonstrate that predation and disturbance interact, and highlight a previously undocumented interaction between ducks and whelks. My results reinforce the importance of doing long-term manipulative field shidies. As suggested by Yodzis (1988) and Wootton (1 992) (but contrary to the conclusions of Menge 1997), the indirect effects detected in this study were delayed in appearing, and an experiment of ody a few months would have produced very different conclusions.

Overall, 1 have demonstrated that ducks are significant interacting members of intertidai communities. In fùture, when ducks or other vertebrate predators are present in intertidal communities under study, their effêcts should be considered. REFERENCES

Altrnann, J. 1974. Observationai study of behaviour: sampling methods. Behaviour 49:

227-269.

Ahlén, I., and A Andersson. 1970. Breeding Ecology of an Eider Population on

Spitsbergen. Omis. Scand. 1: 83-106. hg,P. O., G. L. Sharp, and R E. Semple. 1993. Changes in population structure of

Ascophylm nodom (L.) LeJolis due to mechanical harvesting. Hydrobiologia

260/261: 321-326.

Baird, D., and H. Milne. 1981. Energy flow in the Ythan Estuary, Aberdeenshire,

ScotIand. Estuar. Coast. Shelf Sci. 13: 455-472.

Ball, J. P. 1990. Active diet selection or passive reflection of changing food availability:

the underwater foraging behaviour of canvasback ducks. NATO AS1 series 20:

95- 107.

. 1994. Prey choice of omnivorous canvasbacks: imperfdy optimal ducks? Oikos 70:

233-244.

Barras, S. C., R M. Kaminski, and L. A. Breman. 1996. Acom selection by female wood

ducks. J. Wddl. Manage. 60: 592-602.

Bayne, B. L., and C. Scullard. 1978. Rates of feeding by Thais (Nucella) lapiIIus (L.).J.

Exp. Mar. Biol. Ecol. 32: 113-129.

Bazely, D. R, and R L. leffenes. 1986. Changes in the composition and standing crop of

salt-rnarsh communities in response to the removal of a grazer. J. Ecol. 74:

693-706. Beauchamp, G., M. Guillemette, and R Ydenberg. 1992. Prey selection while diving by

wmmon eiders, Sontaferia molZissirna. hAmm.Behav. 44: 4 17.426.

Bédard, J., and J. Munro. 1976. Brood and crèche stability in the Common Eider of the St .

Lawrence estuary. Behaviour 60: 22 1-23 6.

Bédard, J., J. C. Themault, and J. Bérubé. 1980. Assessment of the importance of nutrient

recychg by seabirds in the St. Lawrence Estuary. Cm.J. Fish. Aquat. Sci. 37:

583-588.

Begon, M., J. L. Harper, and C. R Townsend. 1986. Ecology: Individuals, Populations

and Communities. Sinauer Associates Inc., Sunderland, Massachusetts.

Bender, E. A, T. J. Case, and M. E. Gilpin. 1984. Perturbation experiments in cornmunity

ecology: theory and pracîice. Ecology 65 : 1- 13.

Beukema, J. J. 1993. Increased mortality of alternate bivalve prey during a penod when

the tidal flats of the Dutch Wadden Sea were devoid of mussels. Neth. J. Sea Res.

31: 395-406.

Bronmark, C. 1988. Effects of vertebrate predation on freshwater gastropods: an

exclosure experiment. Hydrobiologia 169: 363-370.

Bousefield, E. L. 1960. Canadian Atlantic Se.Shells. The Queen's Press, Ottawa.

. 1973. Shailow water gammaridean Arnphipoda of New England. CorneU University

Press, Ithica, New York.

Bustnes, J. 0. 1993. Exploitation of others' viaance by the Common Eider Somateria

mollissima. Wddfow1 44: 10 8- 11 0. . 1996.1s parental Gare a constraint on the habitat use of common eider femaies?

Condor 98: 22-26.

Bustnes, J. 0.and KE. Erikstad. 1990. Size selection of common rnussels, MNsedirIis,

by common eiders, Somateria mollissirno: energy maximizrition or sheli weight

minimi7iition? Can. J. 2001. 68: 2280-2283.

. 199 1a. Parental are in the cornmon eider (Somateria moIIissima): factors affecthg

abandonment and adoption of young. Can J. 2001. 69: 153 8-1 545.

. 199 1b. The role of failed nesters and brood abandoning females in the crèching

system of the Common Eider Somatena molIissima. Omis Scand. 22: 33 5-33 9.

Campbell, L. H. 1978. Diumal and tidal behaviour patterns of Eiders winte~gat Leith.

