MECHANISMS UNDERLYING THE EFFECTS OF MARINE HERBIVORES IMPLICATIONS FOR A LOW INTERTIDAL KELP COMMUNITY
by
RUSSELL W. MARKEL
B.Sc, The University of British Columbia, 1992
A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE
in
THE FACULTY OF GRADUATE STUDIES
(Department of B otany)
We accept this thesis as conforming to the required standard
THE UNIVERSITY OF BRITISH COLUMBIA
October 1996
© Russell W. Markel, 1996 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission.
Department of t5T~-rcVlO>v/
The University of British Columbia Vancouver, Canada
Date }Q)bQ>- I I'H &
DE-6 (2/88) MECHANISMS UNDERLYING THE EFFECTS OF MARINE HERBIVORES: IMPLICATIONS FOR A LOW INTERTIDAL KELP COMMUNITY
ABSTRACT
Rocky low intertidal communities structured by the canopy forming kelp
Hedophyllum sessile and the generalist herbivore Katharina tunicata are typical of semi- exposed coasts of British Columbia. Katharina tunicata is capable of removing the community supporting canopy formed by H. sessile . Mechanisms which determine and mediate the interaction between this alga and herbivore are investigated.
Densities of Katharina tunicata were manipulated within the range of densities found on the shore, without the use of artificial barriers, and the percent cover, density, population structure and demography of Hedophyllum sessile were monitored to determine the effects of these manipulations. Differential susceptibility of H. sessile holdfast size classes are shown to account for the rapid decline of H. sessile abundance in areas of high chiton density. These data are used to quantify the mechanism referred to as 'an escape in size.' Several lines of evidence suggest that the mechanism of K. tunicata interaction with
H. sessile adults is an indirect effect whereby holdfast integrity is degraded, making individuals of H. sessile more susceptible to wave-induced mortality. The implications of this mechanism for observed geographic variation in K. tunicata interaction strength are discussed.
The potential role of polyphenolics (anti-herbivore secondary metabolites) in the interaction between Hedophyllum sessile and Katharina tunicata is examined. Three experiments determined that: (1) H. sessile is a phenolic-rich species (5-7% H. sessile dry weight), (2) induction of phenolic production does not occur within three days of simulated wounding, (3) individuals of H. sessile from areas of naturally high and low herbivore densities do not differ in phenolic content, and (4) juvenile H. sessile tissue contains significantly more phenolics than adult vegetative tissue but adult reproductive and
ii vegetative tissues do not differ. These results are discussed in terms of both present-day and evolutionary ecological importance.
By monitoring the Hedophyllum sessile understory community during the chiton density manipulation experiment, and performing descriptive investigations during the same time period, several direct and indirect interactions were revealed which are potentially important determinants of the structure of this community. These interactions are used to construct three interaction webs which distinguish between the interactions experienced by juvenile and adult H. sessile. A model of the annual successional trajectory of this low intertidal kelp community is used to summarize the findings of this thesis.
iii TABLE OF CONTENTS
Page
ABSTRACT ii
LIST OF FIGURES v
LIST OF TABLES vii
ACKNOWLEDGEMENTS ix
CHAPTER 1: General introduction to algal/herbivore interactions , 1
CHAPTER 2: Effects of herbivore density on the demography and population structure of an intertidal kelp, Hedophyllum sessile (C. Agardh) Setchell 9
CHAPTER 3: Polyphenols content of the intertidal kelp Hedophyllum sessile: present-day implications and evolutionary frameworks • 49
CHAPTER 4: Direct and indirect interactions regulating a low intertidal kelp community: implications for juveniles vs. adult kelps 67
CHAPTER 5: General conclusions and summary: A successional model of the mechanisms and processes regulating a low intertidal kelp community 103
iv LIST OF FIGURES
Page Figure 2.1 Katharina tunicata density (#/m2) within low, control and high chiton density manipulation areas. Data are the mean ± SE of 6 replicates for density manipulations at each sampling date. Sampling date abbreviations are: JL95 (July 1995), S95 (September 1995), 095(October 1995), D95 (December 1995), F96 (February 1996), A96 (April 1996), JU96 (June 1996), JL96 (July 1996) 30 Figure 2.2 Hedophyllum sessUe percent cover within low, control and high chiton density manipulation areas. Data are the mean ± SE of 6 replicates for density manipulations at each sampling date 31 Figure 2.3 Hedophyllum sessile density within low, control and high chiton density manipulation areas. Data are the mean ± SE of 6 replicates for density manipulations at each sampling date 32 Figure 2.4 Hedophyllum sessile blade number / individualwithin low, control and high chiton density manipulation areas. Mean blade number / individual was averaged over each 25 x 25 cm quadrat. Data are the mean ± SE of 6 replicates for density manipulations at each sampling date 33 Figure 2.5 Hedophyllum sessile blade length / individualwithin low, control and high chiton density manipulation areas. Mean blade length / individual was averaged over each 25 x 25 cm quadrat. Data are the mean ± SE of 6 replicates for density manipulations at each sampling date 34 Figure 2.6 Blade length (a) and blade number (b) of Hedophyllum sessile individuals of the 1-4 cm maximum holdfast diameter size class within low, control and high chiton density manipulation area. Mean blade number and length / individual were averaged over each 25 x 25 cm quadrat. Data are means ± SE OF 1-4 cm size-class individuals pooled over each chiton density manipulation on each sampling date 35
Figure 2.7 Hedophyllum sessile recruitment densities (#/m") within low, control and high chiton density manipulation areas. Data are the mean ± SE of 6 replicates for density manipulations at each sampling date 36 Figure 2.8 Comparison of survivorship curves of the July 1995 cohort of Hedophyllum sessile (a) <1 cm (b) 1-4 cm and (c) >4 cm holdfast size classes of as a function of low, control and high chiton density 37 Figure 2.9 Results of the Kaplan-Meier survival analysis of the Hedophyllum sessile <1 cm, 1-4 cm and >4 cm maximum holdfast diameter size classes as a function of low, control and high chiton densities. Data are mean survival time (weeks) ±95% confidence intervals 38 Figure 2.10 Comparison of survivorship curves of the July 1995 cohort of Hedophyllum sessile within (a) low, (b) control and (c) high Katharina tunicata density treatment areas as a function of holdfast size 39 Figure 3.1 Phenolic content of Hedophyllum sessile wounded and control blades 1, 2, and 3 days after wounding. Data are mean + SE of 14 replicates for days 1 and 3 and of 6 replicates for day 2 65
v Figure 4.1 Articulated coralline algae percent understory cover within low, control and high chiton density manipulation areas. Data are the mean ± SE of 6 replicates for density manipulations at each sampling date 86 Figure 4.2 Crustose coralline algae percent understory cover within low, control and high chiton density manipulation areas. Data are the mean ± SE of 6 replicates for density manipulations at each sampling date 87 Figure 4.3 Bare rock percent understory cover within low, control and high chiton density manipulation areas. Data are the mean ± SE of 6 replicates for density manipulations at each sampling date 88 Figure 4.4 Fugitive algae percent understory cover within low, control and high chiton density manipulation areas. Data are the mean ± SE of 6 replicates for density manipulations at each sampling date 89 Figure 4.5 February 1996 comparison of Hedophyllum sessile percent cover, density, holdfast diameter and blade length between the high and mid H. sessile zones. Letters represent the transition from the high H. sessile zone (A) to the mid H. sessile zone (D) 90 Figure 4.6 Comparison of interaction webs involving juvenile (A) and adult (B) Hedophyllum sessile 91 Figure 4.7 Types of indirections occuring between the chiton Katharina tunicata, adult and juvenile Hedophyllum sessile, and articulated (AC) and encrusting (EC) coralline algae 92 Figure 4.8 Direct and indirect interactions occuring between juvenile and adult Hedophyllum sessile, articulated (AC) and encrusting (EC) coralline algae, Katharina tunicata (Chiton) and wave force 93 Figure 5.1 Summary model of annual successional trajectory of the temperate low intertidal community dominated by the kelp Hedophyllum sessile and the chiton Katharina tunicata 112
vi LIST OF TABLES
Page Table 2.1 Results of ANOVA comparing Katharina tunicata densities between chiton density treatment areas at each sampling date 40 Table 2.2 Results of ANOVA on the treatment effect of Katharina tunicata density on Hedophyllum sessile percent cover 41 Table 2.3 Results of ANOVA on the treatment effect of Katharina tunicata density on Hedophyllum sessile density 42 Table 2.4 Results of ANOVA on the treatment effect of Katharina tunicata density on Hedophyllum sessile blade number 43 Table 2.5 Results of ANOVA on the treatment effect of Katharina tunicata density on Hedophyllum sessile blade length 44 Table 2.6a Results of ANOVA on the treatment effect of Katharina tunicata density on the blade number of Hedophyllum sessile 1-4 cm holdfast size class individuals 45 Table 2.6b Results of ANOVA on the treatment effect of Katharina tunicata density on the blade length of Hedophyllum sessile 1-4 cm holdfast size class individuals 45 Table 2.7 Results of ANOVA on the treatment effect of Katharina tunicata density on the recruitment (#/m2) of Hedophyllum sessile sporophytes 46 Table 2.8 Simple correlation analysis of the relationship between adult Hedophyllum sessile percent cover and juvenile H. sessile recruitment compared between low, control and high chiton density treatment areas 47 Table 2.9 Results of Cox regression analysis comparing Hedophyllum sessile survivorship between chiton density treatments grouped by H. sessile maximum holdfast diameter size classes 48 Table 2.10 Results of Cox regression analysis comparing Hedophyllum sessile maximum holdfast diameter size-class survivorship grouped by chiton density treatment types 48 Table 3.1 Results of ANOVA comparing phenolic content (% tissue dry weight) between juvenile, adult vegetative and adult reproductive tissue 66 Table 4.1 Results of ANOVA on the treatment effect of Katharina tunicata density on the understory percent cover of articulated coralline algae 94 Table 4.2 Results of ANOVA on the treatment effect of Katharina tunicata density on the understory percent cover of articulated coralline algae 95 Table 4.3 Results of ANOVA on the treatment effect of Katharina tunicata density on the understory percent cover of rock 96 Table 4.4 July 1995 comparison of juvenile and adult Hedophyllum sessile and Katharina tunicata density between the High and Mid H. sessile zones 97 Table 4.5 Simple correlation analysis of July 1995 densities of juvenile and adult Hedophyllum sessile and Katharina tunicata 97
vii Table 4.6 July 1995 comparison of the substrata on which Katharina tunicata and juvenile Hedophyllum sessile were found and the relative abundances of those substrata 98 Table 4.7 Results of ANOVA for the July 1995 comparison of the relative abundance of rock, crustose algae and articulated coralline algae in the control plots of the chiton density manipulation experiment 98 Table 4.8 Comparison of the substrata on which the April 1996 cohort of Hedophyllum sessile recruits were found and the relative abundance of those substrata, between low, control and high chiton density treatment areas 99 Table 4.9 February 1996 comparison of Hedophyllum sessile percent cover, holdfast diameter, and blade length and Katharina tunicata density between the High and Mid H. sessile zones 100 Table 4.10 Results of ANOVA of February 1996 comparison of Hedophyllum sessile percent cover, density, holdfast diameter and bladelength between four vertical intertidal levels (A,B,C and D) from the high to low H sessile zones 100 Table 4.11 May 1996 comparison of Hedophyllum sessile canopy cover, Katharina tunicata density, and understory components between the High and Mid H. sessile zones 101 Table 4.12 July 1996 comparison of Hedophyllum sessile canopy cover, Katharina tunicata density, and understory components between the High and Mid H. sessile zones 101 Table 4.13 Simple correlation analysis of May 1996 Hedophyllum sessile canopy cover, Katharina tunicata density, and understory components 102 Table 4.14 May 1996 comparison of Katharina tunicata density and understory components between areas with >75% and <25% cover Hedophyllum sessile 102
viii ACKNOWLEGEMENTS
I am very grateful to my supervisor, Dr. Robert E. DeWreede, who not only stimulated my original interests in marine ecology and phycology, but also encouraged and facilitated my returning to graduate school. Since joining Rob's lab the atmosphere for learning and doing science has always been stimulating and supportive. I also thank my committee members, Dr. Paul G. Harrison and Dr. Roy Turkington for their time, energy and positive input. Doing intertidal field work throughout the entire year can be demanding work and I could not have accomplished this project without the many people who sat in the fog, rain, snow, and crashing waves with me while I took data. These people include my supervisor (Rob DeWreede) several lab mates (Tania Thenu, Andrea Sussman, Kristen Milligan, Ricardo Scrosati) and many volunteer field assistants (including Laura Froc, Seaton Taylor, and Anne-Marie Huang). Special thanks go to Anne Mcintosh for her endless humour while being doused by waves and grinding endless tissue samples for phenolic assays. Other people I would like to thank in particular include, Kristen Milligan, for her insightful discussions, support and friendship, Frank Shaughnessy, for his critical reviews, Carol-Anne Borden, for being flexible about my TA schedules, Jeong-Ha Kim for his advice and comments, and Catriona Hurd and Kathy Durante for their course in Plant-Animal Interactions. The Bamfield Marine Station provided much of the logistical support required to do research in this rugged environment and the Huu-Ayh-at Band generously allowed me-to conduct my study at Prasiola Point. Financial support was provided in part by NSERC grant no. 5-89872 to R.E. DeWreede and a Bamfield Marine Station Graduate Student Scholarship. Finally, many thanks go to Sophie Boizard, who was always there to help when I needed it most, and my parents, Wayne and Lyn Markel, who gave me all forms of support imaginable.