Wildf0wl29: 147-152.

Cantin, M., J. Bédard, and H. Milne. 1974. The food and feeding of common eiders in the

St. Lawrence estuary in sumrner. Can J. 2001. 52: 3 19-334.

Carpenter, S. R, J. F. Kitcheil, and J. R Hodgson. 1985. Cascading trophic interactions

and lake productivity. Bioscience 3 5: 634-63 8.

Coh~,S. L., S. M. Glenn, and D. J. Gibson. 1995. Experimental andysis of intermediate

disturbance and initial floristic composition: decouplhg cause and effkct. Ecology

76: 486-492.

Connell, J-H- 196 1. The influence of interspecific competition and other factors on the

distribution of the bamacle Chthalus stelhtus. Ecology 42: 7 10-723.

. 1978. Diversity in tropical rain forests and coral reefs. Science 199: 1302- 13 i0.

Davis, D.S. 1976. Periwinkles. The Nova Scotia Museum, Halifax, Nova Scotia. Dayton, P. K. 1971. Cornpetition, disturbmce, and community orghtion: the provision

and subsequent utiiization of space in a rocky intertidal community. Ecol. Monog.

41: 351-389.

DeLeeuw, J. J. and M. R VanEeerden. 1992. Size selection in diving tufted ducks Aythya

firligula explaineci by differential handihg of small and large mussels Dreissena

polymopha. Ardea 80: 353-362.

Draulans, D. 1982. Foraging and size selection of mussels by the tufted duck, Aythya

fulgula. J. Anirn. Ewl. 51: 943-956.

. 1984. Sub-optimal musse1 selection by tufted ducks Aylhyafulgda: test of a

hypothesis. Anim. Behav. 32: 1192-1 196.

. 1987. Do T&ed Duck and Pochard select between Werently sized mussels in a

similar way? Wd~owl38:49-54.

Dumas, J. V., and J. D. Witman. 1993. Predation by Herring Gulls (hsmgentatus

Coues) on two rocky intertidai crab species [Cmcimrs maenas (L.) & Cmcer

irrorotus Say]. J. Exp. Mar. Biol. Ecol. 169: 89-101.

Eadie, J. McA., F. P. Kehoe, and T.D.Nudds. 1988. Pre-hatch and post-hatch brood

amalgarnation in North Arnerican Anatidae: a review of hypotheses. Can. J. 2001.

66: 1709-1721.

Egemp, M., and M. 1. H. Laursen. 1992. Aspects of predation on intertidal blue mussels

(Myiius edulis L.) in the Danish Wadden Sea. ICES Sheffish Cornmittee K25. Eliot, J. M 1977. Some methods for the statistical analysis of samples of benthic

invertebrates. Freshwater Biological Association Scientific Publication No. 25.

Titus Wilson & Son Ltd., U.K.

Elner, R W. and R N. Hughes. 1978. Energy maxhkation in the diet of the shore crab,

Carcimi..maenas. J. Anim. Ecol. 47: 103-1 16.

Elton, C.S. 1927. Animal Ecology. MacMillan, New York.

Fairweather, P. G. 1988. Correlations of predatoq whelks with intertidai prey at several

scales of space and time. Mar. Ecol. Prog. Ser. 45: 237-243.

Farreil, T. M. 1988. Community stability: effects of limpet removal and reintroduction in a

rocky intertidal community. Oecologia 75: 190- 197.

Good, T. P. 1992. Experimental assessrnent of guii predation on the Jonah crab Cancer

borealis (Stimpson) in New England rocky intertidd and shallow subtidal zones. J.

Exp. Mar. Biol. Ecol. 157: 275-284.

Gollop, J. B., and W. H. Marshall. 1954. A guide for aging duck broods in the field. Miss.

Flyway Counc. Tech. Sect. 14 pp.

Gorman, M. L., and H. Milne. 1972. Creche Behaviour in the Cornmon Eider. Omis

Scand. 3: 2 1-25.

Gomian, M., and D. RafEaelli. 1993. The Ythan Estuary. Biologist 40: 10- 13.

Gosner, K. L. 197 1. Guide to identikation of marine and estuarine invertebrates. John

Wdey & Sons Inc., New York.

Goudie, R L, and C. D. Anlaiey. 1986. Body sue, activity budgets, and diets of sea ducks

wintering in Newfoundland. Ecology 67: 1475- 1482. Guillemette, M. G., J. H. Himrnelman, C. Barette, and A Reed. 1993. Habitat selection by

common eiders in winter and its interaction with flock size. Can. J. 2001. 71 :

1259-1266.

Guillemette, M., A Reed, and J. H. Himmelman. 1996. Availability and consumption of

food by common eiders wintering in the Gulfof St. Lawrence: evidence of prey

depletion. Can. J. 2001. 74: 32-38.

Guillemette, M., R C. Ydenberg, and J. H. Hunmelman. 1992. The role of energy intake

rate in prey and habitat selection of common eiders Somaferia mollissima in

winter: a risk-sensitive interpretation. J. Anim Ecol. 6 1: 599-6 10.

Hahn, T., and M. Demy. 1989. Tenacity-mediated selective predation by oystercatchers

on intertidal hpets and its role in maintainhg habitat partitionhg by Collisella

scabra and Lotth digitailis. Mar. Ecol. Prog. Ser. 53 : 1- 10.

Hairston, N. G., F. E. Smith, and B. Slobodkin. 1960. Comrnunity structure, population

control, and cornpetition. Am. Nat. 94: 42 1-425.

Hall, S. J., D. Raffaelli, and W. R Turreli. 1990. Predator-caging experiments in marine

systems: a reexarnination of their value. Am. Nat. 13 6: 657-672.

Hamilton, D. J., C. D. Ankney, and R C. Bailey. 1994. Predation of zebra mussels by

diving ducks: an exclosure study. Ecology 75 : 52 1-53 1.

Kilgerloh, G. 1997. Predation by birds on blue musse1 MytiIus edulis beds of the tidal flats

of Spiekeroog (southem North Sea). Mar. Ecol. Prog. Ser. 146: 6 1-72.

Hughes, R N. 1979. Optimal diets under the energy maximhtion premise: the effects of

recognition tirne and learning. Am. Nat. 113 : 209-22 1. Hughes, R N., and S. d. B. Dunkin. 1984. Behavioural cornponents of prey selection by

dogwheiks, Nucelh @iIIus (L.), feeding on mussels, Mytius edulis L., in the

laboratory. J. Exp. Mar. Biol. Ecol. 77: 45-68.

Hulberg, L. W., and J. S. Oliver. 1980. Caging manipulations in marine soft-bottom

communities: importance of animal interactions or sedimentary habitat

modifications. Can. J. Fish. Aquat. Sci. 37: 1 130- 1 139

Kehoe, F. P. 1989. The adaptive signincance of crèching behaviour in the white-winged

scoter (Melimittafirsca aleglcatdi)).Cm J. Zool.67: 406-41 1.

Keiler, V. E. 199 1. Effects of Hurnan Disturbance on Eider Ducklings Somateria

molZissima in an Estuarine Habitat In Scotland. Biol. Conserv. 58: 2 13-228.

Keser, M., and B. R Larson. 1984. Colonization and growth of Ascophyllum nodomm

(Phaeophyta) in Maine. J. Phycol. 20: 83-87.

Keser, M., R L. Vadas, and B. R. Lawson. 198 1. Regrowth of Ascophyhm nodomm and

Fucus vesicülosüs under various harvesting regimes in Maine, U. S. A. Bot. Mar.

24: 29-38.

Koehn, R. K., J. G. Hall, D. J. Innes, and A. J. Zera. 1984. Genetic differentiation of

Myn'us edulis in eastern North America. Mar. Biol. 79: 1 17- 126.

Korschgen, C. E. 1977. Breeding stress of female eiders in Maine. J. Wddl. Manage. 4 1:

360-3 73.

Lindemann, R 1942. The trophic-dynamic aspect of ecology. Ecology 23 : 399-4 18.

Lively, C. M., P. T. Raimondi, and L. F. Delph. 1993. Intertidal community structure:

space-time interactions in the northem Gulf of California. Ecology 74: 162- 173. Lubchenco, J. 1978. Plant species divemty in a marine intertidal wmfllWLity: importance

of herbivore food prefereme and algal cornpetitive abilities. Am. Nat. 1 12: 23-39.

. 1980. Aigal zonation in the New England rocky intertidal community: an

experimental dysis. Ewlogy 61 : 333-344.

. 1983. Litton'm and Fucus: effécts of herbivores, substratum heterogeneity, and plant

escapes during succession. Ecology 64: 1 116- 1123.

. 1986. Relative importance of competition and predation: early colonization by

seaweeds in New England. pages 537-55 5 in: Community ecology. J. Diamond

and T. J. Case, editors. Harper and Row Publishers, New York.

Lubchenco, J., and B. A. Menge. 1978. Cornrnunity development and persistence in a low

rocky intertidal zone. Ecol. Monog. 59: 67-94.