ix 1
CHAPTER 1
General introduction to algal/herbivore interactions
The characteristic distribution, diversity, and abundance of benthic marine algae in a given habitat is the result of interactions between biotic and abiotic forces. The challenge of identifying and determining the relative importance of these interactions is considerable.
The relative importance of abiotic forces, including wave exposure, light intensity, salinity, water and air temperature, must be determined and compared with those of biotic forces.
Terms such as, competition (for resources, including available substratum and light), grazing( by phylogenetically diverse herbivores), and predation, are simplified descriptors of complex biological forces shaping benthic marine communities.
Algal/herbivore interactions have generated a tremendous amount of attention over the past 20 years as they are believed to exert a major controlling influence over many benthic marine communities (Lubchenco 1978; Lubchenco and Gaines 1981; Hawkins and
Hartnoll 1983; Hay 1984; Duggins and Dethier 1985; Littler et al. 1986; Foster 1992;
Andrew and Underwood 1993; Tegner et al. 1995; Hay and Hixon 1996). These interactions often affect the fitness of those species involved, both algal and herbivore, and thus have the potential to be strong selective agents (Littler and Littler 1980; Lubchenco and
Gaines 1981; Hay and Fenical 1988; Duffy and Hay 1990; John and Lawson 1990;
Steneck 1992).
As evidence accumulates for the controlling influences of marine herbivores as determinants of community structure, studies are becoming increasingly refined in order to determine the mechanisms by which algal/herbivore interaction strength is determined. The importance of interaction, or linkage, strength stems from its utility to community ecologists as descriptors of the importance of various trophic relationships when constructing food webs. MacArthur (1972), notes Paine (1980), refers to consumers as 2 being strong interactors if in their absence, pronounced changes ensue. Removal of a
weakly interacting species will yield little or no change. The nature of this definition,
combined with the pioneering work of Paine (1966), has driven an entire generation of
experimental ecology which has fully utilized the addition and removal experiments so
integral to studies of herbivory and predation. However, while strength of interactions
may become clear through the removal of herbivores or predators, the mechanisms responsible for interactions identified in this way may remain obscured.
The mechanistic approach taken by Lubchenco and Gaines (1981) identifies many
of the algal and herbivore characteristics which mediate the outcome of algal/herbivore
interactions. The probability that a herbivore will encounter an alga is a function of alga
size, abundance (apparency), distribution, chemical stimuli, and life span, as well as
herbivore sensory capabilities, mobility and density. However, once an alga has been
detected another subset of characteristics which determine herbivore feeding preferences
can also be identified. Specific algal characteristics determining herbivore feeding preferences include morphological and chemical resistance which may determine
digestibility and palatability, and caloric or nutritional value (Lubchenco and Gaines'1981;
Hay 1984; Littler et al. 1986). Herbivore characteristics determining feeding preference
include physical condition (i.e. starved or satiated, Vadas 1977), relative body size
(Steneck and Watling 1982), functional morphology of feeding apparatus (Padilla 1985,
1989), the resulting handling time and feeding rate (Himmelman and Carefoot 1975;
Steneck and Watling 1982; Gaines 1985), and the probability of being attacked by ones
predators while feeding (Lubchenco and Gaines 1981; Hay 1984).
Implied in the separate treatment of algal and herbivore characteristics (which
determine the probability of encounter) is the importance of a variety of spatial refuges in
which algae may escape herbivory. Non-coexistence refuges (Lubchenco and Gaines
1981; Menge and Lubchenco 1981), or absolute refuges (Hixon and Menge 1991), can be
described as any location where an alga's survival, and therefore fitness, is high relative to 3 other locations because of the non-uniform impact of herbivory (Pfister and Hay 1988).
Effectively, in marine intertidal habitats, spatial refuges will constitute those areas where herbivores cannot forage efficiently. For example, patches of barnacles or turf-forming algae may inhibit the mobility of gastropod molluscs and therefore algae recruiting into these areas will suffer few or no losses to herbivory (Hay 1981; Hawkins and Hartnoll
1983; Dungan 1986; Van Tamelen 1987; Farrell 1991).
Closely allied with spatial refuges is the concept of size escapes (or coexistence refuges, Lubchenco and Gaines 1981; Harris et al. 1984). While the damage imposed by herbivores to juvenile algal forms may easily result in loss of the entire thallus, those individuals which survive and grow to sizes at which the amount of tissue lost to herbivores is no longer significant can be said to have reached a refuge in size. This process also describes the scenario referred to as 'herbivory bottlenecks' wherein algal survivorship increases in direct proportion to size (DeWreede 1984; Dean et al. 1989;
Denton et al. 1990; Ang, Jr. 1991).
The algal and herbivore characteristics described above, and their mediation by spatial and size-related refuges, govern the interaction strength, and therefore ecological importance, of algal/herbivore interactions in a given community. Ecology has been criticized for its lack of predictive ability due to the ubiquity of variation and lack of ability to generalize from specific experimental results (Peters 1991). Paine (1980) observed that
'pattern is generated by process'. Accordingly, I believe that in many cases the inability to interpret variation stems from a lack of understanding of the subtle mechanisms which underlie more obvious effects. Thus, two general goals directed this thesis: (1) to account for variation in interaction strength of algal/herbivore interactions by determining mechanisms which underlie effects, and (2) to use the quantitative nature of population structure and demography data to identify and assess the importance of these mechanisms in determining benthic algal community structure. 4
The thesis is organized into five chapters, including this introductory chapter
(Chapter 1) and a summary chapter. The general introduction to algal/herbivore interactions of this chapter is intended to give the reader a feel for the issues surrounding this topic and to act as a framework upon which following chapters can be placed.
In Chapter 21 present an experimental field study in which I manipulate the density of the principal herbivore of a low intertidal kelp community and monitor ensuing changes in population structure and demography of the dominant kelp species. Whereas studies of benthic algal population structure and demography are rare in themselves, none have been performed in concert with manipulation of herbivore densities. The intent of this experiment was to determine mechanisms underlying the effects of a marine herbivore while testing the utility of monitoring algal population structure and demography in order to determine mechanisms as compared to more traditional measures of percent cover and density.
Chapter 3 considers how chemical attributes, specifically secondary metabolites called polyphenolics, may influence the outcome of interactions between a kelp and its principle herbivore. A survey of intraspecific variation of polyphenolic content, a wounding experiment to assess potential induced responses, and a comparison of polyphenolic content of individuals collected from sites differing in herbivore density, are used to assess this interaction. The results of these experiments are discussed with respect to how they may impact kelp population dynamics and how they fit into an emerging general framework of the relationships between herbivory and brown algal polyphenolic production.
Chapter 4 uses community data collected from the herbivore density manipulation experiment described in Chapter 2, and additional descriptive data collected during the same time period, to construct and support two models of interaction webs which may account for structure, development of, and variation within, the same low intertidal kelp community. These two models distinguish differences and similarities of interspecific interactions involving juvenile and adult forms of the kelp species in question. A third interaction web is proposed to examine the effect of adults kelp forms on juvenile forms.
Finally, as a summary of what has been learned during the course of this thesis, experimentally, descriptively, and intuitively, Chapter 5 is presented as a model of the mechanisms and processes which drive the annual succession and maintenance of a low intertidal kelp community. I end this chapter, and the thesis, with some brief comments concerning future research directions and experimental design in community ecology. 6
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Denton A, Chapman ARO, Markham J (1990) Size-specific concentrations of phlorotannins (anti-herbivore compounds) in three species of Fucus. Mar Ecol Prog Ser 65: 103-104.
DeWreede RT (1984) Growth and age class distribution of Pterygophora californica (Phaeophyta). Mar Ecol Prog Ser 19: 93-100.
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Dungan ML (1986) Three-way interactions: barnacles, limpets, and algae in a Sonoran Desert rocky intertidal zone. Am Nat 127(3): 292-316.
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Gaines SD (1985) Herbivory and between-habitat diversity: The differential effectiveness of defenses in a marine plant. Ecology 66 (2): 473-485.
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Hay ME (1984) Predictable spatial escapes from herbivory: how do these affect the evolution of herbivore resistance in tropical marine communities? Oecologia (Berlin) 64: 396-407.
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John, S.J. Hawkins, and J.H. Price), Systematics Association Special Volume No. 46, pp. 61-85. Clarendon Press, Oxford, 1992
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CHAPTER 2
Effects of herbivore density on the demography and population structure of an intertidal kelp, Hedophyllum sessile (C. Agardh) Setchell
INTRODUCTION
Herbivores have been shown to exert significant controlling influences upon species abundance and diversity in some benthic marine communities (reviewed by
Lubchenco and Gaines 1981; Hawkins and Hartnoll 1984; John et al. 1992). These effects, however, are often highly variable in space and time (Foster 1992). Whereas many studies have examined changes at the population and community level resulting from coarse manipulation of factors (i.e. herbivore addition or removal), closer examination of the mechanisms which mediate algal/herbivore interactions is required in order to interpret variable results.
Herbivore density is an important determinant of algal/herbivore interaction strength. However, while the effects of dramatic, and often unrealistic differences in herbivore density have been well documented experimentally (Lubchenco 1978;
Himmelman et al. 1983; Duggins and Dethier 1985), the effects of smaller changes in herbivore density remain less well understood (Andrew and Underwood 1993). While presence/absence studies may demonstrate potential effects of experimentally maintained herbivore densities, such studies give little insight as to effects of herbivore densities which are pertinent to natural community operation. Andrew and Underwood (1993) manipulated a subtidal urchin population in Australia at 0%, 33%, 66% and 100% of naturally occurring densities and found that the effects of grazing were not linearly related to density.
Behavioural compensation, differences in herbivore body size, differential susceptibility, and differences in recruitment and growth rates of algal forms all may have been contributing mechanisms accounting for these non-linear results. 10
The ability of algae to survive and grow to sizes at which they become significantly less susceptible to the effect of herbivores, termed escape in size (Duffy and Hay 1990), coexistence refuge (Lubchenco and Gaines 1981), or herbivory botdeneck (Menge and
Lubchenco 1981; Farrell 1988; Dean et al. 1989; Denton et al. 1990), is regarded as a crucial mechanism which determines the influence of herbivory on algal distribution and intertidal community structure (Hawkins and Hartnoll 1983). However, while most experimental studies of herbivory have measured algal percent cover, or less often density, these measures may provide little mechanistic information as to causal processes because they are insensitive to changes in algal age or size-specific survivorship. Consequently, the mechanism of 'size escape' remains unquantified.