Marsh, C. P. 1986a. Rocky intertidal community organization: the impact of avian

predators on musse1 recruitment. Ecology 67: 77 1-786.

. 1986b. Impact of avian predators on hi& htertidal limpet populations. J. Exp. Mar.

Biol. Ewl. 104: 185-201.

Mathieson, A C., J. W. Shipman, J. R O' Shea, and R C. Hasevlat. 1976. Seasonal

growth and reproduction of estuarine fucoid algae in New England. J. Exp. Mar.

Biol. Ecol. 25: 273-284.

McCook, L. J., and A. R O. Chaprnan. 1993. Community succession foilowing massive

ice-scour on a rocky intertidal shore: recnitment, competition and predation

during early, primary succession. Mar. Biol. 1 15: 565-575. McCullough, G. B. 198 1. Migrant waterfowl utilkation of the Lake Erie shore, Ontario,

near the Nanticoke industriai development. J. Great Lakes Res. 7: 117- 122.

Meese, R J. 1993. Effects of predation by birds on gooseneck bamacle Polliczpes

po&mems Sowerby distribution and abundance. J. Exp. Mar. Biol. Ecol. 166:

47-64.

Meire, P. M. 1993. The impact of bird predation on marine and estuarine bivalve

populations: a selective review of patterns and underlying causes. pages 197-243

in: Bivalve filter feeders in estuarine and coastal ecosystem processes. R F. Dame,

editor. NATO AS1 Series G: Ecological sciences, Vol. 33. Springer-Verlag. Berlin,

Gerrnany .

Meire, P. M. and A Ervynck. 1986. Are oystercatchers (Haernatopus ostralegus)

selecting the most profitable mussels (Mytils edulis)? Anim. Behav. 34:

1427-1435.

Mendenhall, V. M. 1979. Brooding of young ducklings by femaie Eiders Somateria

moliissima. Omis Scand. 10: 94-99.

Mendenhall, V. M., and H. Milne. 1985. Factors affeaing duckling amival of Eiders

Somaleria mollissima in northeast Scotland. Ibis 127: 148-158.

Menge, B. A 1978a. Predation intensity in a rocky intertidal community. Relation

between foraging activity and environmental harshness. Oecologia 34: 1-16. -. 1978b. Predation intensity in a rocky intertidal comrnunity. Effect of an algal canopy,

wave action and dessication on predator feeding rates. Oecologia 34: 17-3 5. . 1983. Components of predation intensity in the low zone of the New England rocky

intertidai zone. Oecologia 58: 14 1- 155.

. 1991. Relative importance of recruitment and other causes of variation in rocb

intertidal community structure. J. Exp. Mar. Biol. Ecol. 146: 69-100.

. 1995. Indirect effeçts in marine rocky intertidal interaction webs: patterns and

importance. Ecol. Monog. 65 : 2 1-74.

-. 1997. Detection of direct versus indirect effècts: were experiments long enough?

Am. Nat. 149: 801-823.

Menge, B. A, E. L. Berlow, C. A Blanchette, S. A Navarrette, and S. B. Yamada. 1994.

The keystone species concept: variation in interaction strength in a rocky intertidal

habitat. Ewl. Monog. 64: 249-286.

Menge, B. A, and T. M. Farrell. 1989. Community structure and interaction webs in

shallow marine hard-bottom comrnunities: tests of an environmental stress model.

Adv. Ecol. Res. 19: 189-262.

Menge, B. A and J. Lubchenco. 1981. Community organization in temperate and tropical

rocky intertidal habitats: prey refuges in relation to consumer pressure gradients.

Ecol. Monog. 5 1: 429-450.

Menge, B. A, J. Lubchenco, L. R Ashkenas, and F. Ramsey. 1986. Experimental

separation of effects on sessile prey in the low zone of a rocky shore in the Bay of

Panama: direct and indirect consequences of food web complexity. J. Exp. Mar.

Biol. Ecol. 100: 225-269. Menge, B. A, and J. P. Sutherland. 1976. Species diversity gradients: synthesis of the

roles of predation, cornpetition, and temporal heterogeneity. Am. Nat. 110:

35 1-369.

. 1987. Community regdation: variation in disturbance, cornpetition, and predation in

relation to environmental stress and recruitrnent. Am. Nat. 130: 730-757.

Mercier, F., and R McNeil. 1994. Seasonal variations in intertidal density of invertebrate

prey in a tropical lagoon and effects of shorebird predation. Can. J. 2001.72:

1755-1763.

Mills, L. S., M. E. Sodé, and D. F. Doak. 1993. The keystone-species concept in ecology

and conservation. Bioscience 43 : 2 19-224.

Milne, H., and G.M. Dunnet. 1972. Standing crop, productivity and trophic relations of

the fauna of the Ythan Estuary. pages 86-106 in: The estuarine environment. R S.

K. Barnes and G. M. Dunnet, editors. Applied Science Publishers, England.

Minot, E. 0. 1980. Tidal, diumal, and habitat influences on Cornmon Eider rearing

activities. Omis Scand. I 1: 165-172.

Munro, J., and J. Bédard. 1977a. Gu11 predation and crèching behaviour in the comrnon

eider. J. Anim. Ecol. 46: 799-810.

. 1977b. Crèche formation in the cornmon eider. Auk 94: 759-771.

Navarrete, S. A 1996. Variable predation: effects of whelks on a mid-intertidal

successional community. Ecol. Monog. 66: 30 1-32 1.

Navarrete, S. A., and B. A Menge. 1996. Keystone predation and interaction strength:

interactive effects of predators on their main prey. Ecol. Monog. 66: 409-429. Ndsson, L. 1969. Food consumption of diving ducks wintering at the coast of South

Sweden in relation to food resources. Oikos 20: 128- 135.

. 1972. Habitat seldon, food choice, and feeding habitats of diving ducks in coastal

waters of south Sweden during the non-breeding season. Omis. Scand. 3 : 55-78.

. 1980. Wmte~gdMng duck populations and avaiiable food resources in the Baltic.

Wddfow131: 131-143.

Nudds, T. D. 1992. Patterns in breeding waterfowl cornmunities. pages 540-567 in:

Ecology and management of breeding waterfowl. B. D. H. Batt, A D. Mon, M.

G. Anderson, C. D. Ankney, D. H. Johnson, J. A Kadlec, and G. L. Krapu,

editors. University of Minnesota Press, biinneapolis, Minnesota-

Nystfim, K. G. K., O. Pehrsson, and D. Broman. 199 1. Food of juveniie common eiders

(Somateria mollisrima) in areas of hi& and low salinity. Auk 108: 250-256.

Oksanen, L., S. D. Fretwe11, J. Amida, and P. Nilemel& 198 1. Exploitation ecosystems in

gradients of prhary productivity. Am. Nat. 11 8 : 240-26 1.

Paine, R T. 1966. Food web wmplexity and species diversity. Am. Nat. 100: 65-75.

. 1969. A note on the trophic complexity and community stability. Am. Nat. 103: 91-

93.

. 1974. Experimental studies on the relationship between a dominant predator and its

principal cornpetitor. Oecologia 15 : 93- 120.

- 1980. Food webs: Mage, interaction strength, and cornmunity intiastructure. J.

Anim. Ecol. 49: 667-685. Paine, R T., and S. A. Levin. 1981. Interticid landscapes: disturbance and the dynamics of

predation. Ecol. Monog. 5 1: 145- 178.

Petraitis, P. S. 1987. Factors organizing rocky intertidal communities of New England:

herbivory and predation in sheltered bays. J. Exp. Mar. Biol. Ecol. 109: 1 17- 136.

. 1995. The role of growth in maintaining spatial dominance by mussels wwIus

edulis). Ecology 76: 1337-2346.

Pimm, S. L., J. H. Lawton, and J. E. Cohen. 1991. Food web patterns and their

consequences. Nature 3 50: 669-674.

Quammen, M. L. 1984. Predation by shorebirds, fish, and crabs on invertebrates in

intertidal mudfiats: an experimental test. Ecology 65: 529-537.

RafEaelli, D., V. Falcy, and C. Galbraith. 1990. Eider predation and the dynamics of

mussel-bed cornmunities. pages157- 169 in: Trophic relationships in the marine

environment. M. Barnes, and R N. Gibson, editors.: Aberdeen University Press,

Aberdeen, UK.

Maelii, D., and S. J. Hali. 1992. Compartments and predation in an estuarine food web.

J. A&. Eco~.61: 55 1-560.

Rangeley, R W. 1994. The effécts of seaweed harvesting on fishes: a critique. Env. Biol.

Fish. 39: 3 19-323.

Robles, C. and J. Robb. 1993. Varieci wnivore effects and the prevalence of intertidal

algal turfs. J. Exp. Mar. Biol. Ecol. 166: 65-91.

Robles, C., R Sherwood-Stephens, and M. Alvarado. 1995. Responses of a key intertidal

predator to varying recniitment of its prey. Ecology 76: 565-5 79. SAS Institute, Inc. 1989. SASISTAT User's guide, Version 6, fourth ediaon. Cary, NC:

SAS Institute Tnc.