In this chapter, I examine the interaction between the herbivorous chiton Katharina tunicata (Wood) and the intertidal kelp Hedphyllum sessile. Dayton (1975) and Duggins and Dethier (1985) suggest that K. tunicata is likely to have its greatest effect on H. sessile populations by grazing juvenile sporophytes before they are able to reach an escape in size.
Lack of replacement of adults lost to winter storms thus results in dramatic declines of H. sessile populations. However, while the ability of K. tunicata to suppress the canopy formed by H. sessile is clear in Washington State (Duggins and Dethier 1985), K. tunicata has little or no effect on the dominant kelp species in a similar community in Alaska
(Dethier and Duggins 1988). Further, the mechanism responsible for this variation in interaction strength was not determined.
Therefore, to test and quantify the importance of escapes in size for populations of
Hedophyllum sessile and to identify other mechanisms which may mediate interaction strength between Katharina tunicata and H. sessile, I describe here an experiment wherein I manipulate the density of this herbivore, within the range of densities naturally occurring on the shore, while monitoring the population structure and demography of H. sessile.
While studies that deal with survivorship in marine algae are relatively few, none have been performed in concert with experimental manipulation of herbivore density. 11
Community description. Low intertidal to shallow subtidal, semi-wave protected, areas of the Northeast Pacific are characterized by diverse assemblages of algal species.
Kelp species typically dominate these communities and exhibit a predictable pattern of zonation (Dayton 1975; Duggins and Dethier 1985). Hedophyllum sesstie occupies the highest position of these kelps, extending 1 m above mean low water, and seasonally forms a competitively dominant, monospecific canopy which controls the abundance and diversity of understory algal and invertebrate species (Dayton 1975, Paine 1984; Dethier and Duggins 1984).
The most conspicuous herbivore in this community is the chiton, Katharina tunicata, a generalist grazer which may reach >10 cm in length. This chiton (haay'ustup) is a traditional resource actively harvested by the First Nations people of the Barkley Sound area (the Huu-Ayh-at). The Polyplacophoran (chiton) radula has been described as a
"multi-purpose tool' (Steneck and Watling 1982), well adapted for grazing on crustose algae, microalgae and macrophytes. Himmelman and Carefoot (1975) report that K. tunicata prefers grazing the kelp Hedophyllum sessile, and Duggins and Dethier (1985) found that K. tunicata is capable of controlling the abundance of H. sessile, and therefore the entire understory community, along the coast of Washington but not Alaska (Dethier and Duggins 1988).
While Katharina tunicata is the most abundant chiton in this community, several other chitons are also present but uncommon, including Tonicella lineata (Wood), Mopalia mucosa (Gould) and Cryptochiton stelleri (Middendorf). Limpets are common in the understory, including Acmaea mitra (Rathke), Collisella pelta (Rathke), and Notoacmaea scutum (Rathke). Although Dethier and Duggins (1984) describe an 'indirect commensalism' involving limpets, K. tunicata, macroalgae and microalgae, these herbivores, aside from K. tunicata do not appear to exert any influence over macroalgal abundance (Paine 1984, Duggins and Dethier 1984). The urchin, Stronglycentrotus purpuratus (A. Agassiz), is found occasionally under the H. sessile canopy but is not 12 consider it to be an important regulator of the H. sessile abundance due to its rarity (Paine
1984)
METHODS
Study site. The study area was the rocky low intertidal zone at Prasiola Point located in
Barkley Sound on the west coast of Vancouver Island, British Columbia, Canada (48° 49'
N, 125° 10' W). The portion of Prasiola Pt. used for the experiment was semi-exposed to waves from the northwest. Biological indicators of consistent wave exposure , such as
Lessoniopsis littoralis (Dayton 1975), are absent from this site. Experimental plots were located on rocky substratum with moderate to gentle slopes throughout the low intertidal zone ( 0.5 to 1.0 m above mean lower low tides; Canadian Chart Datum), and over a horizontal distance of approximately 100 m.
Experimental design. Densities of the chiton Katharina tunicata were manipulated within density ranges observed in the field. To avoid using artificial barriers to maintain herbivore density manipulations, treatment areas within the Hedophyllum sessile zone were selected on the basis of the presence of natural topographic features which would limit herbivore emigration and immigration. Katharina tunicata is rarely found above or below the H. sessile zone, therefore, natural isolating features were required primarily to limit horizontal movement. These included small surge channels, isolated rocks and outcrops of the shoreline. Each of 18 selected treatment areas was between two and four meters square.
For each treatment area the number of potential locations for a 25 x 25 cm quadrat was determined using tape measures to construct a grid. Using the number of potential locations where a quadrat could be placed and a random number table, a location within each treatment area was randomly selected to place and mark a permanent 25 x 25 cm 13 quadrat. Quadrat locations were marked by drilling 2" deep and 1/4" diameter holes at two of the four corners using a Ryobi gas rock drill. A #8-10 plastic dry wall anchor was hammered into each hole and a 2 1/2" stainless wood screw was screwed into each. In this way quadrat positions were accurately re-located on each sampling date.
Naturally occurring densities of the chiton Katharina tunicata within each 25 x 25 cm permanent quadrat were recorded and average over the 18 treatment areas. The 25th and 75th percentiles of this mean were then used to determine density ranges for, respectively, low and high chiton density treatment areas.
A random block design was used to determine the treatment type for each treatment area and to control for small scale variation in physical factors such as wave exposure .
Treatment areas were grouped into blocks of three quadrats, and each quadrat within a block was randomly assigned as either a low, control, or high chiton density treatment area.
At the beginning of July 1995 chiton density treatments were applied by haphazardly placing 25 x 25 cm quadrats within treatment areas and either removing or adding chitons until the desired density ranges were achieved. Relatively large treatment areas acted to limit chiton emigration and immigration, thereby maintaining density manipulations within sampling areas. Control density plots were not manipulated but to control for handling effects in the low and high chiton density areas, all chitons were picked up and placed down again. Manipulations were checked and maintained every two months at regular sampling dates between July 1995 and July 1996.
Sampling. Initial sampling took place just prior to chiton manipulation in July 1995 and continued approximately bimonthly over the following year. At each permanent quadrat, the percent cover of Hedophyllum sessile was recorded. A 25 x 25 cm quadrat was divided with monofilament line at 2.5 cm intervals such that each 2.5 x 2.5 cm square represented 1% of total cover. The percent cover of H. sessile and other kelp canopy 14 species was recorded by counting the number of squares occupied by each species. Within each quadrat the density of H. sessile was recorded as the number of individuals with distinctly independent holdfasts. For each individual of H. sessile , the maximum holdfast diameter, number of blades and lengths of blades was recorded.
Using a 25 x 25 cm quadrat and the monofilament grid described above as a coordinate system, a map of all Hedophyllum sessile individuals was created for each permanent plot and updated on each sampling date. By comparing maps between successive sampling dates a measure of survivorship and recruitment rates was obtained for each chiton density treatment. Holdfast diameters at the initial sampling date were used to place all individuals into three size classes. Juveniles were defined as those plants with holdfasts <1 cm in diameter and adults were split into those with holdfasts 1-4 cm and >4 cm in diameter. For survivorship analysis individuals of each size class were pooled over the six replicates of each chiton density manipulation.
Data analysis. Data for all measures except survivorship were compared between treatment groups on each sampling date using one-way analysis of variance with
'treatment' and 'block' as independent variables. Tukey's HSD test was used for post hoc comparisons. For the sampling dates of April 1996 to July 1996, variables were compared using 2-tailed unpaired t-tests because there were no surviving Hedophyllum sessile individuals in the high chiton density areas. For blade number and blade length, the mean of each individual was averaged over each permanent plot. The effect of adult H. sessile cover on sporeling recruitment was further examined using simple correlation analysis.
Fmax-tests were used to check for homogeneity of variances. In some cases this assumption was violated and the transformations used have been indicated in the 'Results' section. Occasionally transformations were not successful in which case non-parametric analyses were employed. Data were examined for approximate normality (Underwood 15
1981) by constructing probability plots. These data were analyzed using SYSTAT version
5.2.1 for Macintosh (Wilkinson et al. 1992).
Blade lengths and blades number of the 1-4 cm holdfast size class of each treatment group were further analyzed by extracting these data from the original analysis and pooling them on each sampling date. This was done in an effort to eliminate some of the variation associated with comparing blade lengths and numbers of all three holdfast size classes simultaneously. However, in treating these data in this way I have committed pseudo- replication and therefore the conclusions drawn from this analysis should be viewed with caution (Hurlbert 1984). Where necessary, some replicates were randomly removed from the analyses to have equal sample sizes in all treatments. The data were compared between chiton density treatment groups using one-way ANOVA and Tukey's HSD tests was used for post hoc comparisons.
Survivorship of Hedophyllum sessile was compared between the treatment groups in three ways. First, survivorship curves for each size class were constructed by calculating and plotting as a percentage the proportion of individuals of the initial cohort surviving to each sampling date. Second, a Kaplan-Meier survival analysis (Collett 1994) was used to determine the mean survival time with 95% confidence intervals of individuals within each size class for each treatment group. Finally, the Cox proportional hazards model (Collett 1994; Cox regression model, Norusis/SPSS 1994), was used to compare survival times of H. sessile between chiton density manipulations. The assumption of proportional hazards for the Cox regression was tested by constructing the model using the independent variable 'chiton density' as a time-dependent covariate. This assumption was met in all cases. All survivorship analyses were performed using SPSS version 6.0 for
Macintosh (Norusis/SPSS 1994). 16
RESULTS
Katharina tunicata density. Densities of K. tunicata in each permanent plot were
expressed on a per m basis. Data were square-root transformed to equalize variances for
the sampling date of September 1995. Chiton density manipulations were intended to
represent small differences in densities naturally occurring throughout the intertidal.
Consequently, K. tunicata densities between low, control and high K. tunicata treatment
areas were significantly different on only three of the eight sampling dates between July
1995 and July 1996 (Figure 2.1; Table 2.1).
Hedophyllum sessile percent cover. No differences in H. sessile canopy cover were
present prior to chiton manipulation in July 1995 but by October 1995 there were
significant (p=0.045) differences between the treatment groups which continued through
the remainder of the experiment. Data were log transformed to equalize variances for the
sampling dates of February, April and June 1996 (Figure 2.2; Table 2.2)
Hedophyllum sessile density. The number of individuals within each permanent plot
was expressed on a per m basis on each sampling date . New recruits appearing after the
initial sampling date were not included in this analysis but are presented separately as
'Sporophyte recruitment.' By February 1996 differences in H. sessile density between
low and high treatment areas approached significance (p=0.080). By April 1996 there
were no surviving individuals in any of the six replicate high chiton density treatment areas.
Transformations were not required for analysis on any sampling date (Figure 2.3; Table
2.3)
Blade length and blade number. No significant differences in blade number were
observed between July 1995 and February 1996 (Figure 2.4; Table 2.4). Differences in 17 blade length (Figure 2.5; Table 2.5) approached significance by December 1995 (p=0.069) and became significantly different by February 1996 (low and control > high, p=0.011).
By April 1996 there were no surviving individuals in any of the high chiton density manipulation areas, hence, there are no values past this date. Transformations were not required on any sampling date.
Examined in isolation, blade numbers of the 1-4 cm holdfast size class did not differ among the three treatment groups prior to the disappearance of the 1-4 cm holdfast size class in high chiton density areas by December 1995. In July 1995, blade length of
1-4 cm holdfast individuals were initially longer in the control group than the low (N=14; p=0.012) and high (p=0.05) chiton density groups, but did not differ again until December
1995 when Hedophyllum sessile in the high chiton density areas disappeared (Figure 2.6;
Table 2.6). At this date, individuals in the control and low chiton density areas did not differ in blade length or blade number (N=5; 2-tailed unpaired t-test).
Sporophyte recruitment. Sporadic recruitment of Hedophyllum sessile sporophytes was observed between July 1995 and April 1996 in the low and control, but not the high, chiton treatment areas. High recruitment was observed in April 1996, with recruits continuing to appear through August 1996. Recruitment was notably patchy throughout the treatment areas and no differences were found between them . August 1996 data required log transformation to equalize variances prior to analysis (Figure 2.7; Table 2.7).
Although no significant correlations were found between recruitment and H. sessile percent cover, a negative interaction was indicated for all chiton density treatment groups (Table
2.8).
Hedophyllum sessile survivorship. Survivorship curves for the three size classes of
H. sessile holdfasts (Figure 2.8) indicate that survivorship of individuals in the <1 cm and
1-4 cm size classes, but not the >4 cm size class, was less in the high than in the control 18 and low chiton density treatment areas. The results of the Kaplan-Meier survival analysis agree with these observations (Figure 2.9). Ninety-five percent confidence intervals for mean survival time of the <1 cm and 1-4 cm size classes in high chiton treatment areas do not overlap with those for the low and control treatment areas.
The Cox regression analysis further supports and adds to the certainty of these differences in survivorship. The Cox regression uses one dependent variable as a reference variable to which to compare other dependent variables. For analysis of survival time between the three size classes of Hedophyllum sessile, the 1-4 cm size class was the reference variable. The relative risk of mortality, denoted Exp(B) in Tables 2.9 and 2.10, describes the percentage change in the relative hazard rate associated with each H. sessile holdfast size class (Norusis/SPSS 1994). For <1 cm holdfast diameter individuals in the high chiton density areas the relative risk of mortality was 2.25 times that of <1 cm individuals in the low chiton density areas (Table 2.9). The 95% confidence intervals for
Exp(B) are interpreted as being significant when they do not include 1 (Collett 1994;
Norusis/SPSS Inc. 1994). Positive regression coefficients indicate decreased survival as compared to the reference variable whereas negative values indicate increased survival.
The significance level of B, the regression coefficient, was determined using the Wald statistic which has a Chi-square distribution (Norusis/SPSS Inc. 1994). The Wald statistic is B, divided by its standard error, squared (i.e. [B/S.E]2). Hedophyllum sessile individuals of the 1-4 cm holdfast size class in the high chiton density areas were 1.72 times more likely to die than the same size class individuals in the low chiton treatment areas. The 95% confidence interval of this value did not include 1 and the positive regression coefficient was significant. No differences in survival time for individuals in the
>4 cm holdfast size class were found between treatment groups.
Hedophyllum sessile survivorship data were also compared within chiton density treatments between size classes. Comparison of survivorship curves for low, control and high chiton density areas indicates that survivorship of the three size classes did not differ 19 between low and control density areas, but in high chiton density areas survivorship significantly increases with increasing holdfast size (Figure 2.10). This relationship is also supported by the Kaplan-Meier survival analysis (Figure 2.9).
For Cox regression analysis of survival time of size classes within the three chiton density manipulations, low chiton density was the reference variable. Survival times of size classes differed within the high, but not the low and control chiton density areas (Table
2.10). In areas of high chiton density individuals within the 1-4 cm holdfast size class lived significantly longer than those in the <1 cm holdfast size class and significantly fewer weeks than >4 cm holdfast size-class individuals. Individuals of the <1 cm holdfast size class were 2.12 times more likely to die, and >4 cm individuals only 0.38 times as likely to die, as 1-4 cm individuals.
DISCUSSION
The percent cover and density of Hedophyllum sessile declined dramatically in the high Katharina tunicata density manipulation areas. However, these measures alone yielded little insight as to the mechanism underlying this effect. Duggins and Dethier
(1985) describe similar changes in H. sessile percent cover between their K. tunicata addition and removal areas. In my study, however, the manipulated changes in chiton density were much smaller, giving some indication as to the range of chiton densities that are required to cause these declines in H. sessile abundance. For nearly all variables no significant differences were found between low and control chiton density treatment areas but in many cases significant differences were found between low and high chiton treatment areas. These results indicate non-linear effects of K. tunicata and perhaps a threshold which chiton densities must reach before significant changes ensue in H. sessile population structure and demography. Averaging mean K. tunicata densities at each sampling date over the one year course of the experiment indicates that such a threshold 20 would exist between 17 and 31 chitons Im. Interestingly, K. tunicata densities are known to persist at significantly higher densities in areas of high wave exposure (Stebbins 1988), suggesting that some mechanism may be responsible for changing this threshold value of
K. tunicata density. This question will be returned to in following sections.
This study supports the previously unquantified phenomena termed size escape
(Duffy and Hay 1990), refuge in size, or coexistence escape (Lubchenco and Gaines
1981). Although Katharina tunicata density had no significant effect on recruitment, this chiton is associated with decreased juvenile survivorship. In the high chiton density areas individuals of the 1-4 cm maximum holdfast diameter size class survived significantly longer than <1 cm holdfast individuals and significantly less than those in the >4 cm holdfast size class. This process is regarded as a crucial mechanism which determines in part the influence of grazing on algal distribution and intertidal community structure
(Hawkins and Hartnoll 1983). Lubchenco and Gaines (1981) calculate a theoretical measure of 'expected herbivore damage' by multiplying the probability that a plant will be encountered by a herbivore, by the conditional probability the herbivore will eat at least part of the plant, by the expected change in fitness of the plant given that it is encountered and at least partially consumed. Size-related escapes impact this equation directly, affecting both the conditional probability that the herbivore will feed and the expected change in fitness.
In the case of Hedophyllum sessile, the probability that Katharina tunicata will feed when it encounters adult blades is high, but the expected change in algal fitness is low. Juvenile sporophytes, on the other hand, are likely to suffer dramatic changes in fitness should grazing occur. The finding of increasing survivorship for successively larger sizes of
H. sessile individuals in this study represents an important quantification of this ecologically important mechanism which mediates the outcome of plant-animal interactions.
Most winters Hedophyllum sessile populations are dramatically reduced by storms which either removed entire individuals or leave only perennating holdfasts with a few blades of greatly reduced length (Duggins and Dethier 1985). Previous investigators of 21 this community (Dayton 1975; Duggins and Dethier 1985) have speculated that Katharina tunicata inhibits the annual recovery of the H. sessile canopy by grazing juvenile sporophytes before they are able to grow and reach a refuge in size. Grazing of adult sporophytes, particularly during low tides when blades are most accessible, also occurs.
However, adults were thought to be tolerant of K tunicata grazing as a result of their size.
Following this scenario, lack of replacement of adult H. sessile lost annually to wave force by juveniles would eventually lead to the differences in canopy cover observed between areas of low and high K. tunicata density (Duggins and Dethier 1985). Although juvenile survivorship was found to be lower in high chiton density areas, this scenario is not completely supported by the results of this study.
The survivorship analysis also showed that although the Hedophyllum sessile canopy does decline dramatically each winter due to battering by waves, the system is not completely reset every year. In fact, during the 1994/95 winter, the canopy showed little sign of deterioration at all. However, during the 1995/96 winter over which the experiment was performed, the particularly stormy weather resulted in an approximately 60% decline in the percent cover H. sessile blades in the control chiton density treatment areas.
Survivorship data for individuals within the low and control chiton density treatment areas show that many plants survived the winter and contributed to the recovery of the canopy the following spring. In many cases individuals survived as only holdfasts which regrew new blades, also aiding canopy recovery. These facts indicate that adult survivorship, as well as juvenile survivorship, is necessary for canopy recovery each year.
If survival of adult Hedophyllum sessile through winter storms, as well as replacement of adults by juveniles in the spring, are both important to the annual recovery of the H. sessile canopy, one must question if Katharina tunicata also affects adult
H. sessile survivorship. Survivorship analysis of individuals within high chiton density areas shows that young adults with holdfast diameters of 1-4 cm at the start of the study survived significantly fewer weeks than comparable adults in the control and low chiton 22 density areas. All of these 1-4 cm holdfast diameter individuals were lost by December
1995. The mechanism by which K. tunicata can remove juvenile (<1 cm size class)
H. sessile is clear: the small delicate blades are easily grazed to mortality. However, the mechanism by which adult individuals are removed, which have supposedly reached a refuge in size from grazing, is less obvious.
Circumstantial evidence strongly suggests that the 1-4 cm holdfast individuals in the high chiton density areas were lost due to the combined effects of Katharina tunicata- induced degraded holdfast integrity and increased wave force from September 1995 to
December 1995. The rate at which these individuals were lost strongly suggests that some factor in addition to chiton grazing must have played a contributing role in their demise. It is not plausible to expect that these metabolically low and slow moving gastropods could have consumed this entire size class between July 1995 and December 1995. If this were the case, one would expect to have found evidence of grazing at sampling dates prior to
December 1995 in the form of significant decreases in blade number and/or blade length.
However, neither blade length nor blade number differed when all three holdfast size classes were compared together (Figures 2.4 and 2.5; Tables 2.4 and 2.5). It is possible that variation within these data imposed by comparing all three size classes together may have masked intermediate stages. This was the rationale for also examining the blade lengths and blade numbers of the specific individuals composing the 1-4 cm holdfast size class. This analysis clearly also suggests that individuals of the 1-4 cm holdfast size class within high chiton density areas did not differ in blade length or blade number prior to their disappearance by December 1995 (Figure 2.6; Table 2.6). Together, these analyses strongly suggest that K. tunicata was indirectly responsible for the loss of the 1-4 cm holdfast size class by degrading holdfast integrity, thus making 1-4 cm holdfast individuals in high chiton density areas extremely susceptible to wave-induced mortality.
One additional line of evidence to support this hypothesis is that in the low and control treatment groups several individuals of Hedophyllum sessile survived as only 23
holdfasts. These individuals regrew new blades in the spring. No such individuals were
found in high chiton density treatment areas, suggesting that some mechanism was
responsible for loss of entire plants.
Duggins and Dethier (1985) pointed out the importance of abiotic disturbance in this
community. The seasonal increase in hydrodynamic forces imposed upon Hedophyllum sessile is directly responsible for attrition of the canopy and thus prevents the development
of a competitively formed spatial monopoly (Paine 1994). However, the indirect effects of
herbivores on the population dynamics of this kelp were not realized. This study presents
evidence that high densities of Katharina tunicata significantly decrease the survivorship of
young adult (1-4 cm holdfast size class) H sessile. As suggested earlier, this mechanism
may also contribute to the observation of higher K. tunicata densities in areas of high wave
exposure (Stebbins 1988): consistent wave exposure may decrease desiccation and K.
tunicata's requirement for H. sessile cover, and therefore decrease the level of interaction
between this alga and herbivore.
Herbivory and hydrodynamic forces represent the two primary sources of mortality
to benthic algal populations of wave-swept, low intertidal and shallow subtidal
communities. Critical investigations of the roles that hydrodynamic forces play in shaping benthic algal communities of wave-swept shores are relatively recent and few (Denny 1988;
Gaylord et al. 1994; Shaughnessy et al.1996). By directly impacting survivorship and
reproductive output, the combined effects of drag, lift and acceleration resulting from water
movement are strong selective agents acting on both form (Shaughnessy et al. 1996) and
reproductive strategies (DeWreede and Klinger 1988). Similarly, marine herbivores are
well known for their effects on benthic algal population and community structure and
several investigations also have attempted to attribute morphological, structural, chemical
and reproductive features of algal thallus forms to their effects (Littler and Littler 1980;
Padilla 1984; Paul and Hay 1986; Hay and Fenical 1988; Duffy and Hay 1990). However,
recent attention to the indirect relationship between herbivores and macroalgae by way of 24 wave exposure (Biedka et al. 1987; DeWreede et al. 1992; Padilla 1993; Tegner et al.
1995) suggests that determining the relative importance of these factors separately may be inappropriate.
As pointed out by Tegner et al. (1995), the hydrodynamic models of Gaylord et al.