Scheiner, S. M. 1993. MANOVA:Multiple response variables and multispecies

interactions. pages 94- 1 12 in: Design and analysis of ecological field experiments.

S. M. Scheiner and J. Gurevitch, editors. Chapman and Hall, New York.

Schmitz, O. J. 1992. Exploitation in mode1 food chains with mechanistic

consumer-resource dynarnics. Theor. Pop. Biol. 4 1 : 16 1- 183.

Schmutz, J. K., R J. Robertson, and F. Cooke. 1982. Female sociality in the cornmon

eider duck during brood rearing. CmJ. 2001. 60: 3326-333 1.

Schneider, D. 1978. Equhtion of prey numbers by migratory shorebirds. Nature 271:

353-354.

Seddon, L. M. and T. D. Nudds. 1994. The cost of raising nidifigous offspring: brood

rearing by giant Canada geese (Branta cdemsmmima). Can. J. 2001. 72: 533 -

540.

Seed, R 1993. Invertebrate predators and their role in stmcturing coastal and estuarine

populations of filter feeding bivalves. pages 149- 195 in: Bivalve filter feeders in

estuarine and coastai ecosystem processes R F. Dame, editor. Springer-Verlag,

New York.

Sharp, G. J. 198 1. An assessrnent of AscophyIZum noabmm harvesting methods in

southwestern Nova Scotia. Can. Tech. Rep. Fish. Aquatic. Sci. 1012: v + 28 p. Sih, A, P. Crowley, M. McPeek, J. Petranka, and K. Strohmeier. 1985. Predation,

cornpetition, and prey communities: a review of field experiments. Ann. Rev. Ecol.

Syst. 16: 269-3 1 1.

Smith, L. M., L. D. Vangiider, R T. Hoppe, S. J. Morreale, and 1. L. Jr. Brisbin. 1986.

Effect of diving ducks on benthic food resources during winter in South Carolina,

U.S.A. Wddfow137: 136-141.

Sousa, W. P. 1979a. Disturbance in marine intertidal boulder fields: the nonequlibnum

maintenance of species diversity. Ecology 60: 1225- 123 9. -. 1979b. Experimental investigations of disturbance and ecological succession in a

rocky intertidal algal cornmunity. Ecol. Monog. 49: 227-254.

Stephens, D. W. and J. R Krebs. 1986. Foraging theory. Princeton University Press,

Princeton, New Jersey.

Stott, R S., and D. P. Olson. 1973. Food-habit relationship of sea ducks on the New

Hampshire coastline. Ecology 54: 996- 1007.

Strauss, S. Y. 1991. Indirect effects in comrnunity ecology: their definition, study, and

importance. Trends in Ecol. Evol. 6: 206-2 10.

Strong, D. R 1997. Quick indirect interactions in intertidal food webs. Trends in Ecol.

Evo~.12: 173-174.

Suchanek, T. H. 1978. The ecology ofMPIus edulis L. in exposed rociq intertidal

communities. J. Exp. Mar. Bioi. Ecol. 3 1: 105-120.

Swennen, C. 1989. Gu11 predation upon eider Somateria mollissirna ducklings: destruction

or elimination of the unfit? Ardea 77: 2 1-44. Swemen, C., G. Nehls, and K Laursen. 1989. Numbers and distribution of eiders

Sontateria moIIi&mu in the Wadden SaNeth. J. Sea Res. 24: 83-92.

Székely, T., and 2. Barnberger. 1992. Predation of waders (CharaLini) on prey

populations: an exclosure experirnent. J. Anim. Ewl. 6 1 : 447456.

Tabachnick, B.G.and L.S.Fidell. 1989. Using multivariate statistics. Second Edition.

Harper Collins, New York.

Thomas, M. L. H.,D. C. Arnold, and A R A Taylor. 1993. Rocky intertîdal

comrnunities. pages 35-73 in: M. L. H. Thomas, editor. Marine and coastal

systems of the Quoddy region. Can. Spec. Publ. Fish. Aquat. Sci. 64: 306 p.

Underwood, A J., and P. S. Petraitis. 1993. Structure of intertidal assemblages in

different locations: how can local processes be interpreted. pages 39-5 1 in: Species

diversity in ecological communities- historical and geographic perspectives. R E.

Ricklefs and D. Schluter, editors. University of Chicago Press, Chicago, Illinois.

Vadas, R L., W.A. Wright, and S. L. MiUer. 1990. Recruitment ofAscophyI~um

ndosfnn:wave action as a source of mortality. Mar. Ewl. Prog. Ser. 61 : 263-272.

VanWuike, W. 1970. Effect of environmental factors on byssal thread formation. Mar.