(1994) predict that as individual macroalgae increase in size the probability of mortality due to holdfast failure or breakage becomes much higher. The finding in this study that smaller adults (1-4 cm holdfast size class), not the largest adult (>4 cm holdfast size class), are lost first contradicts this prediction and may give some insight as to the trade-offs that macroalgae face. Specifically, Hedophyllum sessile juveniles (<1 cm holdfast size class) may reach an escape in size from significant damage to blades when they reach the 1-4 cm holdfast size class, however, at this size their holdfasts are still susceptible to degradation by high densities of Katharina tunicata. Growing larger yet into the >4 cm holdfast size class may make these algae less susceptible to the actions of K. tunicata, but may also push them past the maximum size at which they can tolerate local and seasonal hydrodynamic forces. However, surpassing a yearly optimal size may not be an issue for H. sessile.
Gaylord et al. (1994) point out that there may be a benefit for perennating algae, such as
H. sessile, to 'take a chance' and grow larger than 'optimal', maximizing reproductive output, particularly if a storm does not hit before the time of reproduction. If manageable hydrodynamic forces are frequently exceeded prior to reproduction at least a portion of the individual will survive to try again next year. The timing of reproduction of H. sessile is unique among the kelps in that it begins to release spores as early as November and sporelings begin to appear in March. This timing of reproduction combined with the seasonal decline of H. sessile abundance may reflect a strategy to maximize size annually to escape the direct and indirect effects herbivores and to maximize reproductive output.
The interaction between Katharina tunicata and the holdfast of Hedophyllum sessile may also explain observed geographic variation in interaction strength between these species. Duggins and Dethier (1988) found that although K. tunicata interacts strongly 25 with the dominant mid-low intertidal canopy forming kelp in Washington state (USA), it has no such effect in a similar community in the northerly state of Alaska (USA).
However, although Duggins and Dethier (1988) describe differences between these communities as subtle, dramatic morphological differences in the dominant kelp species
found in each of these areas may account for the observed differences in interaction
strength. Based on their descriptions, the community studied in Washington was
dominated by H. sessile and is very similar to that found at Prasiola Point. However, the
Alaskan community was dominated by the winged-kelp, Alaria marginata. Functionally,
A. marginata appears to play an ecologically equivalent role in Alaska as H. sessile does in
Washington and British Columbia: a competitively dominant canopy that supports a diverse
algal and invertebrate understory community. However, whereas H. sessile individuals possess a large (occasionally >10 cm), perennating holdfast composed of interwoven
haptera which supports a diverse invertebrate community, A. marginata possesses a small,
stout, annual holdfast (Scagel 1972) which appears impenetrable to most burrowing
invertebrates. This difference in penetrability likely explains K. tunicata's variable effects
on these kelp communities. Tegner et al. (1995) describe a similar interaction between sea
urchins and the holdfasts of Macrocystis pyrifera, in which cavitation, or burrowing, by
sea urchins into the large holdfasts resulted in a higher probability of dislodgment by wave
action.
Paine (1984) found in Washington that if Katharina tunicata were removed
Alaria marginata became the dominant canopy species. Although this result was clear, no
explanation could be offered as to the underlying mechanisms involved. Differential
interaction of K. tunicata with the holdfasts of A. marginata and Hedophyllum sessile may,
at least in part, be responsible for this result. Together, the H. sessile canopy and
K. tunicata are able to suppress A. marginata, but if A. marginata does manage to establish,
excluding H. sessile, K tunicata is ineffective in affecting its dominance. I suggest that 26
K. tunicata's ability, or lack thereof, to control the abundance of these kelps stems from the
nature of its interaction with their holdfasts.
This study capitalized on the advantages of monitoring benthic algal population
structure and demography in combination with experimental manipulation of herbivore
densities to determine the mechanisms by which a principal herbivore affects a temperate
low intertidal kelp community. This mechanism emphasizes the combined importance of
hydrodynamic forces subsequent to herbivore-induced tissue damage and, potentially
explains geographic variation in interaction strength of this herbivore. Finally, the
importance of algal size was emphasized with the quantification of the ecologically
important mechanism of size-related escapes. 27
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JL95 S95 095 D95 F96 ' A96 JU96 JL96
SAMPLING DATE (MONTH/YEAR)
Figure 2.1 Katharina tunicata density (#/m2) within low, control and high chiton density manipulation areas. Date are the mean+SE of 6 replicates for density manipulations at each sampling date. Sampling date abbreviations are: JL95 (July 1995), S95 (September 1995), 095 (October 1995), D95 (December 1995), F96 (February 1996), A96 (April 1996), JU96 (June 1996), JL96 (July 1996). Results of Tukey tests for significant ANOVA are labeled. Letters sharing an underline are not significantly different (P>0.05). 31
Figure 2.2 Hedophyllum sessile percent canopy cover within low, control and high chiton density manipulation areas. Data are the mean+SE of 6 replicates for density manipulations at each sampling date(see Figure 2.1 for abbreviation). Results of Tukey tests for significant ANOVA are labeled. Letters sharing an underline are not significantly different (P>0.05). 32
140
E 120 cr & 100 co z UI 80 O Uj
55 60
^ 40
20
JL95 S95 095 D95 F96 A96 JU96 JL96 SAMPLING DATE (MONTH/ YEAR)
Figure 2.3 Hedophyllum sessile density (#/m2) within low, control and high chiton density manipulation areas. Data are the mean+SE of 6 replicates for density manipulations at each sampling date (see Figure 2.1 for abbreviations). No significant differences (p<0.05) within any sampling dates were found (* indicates no individuals in the high chiton density areas survived to these dates. Remaining low and control groups were compared using 2-tailed unpaired t-tests). 33
Figure 2.4 Hedophyllum sessile blade number / individual within low, control and high chiton density manipulation areas. Mean blade number / individual was averaged over each 25 x 25 cm quadrat. Data are the mean+SE of 6 replicates for density manipulations at each sampling date (see Figure 2.1 for abbreviations). No significant differences (p<0.05) within any sampling dates were found (* indicates no individuals in the high chiton density areas survived to these dates. Remaining low and control groups were compared used using 2-tailed unpaired t-tests). 34
JL95 S95 095 D95 F96 A96 JU96 JL96 SAMPLING DATE (MONTH/ YEAR)
Figure 2.5 Hedophyllum sessile blade length / individual within low, control and high chiton density manipulation areas. Mean blade number / individual was averaged over each 25 x 25 cm quadrat. Data are the mean+SE of 6 replicates for density manipulations at each sampling date (see Figure 2.1 for abbreviations). Results of Tukey tests for significant ANOVA are labeled. Letters sharing an underline were not significantly different (P>0.05) (* indicates no individuals in the high chiton density areas survived to these dates. Remaining low and control groups were compared used using 2-tailed unpaired t-tests). 35
Figure 2.6 Blade length (a) and blade number (b) of individuals of the 1-4 cm maximum holdfast diameter size-class within low, control and high chiton density manipulation areas. Mean blade number and length / individual were averaged over each 25 x 25 cm quadrat. Data are means+SE of 1-4 cm size-class individuals pooled over each chiton density manipulation on each sampling date (see Figure 2.1 for abbreviations). Results of Tukey tests for significant ANOVA are labeled. Letters sharing an underline were not significantly different (P>0.05; * indicates no individuals in the high chiton density areas survived to these dates. Remaining low and control groups were compared using 2-tailed unpaired t-tests). 36
Figure 2.7 Hedophyllum sessile recruit densities (#/m2) within low, control and high chiton density manipulation areas. Data are the mean+SE of 6 replicates for density manipulations on each sampling date (see Figure 2.1 for abbreviations). No significant differences (p<0.05) were found within any sampling date. 37 a.
JL95 S95 095 D95 F96 A96 JU96 JL96
Figure 2.8 Comparison of survivorship curves of the July 1995 cohort of Hedophyllum sessile (a) <1 cm, (b) 1-4 cm, (c) >4 cm holdfast size classes as a function of low, control and high chiton density. 38
45
<1 cm 1 -4 cm >4 cm HOLDFAST SIZE CLASS
Figure 2.9 Results of the Kaplan-Meier suvival analysis of the Hedophyllum sessile <1 cm, 1-4 cm and >4 cm maximum holdfast diameter size-classes as a function of low, control and high chiton densities. Data are mean survival time (weeks) +95% confidence interval (refer to Figure 2.8 for respective sample sizes). 39
a.
o i 1 JL95 S95 095 D95 F96 A96 JU96 JL96
b.
c.
JL95 S95 095 D95 F96 A96 JU96 JL96
Figure 2.10 Comparison of survivorship curves of the July 1995 cohort of Hedophyllum sessile within (a) low, (b) control and (c) high chiton density treatments areas as a function of holdfast size class. 40
Table 2.1 Results of ANOVA comparing Katharina tunicata densities between chiton density treatment areas at each sampling date. Abbreviations indicate degrees of freedom (df), mean square (MS), F ratio (F), and the probability of a Type 1 error (p).
Source of variation df MS F p
A. July 1995 Treatment 2 128.000 0.283 0.759 Block 5 93.867 0.208 0.952 Error 10 452.267
B. Sept. 1995 (Square-root transformed) Treatment 2 55.207 15.056 0.001 Block 5 2.157 0.588 0.710 Error 10 3.667
C. Oct. 1995 Treatment 2 1322.667 3.163 0.086 Block 5 196.267 0.469 0.791 Error 10 418.133
D. Dec. 1995 Treatment 2 355.556 1.404 0.290 Block 5 628.622 2.483 0.104 Error 10 253.156
E. Feb 1995 Treatment 2 1038.222 7.766 0.009 Block 5 389.689 2.915 0.071 Error 10 133.689
F. April 1996 Treatment 2 14.222 0.122 0.886 Block 5 261.689 2.244 0.130 Error 10 116.622
G. June 1996 Treatment 2 298.667 1.129 0.361 Block 5 93.867 0.355 0.868 Error 10 264.533
H. July 1996 Treatment 2 8.167 8.448 0.007 Block 5 0.400 0.414 0.829 Error 10 0.967 41
Table 2.2 Results of ANOVA on the treatment effect of Katharina tunicata density on Hedophyllum sessile percent cover. Abbreviations indicate degrees of freedom (df), mean square (MS), F ratio (F), and the probability of a Type 1 error (p).
Source of variation df MS
A. July 1995 Treatment 2 165.722 0.358 0.708 Block 5 808.322 1.744 0.212 Error 10 463.522
B. Sept. 1995 Treatment 2 359.722 0.295 0.751 Block 5 900.322 0.738 0.612 Error 10 1219.789
C. Oct. 1995 Treatment 2 1992.667 4.218 0.047 Block 5 312.533 0.662 0.661 Error 10 472.400
D. Dec. 1995 (Log transformed) Treatment 2 10.419 7.172 0.012 Block 5 2.911 2.004 0.164 Error 10 1.453
E. Feb 1996 (Log transformed) Treatment 2 12.111 15.736 0.001 Block 5 1.941 2.522 0.100 Error 10 0.770
F. April 1996 (Log transformed) Treatment 2 15.871 8.499 0.007 Block 5 2.857 1.530 0.265 Error 10 1.867
G. June 1996 (Log transformed) Treatment 2 16.975 6.413 0.016 Block 5 2.805 1.060 0.436 Error 10 2.647
H. July 1996 Treatment 2 4578.722 5.324 0.028 Block 5 152.222 0.174 0.960 Error 10 874.856 42
Table 2.3 Results of ANOVA on the treatment effect of Katharina tunicata density on Hedophyllum sessile density. Abbreviations indicate degrees of freedom (df), mean square (MS), F ratio (F), and the probability of a Type 1 error (p).
Source of variation df MS F P
A. July 1995 Treatment 2 170.667 0.066 0.936 Block 5 6169.600 2.394 0.230 Error 10 2577.067
B. Sept. 1995 Treatment 2 1578.667 1.412 0.288 Block 5 2969.600 2.656 0.089 Error 10 1117.867
C. Oct. 1995 Treatment 2 896.000 0.802 0.475 Block 5 1117.867 1.000 0.465 Error 10 1117.867
D. Dec. 1995 Treatment 2 1763.556 1.197 0.342 Block 5 1046.756 0.710 0.629 Error 10 1473.422
E. Feb 1996 Treatment 2 1038.222 7.766 0.080 Block 5 389.689 2.915 0.071 Error 10 133.689 43
Table 2.4 Results of ANOVA on the treatment effect of Katharina tunicata density on Hedophyllum sessile blade number. Abbreviations indicate degrees of freedom (df), mean square (MS), F ratio (F), and the probability of a Type 1 error (p).
Source of variation df MS F P
A. July 1995 2 6.000 0.520 0.610 Treatment 5 17.330 1.503 0.272 Block 10 11.530 Error
B. Sept. 1995 Treatment 2 5.525 0.985 0.407 Block 5 10.893 1.942 0.174 Error 10 5.610
C. Oct. 1995 Treatment 2 12.888 0.742 0.501 Block 5 16.802 0.967 0.482 Error 10 17.379
D. Dec. 1995 Treatment 2 13.927 1.582 0.253 Block 5 10.387 1.180 0.384 Error 10 8.801
E. Feb 1996 Treatment 2 18.667 2.222 0.159 Block 5 9.833 1.171 0.388 Error 10 8.40 44
Table 2.5 Results of ANOVA on the treatment effect of Katharina tunicata density on Hedophyllum sessile blade length. Abbreviations indicate degrees of freedom (df), mean square (MS), F ratio (F), and the probability of a Type 1 error (p).
Source of variation df MS F P
A. July 1995 Treatment 2 99.056 0.713 0.514 Block 5 231.289 1.664 0.230 Error 10 138.989
B. Sept. 1995 Treatment 2 59.430 0.758 0.494 Block 5 126.318 1.612 0.243 Error 10 78.370
C. Oct. 1995 Treatment 2 83.247 1.044 0.387 Block 5 67.458 0.846 0.547 Error 10 79.726
D. Dec. 1995 Treatment 2 117.084 3.525 0.069 Block 5 33.431 1.007 0.462 Error 10 33.214
E. Feb 1996 Treatment 2 112.500 7.418 0.011 Block 5 22.667 1.495 0.275 Error 10 15.167 45
Table 2.6a Results of ANOVA on the treatment effect of Katharina tunicata density on the blade number of Hedophyllum sessile 1-4 cm holdfast size class individuals. Abbreviations indicate degrees of freedom (df), mean square (MS), F ratio (F), and the probability of a Type 1 error (p).
Source of variation df MS
A. July 1995 (N=14) Treatment 3 4.842 1.869 0.151 Error 38 2.591
B. Sept. 1995 (N=ll) Treatment 2 0 0 1.0 Error 30 4.206
C. Oct. 1995 (N=5) Treatment 2 9.8 0.623 0.553 Error 12 15.733
Table 2.6b Results of ANOVA on the treatment effect of Katharina tunicata density on the blade length of Hedophyllum sessile 1-4 cm holdfast size class individuals. Abbreviations indicate degrees of freedom (df), mean square (MS), F ratio (F), and the probability of a Type 1 error (p).
Source of variation df MS
A. July 1995 (N=14) Treatment 3 710.548 5.008 0.012 Error 38 141.872
B. Sept. 1995 (N=ll) Treatment 2 70.030 0.555 0.580 Error 30 126.206
C. Oct. 1995 (N=5) Treatment 2 12.937 0.179 0.838 Error 12 72.208 46
Table 2.7 Results of ANOVA on the treatment effect of Katharina tunicata density on the recruitment of Hedophyllum sessile sporophytes (#/m2). Abbreviations indicate degrees of freedom (df), mean square (MS), F ratio (F), and the probability of a Type 1 error (p).
Source of variation df MS
A. April 1996 Treatment 2 20266.667 0.566 0.585 Block 5 19754.667 0.552 0.734 Error 10 35797.333
B. May 1996 Treatment 2 78904.889 1.000 0.402 Block 5 21697.422 0.275 0.917 Error 10 78939.022
C. June 1996 Treatment 2 14606.222 0.463 0.642 Block 5 10569.956 0.335 0.880 Error 10 31536.356
C. July 1996 (Log transformation) Treatment 2 15.348 1.953 0.192 Block 5 1.730 0.220 0.946 Error 10 7.858 47
Table 2.8 Simple correlation analysis of the relationship between adult Hedophyllum sessile percent cover and juvenile H. sessile recruitment compared between low, control and high chiton density treatment areas. Abbreviations indicate sample size (N), correlation coefficient (r) and the probability of a Type 1 error (p).
Variables compared (N=6) r P Adult H. sessile percent cover -0.707 0.127 vs. recruitment (low chiton density areas)
Adult H. sessile percent cover -0.431 0.424 vs. recruitment (control chiton density areas)
Adult H. sessile percent cover -0.573 0.259 vs. recruitment (high chiton density areas) 48
Table 2.9 Results of Cox regression analysis comparing Hedophyllum sessile survivorship between chiton density treatments grouped by H. sessile maximum holdfast diameter size classes . Refer to pages 17-18 for explanation of column headings.
VARIABLE EXP(B) 95% CI FOR EXP(B) B COMPARISON (RELATIVE LOWER UPPER (REGRESSION RISK) COEFICIENT) Size class: <1 cm Control vs. low 0.5758 0.2888 1.1479 -0.5520 0.1169 High vs. low 2.2511 1.2073 4.1973 0.8114 0.0107
Size class: 1-4 cm Control vs. low 0.8110 0.5160 1.2775 -0.2084 0.3675 High vs. low 1.7155 1.0699 2.7506 0.5397 0.0250
Size class: >4 cm Control vs. low 1.2541 0.6712 2.3434 0.2264 0.4778 High vs. low 0.9719 0.5082 1.8589 -0.0285 0.9315
Table 2.10 Results of Cox regression analysis comparing Hedophyllum sessile maximum holdfast diameter size-class survivorship grouped by chiton density treatment types. Refer to pages 17-18 for explanation of column headings.
VARIABLE EXP(B) 95% CI FOR EXP(B) B COMPARISON (RELATIVE LOWER UPPER (REGRESSION RISK) COEFICTENT) Chiton density: Low >4 cm vs. 1-4 cm 0.9457 0.5315 1.6827 -0.0559 0.8145 Chiton den: Control >4cmvs. 1-4 cm 1.1953 0.6512 2.21941 0.1784 0.5647 Chiton density: High >4 cm vs. 1-4 cm 0.3766 0.1651 0.8592 -0.9764 0.203 CHAPTER 3 Polyphenolic content of the intertidal kelp Hedophyllum sessile: present-day implications and evolutionary frameworks INTRODUCTION Marine algae use a wide variety of mechanisms to limit the effects of herbivores, including the production of feeding deterrents in the form of secondary metabolites (Hay and Fenical 1988; Paul 1992). Many species of brown algae (division Phaeophyta) produce complex polymers of phloroglucinol (1,3,5-trihydroxybenzene) called polyphenolics, or phlorotannins (herein referred to as 'phenolics'). Phenolics can deter grazing by herbivorous gastropods (Geiselman and McConnell 1981; Steinberg 1985; Van Alstyne 1988) and echinoderms (Norris and Fenical 1982), and are thought to act by reducing digestibility of plant tissue through binding of proteins (Steinberg 1985), affecting nervous system or cardiac functions (Van Alstyne 1988), or causing cell damage (Hay and Fenical 1988), The potential for chemical feeding deterrents to increase the fitness of algae has lead to the implication of herbivores as selective agents driving the production and distribution of compounds which deter grazing. Given the ecological importance of brown algae, and kelps in particular, considerable work has been devoted to testing the relationships between marine herbivores and brown algal phenolic production and interpreting results in the context of plant-defense theory (Steinberg 1985; Tugwell and Branch 1989; Van Alstyne 1988; Pfister 1992; Steinberg 1994; Arnold et al. 1995). However, this work has revealed extensive variation in phenolic content within individual plants, intraspecifically, geographically and taxonomically (Steinberg 1984, 1989; Tugwell and Branch 1989). Further, in addition to herbivory, variation in phenolic content has also been correlated with several seasonally variable abiotic factors including 50 salinity, tidal exposure (Ragan and Glombitza 1986), nutrient availability (Arnold et al. 1995), and irradiance (Yates and Peckol 1993). Attempts to synthesize these present-day ecological patterns into models of natural community operation and evolutionary frameworks have met with limited success. Estes and Steinberg (1988) suggest that variation in phenolic production by brown algae may be the result of differential exposure to herbivory intensity over evolutionary time. Steinberg (1989) expands upon this model, suggesting that the major source of variation in phenolic content within brown algae is due to differences in geography and/or taxonomy, strongly implicating evolutionary history of herbivory intensity and phylogenetic relationships as explanations for patterns of phenotypic variation in phenolic content observed today. In positing this hypothesis, Steinberg (1989, 1992) describes two present-day trends in phenolic production of North American brown algal populations (specifically in the orders Fucales and Laminariales): (1) phenolic content of brown algae tends to decrease from the littoral to sublittoral zones, and (2) a taxonomic trend follows the same bathymetric gradient wherein littoral fucoids are typically phenolic rich and sublittoral kelps (Laminariales) are typically phenolic poor. However, relatively few species of brown algae have been thoroughly examined and more descriptions of species within these orders are needed to either support or reject suspected herbivore-related, taxonomic or geographic sources of phenotypic variation in phenolic production. This study attempts to contribute to this debate by describing the interaction between the intertidal kelp Hedophyllum sessile (C. Agardh) Setchell (order Laminariales), and the chiton Katharina tunicata (Wood) (Class Polyplacophora), and assess the potential for phenolics to influence this interaction. I then review the results in the context of current ecological trends in an effort to provide evidence for or against the herbivory-related evolutionary models of Estes and Steinberg (1988) and Steinberg (1989). The specific questions I test are: (1) Is Hedophyllum sessile capable of exhibiting induced chemical defense responses to wounding? (2) Does variation in phenolic content 51 reflect variation in local herbivore intensity? (3) Does intraspecific variation in phenolic content exist between juvenile, non-reproductive adult, and reproductive tissues of H. sessile! METHODS The study took place at Prasiola Point, (125° 10'W, 48° 51'N) in Barkley Sound, on the west coast of Vancouver Island, B.C., Canada, near the Bamfield Marine Station. In Barkley Sound, in semi-exposed areas, from the mid to low intertidal, Hedophyllum sessile forms a dominant, monospecific canopy which controls the abundance and diversity of understory algal and invertebrate species (Dayton 1975; Paine 1984; Duggins and Dethier 1985). Adult individuals of H. sessile consist of a large holdfast, no stipe, and up to 30 blades which may reach 50-60 cm in length. In this community, the chiton Katharina tunicata is the primary herbivore, capable of completely removing the H. sessile canopy (Chapter 2, this thesis, and Duggins and Dethier 1985, but see Dethier and Duggins 1988). Three experiments were conducted on different dates; these were the wounding experiment (October 1995), the intra-population variation experiment (February 1996) and herbivore density experiment (March 1996). The Folin-Denis Assay for phenolic content was used to determine the percent phenolics of total dry tissue mass (Swain & Hillis 1959). The Folin-Denis Assay is a colorimetric test which measures phenolic content of algal extracts based on absorbances of standard phloroglucinol concentrations. The assay quantifies hydroxylated aromatic groups, and will also measure other non-phenolic metabolites such as ascorbic acid. Thus, non-phenolic compounds included in the assay were assumed to be relatively low in concentration and similar across all treatments (Van Alstyne and Paul 1990). The Folin- Denis assay was used to determine the percent phenolic content expressed as mg/mg dry weight of Hedophyllum sessile. 52 Wounding Experiment: This experiment was designed to examine whether wounding caused by Katharina tunicata would increase phenolic production by Hedophyllum sessile can be induced by Katharina tunicata wounding. Forty-five adult plants (defined as individuals with holdfasts >1 cm diameter and more than two blades) were randomly located and marked using flagging tape. Each plant was alternately placed into one-of three groups (n=15): day 1, day 2, or day 3. Inability to relocate tagged plants and a minor laboratory accident resulted in 14 replicates for days 1 and 3, and six replicates for day 2, of control and wounded blades. Wounding resulting from Katharina tunicata radulae was simulated using a hole punch to place a 8 mm diameter circular 'wound' in the center of one blade, 10 cm from the holdfast. This area was chosen to target the metabolically active meristem which, in kelps, is typically found at the holdfast-blade or stipe-blade junction. A second, unpunched blade from the same plant was used as a control. Approximately 24 hours after initial wounding, day 1 wounded and control blades were collected. Collection involved removing H. sessile blades with a razor at the holdfast-blade junction and storing these blades overnight (approximately 12 hours) in running seawater. This procedure was repeated for day 2 and day 3 plants, 48 and 72 hours after wounding. From each wounded blade a 15 mm square tissue sample surrounding, and including the 6 mm diameter hole created by the holepunch was excised using a razor blade. Control blades had similar samples removed from the same region. These sample were then assayed for phenolic content. The data were tested for normal distribution by constructing probability plots and tested for homoscedasticity of variances using Fmax-tests. No transformations were required for analysis. Treatment and control blades from the same individual were treated as being dependent and therefore paired 2-tailed t-tests were used to compare the phenolic content of wounded and control samples for each day following wounding (Zar 1984). 