Biol. 7: 143-148.

Vistein, R W.1977. The importance of predation by crabs and fishes on benthic Uifauna

in Chesapeake Bay. Ecology 58: 1 199- 12 16.

Ward, D. 199 1. The size selection of clams by Mcan biack oystercatchers and kelp guiis.

Ecology: 72: 5 13-522. Wootton, J. T. 1992. Indirect effects, prey susceptibility, and habitat selection: impacts of

birds on lirnpets and dgae. Ecology 73 : 98 1-99 1.

. 1993a. Indirect effects and habitat use in an intertidal cornmunity: interaction chains

and interaction modifications. Am. Nat. 14i : 7 1-89.

. 1993b. Size-dependent cornpetition: effects on the dynamics vs. the end point of

mussel bed succession. Ecology 74: 195-206.

. 1994a Predicting direct and indirect eEects: an integrated approach using

experiments and path analysis. Ecology 75: 15 1-165.

. 1994b. The nature and consequences of indirect effects in ecological communities.

Ann. Rev. Ecol. Syst. 25: 443-466.

. 1997. Estimates and tests of per capita interaction strength: diet, abundance, and

impact of intertiddy foraging birds. Ecol. Monog. 67: 45-64.

Wmer, B. J., D. R Brown, and K. M. Michels. 1991. Statistical principles in experimental

design. Third Edition. McGraw-Hill, New York. 1057 pp.

Ydenberg, R C. 1988. Foraging by diving birds. pages 22-29 in: Acta XIX Congressus

ùiternationalis Ornithologici. H. Ouellet, editor. University of Ottawa Press,

Ottawa, Canada.

Yodzis, P. 1988. The indet erminacy of ecologicai interactions as perceived through

perturbation experiments. Ecology 69: 508-5 15. -. 1993. EnWonment and trophodiversity. pages 26-38 in: Species diversity in

ecological cornmunities. R E. Ricklefs and D. Schluter, editors. The Univcnsty of

Chicago Press, Chicago, IUinois. Zar, J.H. 1996. Biostatistical analysis. Third edition. Prentice Hall, Upper Saddle River,

New Jerxy.

Zwarts, L. and A-M.Blomert. 1992. Why knot CaIi&s carmtus take medium-sized

Macoma buIthzca when six prey species are available? Mar. Ecol. Prog. Ser. 83:

113-128. APPENDIX 1

Foods eaten by ducks in Passamaquoddy Bay

Below is a list of invertebrates eaten by Common Eider adults and duckhgs, and by White-Wmged Scoters. Data were obtained fiom esophagi and ginards of ducks coliected by hunters. This list includes data fkom 43 eiders and 6 ducklings, and provides the number of ducks which consumed each invertebrate species. Sues of blue mussels eaten by aduit eiders range fiom 1 to 55.4 mm (median = 34.7 mm). These sizes can not be used to infer prey size-selection, because 1 have no information prey availability where ducks were feeding.

- Adult Common Eiders Species eaten Number of ducks

Littorim littorea 19 Littorim saatilis unidentified periwinkles (probably L. littorea) Skeneopsis planorbis

Bucirtum umhm Mytilus eailis unidentified crab Common Eider duc Ming s Littonna Iittorea 3 Littonna obhrsata 3

Gammarus uceanicus 1 Arnpithoe rtlbricata 1 Mbrinogamnmonfinmarchicus 1 unidenti~edamphipad 1 crimgon septemspinom 1 MHIus edalis 1 insects (Coleoptera, Hymenoptera, Diptera) 2 APPENDIX 2

Species List

Following is a list of species 1 found in sarnples collected at Indian Point fiom 1994 to 1996. Most animals were identified to

species, with the exception of rare juveniles, insect larvae, damaged arnphipods, and polychaetes. Num refers to the number of samples

during each collection period in which each species is found (maximum = 24), and biomass refers to total dry tissue biomass (g) of a

species which was collected dunng that period. Small biomasses (<0.0001 g) are referred to as zero.