53 Intra-population variation: In February 1996 reproductive adult, vegetative adult, and juvenile Hedophyllum sessile blades (n=14 for each tissue type) were collected and assayed for phenolic content. Plant age determination was based on blade toughness (not quantified), blade length and holdfast diameter. Juveniles were defined as delicate, single bladed plants less than 15 cm in length and holdfasts <1 cm in diameter. Adults were defined as tough with many blades greater than 30 cm in length and holdfasts >4 cm in diameter. Reproductive plants were identified by the presence of patches of sori on at least one blade. Blades were removed with a knife at the blade-holdfast junction, stored in seawater overnight, then assayed for phenolic content. All tissue samples were taken from the center of the blade, within five centimeters of the holdfast. This distance was necessary to standardize collection of adult vegetative tissue from blades with reproductive sori present. Normality of data and homoscedasticity of variances were tested as described in the wounding experiment and no transformations were required. One-way ANOVA was used to test for differences in phenolic content between the three tissue types. Tukey's HSD test was used for post hoc comparison (Underwood 1981). Herbivore Density: In March 1996, Hedophyllum sessile from sites with naturally high and low Katharina tunicata densities (n=15 for each site) were examined for differences in phenolic content. One blade from each of 15 randomly located adult plants (>1 cm holdfast diameter and more than two blades) was collected from each of the two sites at Prasiola Point. A single tissue sample was excised from the center of each blade, within 5 cm of the holdfast to ensure that only vegetative tissue was collected. Katharina tunicata densities were measured by randomly placing a 25 cm x 25 cm quadrat at 15 random points along a 10 m lead-line transect. 54 Normality and homoscedasticity of data were tested as described previously. Transformation was not required to meet the assumptions of parametric analyses. Concentrations of phenolics and herbivore densities from independent samples from each site were compared using unpaired 2-tailed t-tests (Zar 1984). RESULTS Wounding experiment: No significant differences in phenolic levels were found between wounded and control tissue one day (p=0.410) and two days (p=0.380) after wounding. However, by day 3 wounded tissue contained significantly less (p=0.030) phenolics than control tissue (Figure 3.1). Intra-population variation: In addition to blade toughness, the number of blades/holdfast, and holdfast diameter, blade length was used to differentiate adult and juvenile Hedophyllum sessile. The mean +S.E. juvenile blade length was 7.9 + 0.5 cm with the longest blade 11.4 cm. The shortest adult blade was 27.4 cm long, and the mean adult blade length was 45.7 + 2.3 cm. While reproductive blade identification was based solely on the presence of reproductive sori, blade lengths were recorded and the mean determined to be 24.7 + 4.9 cm. Phenolic levels differed significantly between the three tissue types (Table 3.1). Juvenile tissue contained significantly higher phenolic levels (N=14, mean+SE; 8.10+0.47) than vegetative adult tissue (N=14, 5.72+0.46, p=0.008) and approached having significantly higher phenolic levels than reproductive adult blades (N=14, 6.4+0.64, p=0.095). Adult reproductive and vegetative tissue did not differ (p=0.560). Herbivore density: Densities of Katharina tunicata in the high density and low density sites were (N=15, mean+SE) 53 + 5 Im2 and 33 + 5/m2 , respectively (p<0.05). Phenolic 55 levels did not differ (p=0.790) between the low (N=15, mean±SE; 6.08+0.32 mg/mg dry weight) and high (N=15, 6.23±0.32 mg/mg dry weight) herbivore density sites. DISCUSSION Knowledge of the importance and function of anti-herbivore compounds is critical to understanding the complex interactions and outcomes of algal-herbivore interactions. Regardless of the evolutionary pathways from which they arose, their multiple functions, and the variety of factors which may influence their production, phenolics play a key role in the present-day ecology of at least some benthic marine algal communities by affecting the grazing preferences of herbivores (Steinberg 1985; Hay and Fenical 1988; Van Alstyne 1988). Variation, however, on all scales of phenolic production and the complexity of interacting biotic and abiotic factors has driven the need to examine more algal/herbivore associations in order to establish patterns. Wounding Experiment: The observed decrease in phenolic content of Hedophyllum sessile three days after simulated wounding appears to be an example of a somewhat common but little discussed result that also occurs in other algae. Pfister (1992) found that removing the entire frond of Alaria nana led to a marginal decrease in the percent phenolics in the remaining sporophylls and Steinberg (1994) reports a decrease in phenolic content following experimental clipping of Ecklonia radiata. Van Alstyne (1988) reports an initial herbivore preference for clipped Fucus distichus prior to an induced increase in phenolic content two weeks after clipping. A potential explanation for these results may be the water solubilty of polyphenolics. Although the compounds themselves are slow to break down (Ragan and Glombitza 1986), phenolics are unstable in water and may be diluted in the vicinity of newly wounded tissue. This short-term result may be an ecologically important event if the alga then becomes more vulnerable to grazing. The lack of an induced increase 56 in phenolic production by Hedophyllum sessile is consistent with other similar experiments examining kelps (Pfister 1992; Steinberg 1994). In fact, the only example of an induced increase in brown algae to wounding is that of Fucus distichus by Van Alstyne (1988) and repeated by Yates and Peckol (1993) with Fucus vesiculosus. Herbivore Density: Plant defense theory (Rhoades 1979) dictates that the amount an alga should invest in the production of chemical defenses is in direct proportion to the probability that they will be discovered by herbivores. Similarly, the probability of discovery, or encounter, increases with increasing herbivore density (Lubchenco and Gaines 1981). Assuming there is a cost associated with their production, inducible defenses should therefore be favored, allowing for adjustment of defense production relative to changes in local herbivore density. However, Steinberg (1994) points out that several studies have suggested that spatial variation in herbivore intensity (referring to the effects of herbivore guilds as well as the density of a single species) is often highly predictable and strongly correlated with variation in the frequency of algae which are rich in secondary metabolites (see Hay 1984; Estes and Steinberg 1988; Hay and Steinberg 1992). Steinberg (1994) points out that such patterns suggest there has been long-term selection for constitutive defenses in macroalgae which perhaps lessens the importance of inducible defenses. Although Katharina tunicata densities at the two sites fluctuate annually, relative differences remained constant over the 18 months prior to the experiment. Thus, I find it reasonable to assume that differences in herbivory intensity at these sites have existed for a long period of time. I predicted that individuals of Hedophyllum sessile from high herbivore density sites would have higher rates of wounding than those from low density sites and therefore have higher phenolic contents. However, similar to Steinberg (1995), no differences in phenolic content of H. sessile from sites differing in herbivory density were found, suggesting constitutive phenolic production. This differs from the findings of 57 Van Alstyne (1988), where Fucus distichus collected from sites with high and low densities of Littorina sp. had correspondingly high and low levels of phenolics. Unfortunately, none of the studies which have examined the relationship between grazing intensity (Hay 1984) and secondary metabolite production have simultaneously examined abiotic factors which may influence metabolite production and are often associated with sites differing in herbivore intensity (but see Yates and Peckol 1993). For example, in this study the high herbivore-density site is more exposed to waves. A similar relationship between K. tunicata density and wave exposure is described by Stebbins (1988). Also, Hay (1984) reports herbivore intensity to be greatest near wave-exposed reef crests and least in reef lagoons. Potential differences in nutrient supply, desiccation, salinity, irradiance and herbivore and algal diversity associated with changes in wave exposure need to be considered when describing spatial variation in algal secondary metabolite production. Intra-Population Variation: Steinberg (1984) argues that because plant defense theory predicts that organisms should allocate their internal resources in such a way as to maximize fitness, and because fitness is dependent upon survival of reproductive products, organisms should invest in defenses for protection of reproductive structures. Some investigations of within-individual differences in phenolic content of kelp species have found patterns which support this prediction. Steinberg (1984) and Pfister (1992) report, for Alaria marginata and A. nana, respectively, that the sporophylls of Alaria spp. contain significantly higher levels of phenolics as compared to vegetative tissue. Similarly, Tugwell and Branch (1989) found the sporogenous tissue of the Southern-hemisphere kelps Macrocystis angustifolia and Ecklonia maxima also to have higher levels of phenolics. However, in my study I found that reproductive tissues of Hedophyllum sessile do not differ in phenolic content from vegetative tissue. Similar findings have been reported by Steinberg (1989) for Ecklonia radiata and Tugwell and Branch (1989) report 58 the sporogenous tissue of Laminaria pallida to be lower in phenolic content than vegetative tissue. Juvenile forms of marine and terrestrial plant species often contain higher levels of anti-herbivore compounds (Coley 1988; Hay and Fenical 1988; Steinberg 1989). In this study juvenile Hedophyllum sessile contained significantly higher levels of phenolics than adult vegetative tissue and approached significance over adult reproductive tissue. As discussed above, some have argued that resources should be differentially allocated to protect those parts of adults plants which are critical to survival and reproduction (i.e. holdfasts and sporogenous tissue), however, the chemical defense of juvenile forms has received little attention. DeWreede (1984) suggests that kelp populations are more sensitive to survivorship of juvenile forms than they are to fecundity. Typical of many marine organisms, kelps are characterized as having high fecundity and low juvenile survivorship. Survival during juvenile life stages is disproportionately important to populations in terms of both eventual abundance and selection pressure. In general, juvenile forms of marine macrophytes are thought to pass through a 'herbivory bottleneck', where they are initially highly susceptible to herbivory, but eventually reach a size, or develop morphological toughness, sufficient to make them significantly less susceptible to the effects of herbivores (Menge and Lubchenco 1981; Farrell 1988; Dean et al. 1989; Denton et al. 1990). Size-specific differences in susceptibility to herbivory stem from the inability of herbivores to physically handle or completely consume large macrophytes. So called 'escapes in size' may create coexistence refuges for fast growing species that quickly reach sizes at which herbivores have little impact upon their.survivorship (Lubchenco and Cubit 1980; Lubchenco 1983; Hawkins and Hartnoll 1983; Duffy and Hay 1990). These studies therefore suggest that selection may have favored kelps which invest in the production of defenses which increase the survivorship of juvenile forms. 