Mussel bed (Chaoter 3) h, O

Aug194 Decl94 Aprl95 Augl95 Decl95 Aprf96 Augl96 Decl96 species num biomass nwn biomass num biornass num biomass num biomass num biomass num biomass num biomass phylum Coelenîerata class Anîhozoa juvenile anemone O O O O O O O O O O O O 1 0.003 2 O

phylum PIatyhelminthes class Turbellaria Notoplana atomata 1 0.001 11 0.211 9 0.1289 O O 1 0.001 1 0,003 1 0.005 3 0.01 phylum Mollusca class Gastropoda Acanthodoris pilosa juvenile nudi branch Collisella testudinalis Crucibulurn striaturn Crepidulafornicata Margarites helicinus Skeneopsis planorbis Littorina littorea Littorina obtusata N Littortna smtilis O N Lacuna vincta LunaHa heros Lunatia triseriata Onoba amleus Buccfnum undatum Nucella lapillus

class Bivalvia Mytilus edulis Cerastoderma pinnularum Macoma balthica Mya arenaria Hiatella arcticn

order Amphipoda Ampelisca abdita Corophium volutator Marinogammarus tfinmrchicus Marinogammarus obtusatus Unciola irrorata Muera dame Maera lovenl Gammarus oceanicus Gammarus tigrinus Ampithoe rubricata g Phoxocephalus holbolli Dexamine thea misc. arnphipods

class Insecta misc. insects

phylum Echinodermata class Stelleroidea Asterias Jorbesii 6 0.002 10 0.359 6 0.3785 3 0.6235 2 0.016 3 0.0233 4 0.242 4 0.026 Ophiophois aculeata O O O O O O O O O O O O O O 1 O

class Echinoidc?~ Strongylocentrotus droebachiensis O O O O 1 O. 1765 1 O 3 1.33 2 0,5461 2 0.174 2 0.158 phylum Chordata class Ascidiacea iuvenile tunicate O O O O O O O O 1 O O O O O O O

Rockweed bed (Cha~ter4)

species num biomass num biomass num biomass nurn biomass num biomass num biomass num biornass num biomass phylum Coelenterata class Anthozoa juvenile anernone O O 1 0.03 O O O O O O O O O O O O

O phylum Platyhelmtnthes ~l class Turbellaria Notoplana atomata 7 0.091 2 O 16 0.309 18 0.349 O O 1 0.003 3 0.032 O O

phylum Mollusca class Gastropoda Acanthodoris pilosa O O O O O O O O O O O O O O 1 O juvenile nudi branch O O 8 0.006 14 0.026 11 0.036 4 0.001 4 0.003 1 O O O Collisella testudinalis 18 0.187 14 0,117 17 0.162 18 0.16 20 0.327 24 0.183 12 0.089 19 0.316 Margarites helicinus 1 0.003 O O 1 0,002 1 0.003 O O O O O O O O Skeneopsisplanorbis 21 0.041 19 0.014 17 0.015 18 0.013 3 O 21 0.026 19 0.079 14 0.01 Littorina littorea 23 28.555 24 24.963 24 37.108 23 27.144 23 16.537 23 20.358 19 14.39 24 24.872 LIttorina obtusata 21 2.092 24 5.035 23 4.693 23 4.086 22 4.103 17 4.636 20 2.522 22 3.248 Llttorina saxatilis 4 0.019 10 0.068 4 0.018 6 0.012 1 0.014 5 0.023 11 0.044 5 O Lacuna vincta 1 0.008 2 0.002 1 O O O O O 6 0.005 1 0.001 1 O crl

O

crl

O

APPENDIX 3 Duck Counts Following are counts of ducks present at my study site throughout the experiments. The fist data are monthly averages for adult eiders and ducklings present in the general study area. The second set of counts are for eiders observed either diredy in my shidy area or in the immediate surrounding area. These birds would have been the ones feeding in my experimental sites. Counts were conducted 1 to 2 hours after high tide. Counts of ducklings in my study sites are probably lower than amal usage because rockweed was usually submerged at that tirne. However, overd counts of duckiings are probably an accurate reflection of the local population because crèches clustered near shore around high tide and worked their way out as the tide fell and rockweed was exposed. The third set of data are average counts of eiders and scoters for each sarnpling period, and a caldation of "duck days" - number of ducks x number of days in the sample penod.

Total eider counts Adult eiders hider duckhgs Month Mean Minimum Maximum Mean Minimum Maximum 3 57.4 O 903 33.8 O 114 Counts of eiders in the immediate study area Aduit eiders Eider duckluigs Minimum Minimum O O 5 O O O 72 O 116 O 11 O O O 366 O O O O O O O O O O O O O O O O O O Caldation of relative use of the experirnental area by eiders and scoters Sample penod Mean erder count days duck-days

Total 212589.5

Season Mean scoter count days duck-days Fm4 29.2 101 2953.5 May/9 5 27.3 3 1 845.9 FalV95 30.8 102 3 138.2 Wmt ed96 0.2 67 10.3 Apr-May/96 3.1 61 186.1 Fa96 36.1 88 3 179.5 IMAGE EVALUATION TEST TARGET (QA-3)

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