59 General conclusions: The goal of this study was to assess the present-day relationship between the chiton Katharina tunicata and polyphenolic production by the kelp Hedophyllum sessile, and to interpret the results of these experiments in terms of developing frameworks of brown algal phenolic production. No induced responses or differences in phenolic content between sites differing in Katharina tunicata density were observed in this study. The results in Chapter 2 indicate that K. tunicata may only be indirectly responsible for changes in H. sessile abundance, and then only under conditions of experimentally maintained high chiton densities. Chapter 2 also showed that K. tunicata density significantly decreases the survivorship of juvenile H. sessile (<1 cm holdfast diameter), despite their high phenolic content. Together, these results suggest that phenolics presently may play a small role in H. sessile population dynamics. How then, would this algal/herbivore interaction change if no phenolics were produced by Hedophyllum sessile! Lubchenco (1978) points out the importance of herbivore feeding preferences to algal community structure but, in general, chitons have been the subjects of few feeding preference experiments and therefore little is known of their interactions with phenolics. Although most molluscan feeding appears to be inhibited by phenolics, Robb (1975) reports that, in the field, the chiton Cyanoplax hartwegii (Carpenter, 1855) feeds exclusively on a phenolic-rich brown alga, Pelvetiafastigiata (J.G. Agardh). Also, Himmelman and Carefoot (1975) report that Katharina tunicata prefers to graze the blades of H sessile. Experiments designed to assess the interplay between H. sessile phenolic content, morphology, and size which determine K. tunicata feeding preferences may have helped to determine the nature of these interactions. However, the results of a feeding preference experiment I performed in May 1995 (Markel, unpublished data) were inconclusive due to the failure of K. tunicata to feed under laboratory conditions. Although preference experiments involving chitons are rare, similar difficulties 60 with K. tunicata are described by Himmelman and Carefoot (1975), Dethier (1982) and Gaines (1985). Steinberg (1989) suggests that much of the variation in phenolic production among temperate brown algal species follows taxonomic lines. While the fucoids (Fucales) are generally rich (-4-12% dry weight), and the kelps (Laminariales) generally low (-0.50% dry weight), in phenolic content, noted exceptions within the Laminariales are the phenolic- rich kelps Dictyoneurum californicum (5.41% dry weight; Steinberg 1985) and Agarum cribrosum (5.53% dry weight; Steinberg 1985). The results of this experiment indicate that Hedophyllum sessile (5.60 % dry weight).represents another phenolic-rich exception within the Laminariales. However, although'these phenolic rich species may represent the extremes of the range of phenolic production within the Laminariales, intermediary 'exceptions' exist as well. For example, Steinberg (1985) reports phenolic content values of -1 % dry weight for Macrocystis pyrifera, Egregia menziesii, and Pterygophora californica, 1.65% dry weight for Postelsia palmaeformis, and 3.11% dry weight for Eisenia arborea. Thus, rather than exceptions, the phenolic-rich species within the Laminariales appear to represent extremes of a continuum. Another general trend of phenolic content of Northern-hemisphere temperate brown algae, but not their temperate Australasian counterparts, is the depth-related pattern of phenolic-rich species being found intertidally (primarily fucoids) while phenolic-poor kelps (Laminariales) are most abundant in the very low intertidal or shallow subtidal zones (Steinberg 1989). Among the kelp assemblage characteristic of semi-exposed coasts of Washington and British Columbia, Hedophyllum sessile occupies the upper-most position, extending to the mid intertidal. The finding that H sessile is a phenolic-rich species within the Laminariales supports the model proposed by Estes and Steinberg (1988): both Agarum spp. (a deep subtidal kelp) and H. sessile live in habitats where herbivory is unlikely to be controlled by predation. However, these authors also point out that an alternate hypothesis to phenolic-rich species evolving through exposure to intense herbivory is that plants in 61 physiologically stressful environments may need to defend themselves against herbivores more strongly than plants in less harsh environments. Interestingly, the two kelp species in British Columbia which occupy the extreme ends of kelp habitat distributions are the same species regarded as phenolic-rich exceptions within the Laminariales. While Agarum spp. are found deep in the subtidal under conditions of low light and perhaps suboptimal nutrient regimes (Estes and Steinberg 1988), H. sessile must contend with the rigors of extreme differences in temperature, UV light intensity and desiccation associated with intertidal habitats. Determining the biotic and abiotic processes, both present-day and of the evolutionary past, responsible for the morphological, chemical, or life history, attributes of benthic marine algae is a formidable task of ecologists. Given the evidence and models provided by previous investigators of the ecology of polyphenolics, my study adds to the growing consensus that polyphenolics likely serve a variety of functions and are governed by both biotic and abiotic factors (Yates and Peckol 1993). An inverse relationship between nutrient availability and phenolic levels (Yates and Peckol 1993; Arnold et al. 1995) and moderate to strong irradiance associated with increased phenolic levels may in part explain the phenolic contents of Agarum spp. and Hedophyllum sessile. The proposed continuum of phenolic content of species within the Laminariales and abiotic factors associated with habitats which supports phenolic-rich brown algae may be extendible to include the primarily mid and high intertidal species of the Fucales. Future studies should investigate further the abiotic parameters associated with sites which consistently support conditions of high herbivore intensity. 62 LITERATURE CITED Arnold TM, Tanner CE, Hatch WI (1995) Phenotypic variation in polyphenolic content of the tropical brown alga Lobophora variegata as a function of nitrogen availability. Mar Ecol Prog Ser 123: 177-183. Coley PH (1988) Effects of plant growth rate and leaf lifetime on the amount and type of anti-herbivore defense. Oecologia 74: 531-536. Dayton PK (1975) Experimental evaluation of ecological dominance in a rocky intertidal algal communtiy. EcolMonog45: 137-159. 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Ann Rev Ecol Syst 19: 111-145. Hay ME, Steinberg PD (1992) The chemical ecology of plant-herbivore interactions in marine vs. terrestrial communities, In: Herbivores: their interaction with plant secondary metabolites (Rosenthal GA, Benenbaum MR, eds) 2nd Ed. Academic Press, NY. Himmelman JH, Carefoot TH (1975) Seasonal changes in calorific values of three Pacific coast seaweeds, and their significance to some marine invertebrate herbivores. J Expt Mar Biol Ecol 18: 139-151. Lubchenco J (1978) Plant species diversity: importance of herbivore food preference and algal competitive abilities. Am Nat 112: 23-39. Lubcheno J (1983) Littorina and Fucus: effects of herbivores, substratusm heterogeneity, and plant escapes during sucession. Ecology 64(5): 1116-1123. Lubchenco J, Cubit J (1980) Heteromorphic life histories of certain maine algae as adaptations to variations in herbivory. Ecology 61(3): 676-687. Lubchenco J, Gaines SD (1981) A unified approach to marine plant-herbivore interactions. I. Populations and communities. Ann Rev Ecol Syst 12: 405-437. Menge BA, Lubchenco J (1981) Community organization in temperate and tropical rocky intertidal habitats: prey refuges in relation to consumer pressure gradients. Ecol Monogr 51(4): 429-450. Norris JN, Fenical W (1982) Chemical defenses in tropical marine algae. In: Rutzler K, Maclntyre IG (eds) Atlantic barrier reef ecosystems at Carrie Bow Cay. Belize. I. Structure and communities. Smithsonian Contr Mar Sci 12: 417-431. Paine RT (1984) Ecological determinism in the competition for space. Ecology 65(5): 1339-1348. Paul VJ (1992) Ecological roles of marine natural products. Cornell Press, Ithaca, NY Paul VJ, Hay ME (1986) Seaweed susceptibiltiy to herbivory: chemical and morphological correlates. Mar Ecol Prog Ser 33: 255-264. Pfister CA (1992) Costs of reproduction in an intertidal kelp: patterns of allocation and life history consequences. Ecology 73(5): 1586-1595. Ragan MA, Glombitza KW (1986) Phlorotannins, brown algal polyphenols. Prog Phycol Res 4: 129-141. Robb MF (1975) The diet of the chiton Cyanoplax hartwegii in three intertidal habitats. Veliger (Suppl.): 34-37. Stebbins TD (1988) Variable population structure and tenacity in the intertidal chiton Katharina tunicata (Mollusca: Polyplacophora) in northern California. Veliger 30(4): 351-357. 64 Steinberg PD (1984) Algal chemical defense against herbivore: Allocation of phenolic compounds in the kelp Alaria marginata. Science 223: 405-406. Steinberg PD (1985) Feeding preferences of Tegulafimebralis and chemical defenses of marine brown algae. Ecol Monogr 55(3): 333-349. Steinberg PD (1989) Biogeographical variation in brown algal polyphenolics and other secondary metabolites: comparison between temperate Australasia and North America. Oecologia 78: 373-382. Steinberg PD (1992) Geopgraphical variation in the interaction between marine herbivores and brown algal secondary metabolites. In: Paul VJ (ed) Ecological roles of marine natural products. Chapter 2. Cornell University Press, Ithaca NY. Steinberg PD (1994) Lack of short-term induction of phlorotannis in the Australasian brown algae Ecklonia radiata and Sargassum vestitum. Mar Ecol Prog Ser 112: 129-133. Steinberg PD (1995) Interaction between the canopy dwelling echinoid Holopneustes purpurescens and its host kelp Ecklonia radiata. Mar Ecol Prog Ser 127: 169-181. Swain T, Hillis WE (1959) The phenolic constituents of Prunus domesticus I. The quantitative analysis of phenolic constituents. J Sci Food Agric 1: 63-68. Tugwell S, Branch GM (1989) Differential polyphenolic distribution amoung tissues in the kelps Ecklonia maxima, Laminaria pallida and Macrocystis angustifolia in relation to plant-defense theory. J Exp Mar Biol Ecol 129: 219-230. Underwood AJ (1981) Techniques of analysis of variance in experimental marine biology and ecology. Ocenogr Mar Biol 19: 513-605. Van Alstyne KL (1988) Herbivore grazing increases polyphenolic defenses in the intertidal brown alga Fucus distichus. Ecology 69(3): 655-663. Van Alstyne KL, Paul VJ (1990) The biogeography of polyphenolic compounds in marine macroalgae: temperate brown algal defenses deter feeding by tropical herbivorous fish. Oecologia 84: 158-163. Yates JL, Peckol P (1993) Effects of nutrient availability and herbivory on polyphenolics in the seaweed Fucus vesiculosus. Ecology 74(6): 1757-1766. Zar JH (1984) Biostatistical Analysis (2nd Ed.) Prentice-Hall, Englewood Cliffs, NJ. 65 •Control blades X •Wounded blades X x Day 1 Day 2 Day 3 Figure 3.1 Phenolic content of Hedophyllum sessile wounded and control blades 1, 2 and 3 days after wounding. Data are the mean+SE of 14 replicates for Days 1 and 3 and of 6 replicates for Day 2 (* indicates significant difference, p<0.05; 2-tailed paired t-test). 66 Table 3.1 Results of ANOVA comparing phenolic content (% tissue dry weight) between juvenile, adult vegetative and adult reproductive tissue. Symbols indicate degrees of freedom (df), mean square (MS), F ratio (F), and the probability of a Type 1 error (p). Source of variation df MS F Tissue type 2 20.380 5.251 0.010 Error 39 3.938 67 CHAPTER 4 Direct and indirect interactions regulating a low intertidal kelp community: implications for juveniles vs. adult kelps INTRODUCTION Communities are often structured by disproportionately strong interactions which emphasize the importance of particular species and populations. Thus, a primary goal of community ecology is identifying and determining the relative importance of interspecific interactions (Dungan 1986). The complexity of interactions within and between trophic levels, direct or indirect, positive and negative, has lead to a variety of models which have become integral tools for community ecologists. Whether experimentally or descriptively based, models of the trophic and non-trophic dynamics which determine the structure of communities function to synthesize and communicate ideas and relationships, and to generate hypotheses and predictions (Paine 1994). In this chapter I combine the results of Chapter 2 with understory community data from the same experiment and descriptive data collected over the same period and examine their implications for the factors structuring the temperate low intertidal community dominated by the kelp Hedophyllum sessile and the chiton Katharina tunicata. These data are used to construct and support two proposed interaction webs (Paine 1980; Menge 1995) which distinguish the nature of interspecific interactions experienced by juvenile and adult H sessile. Although many models have characterized the roles of adult canopy forming kelps (Dayton 1975; Estes and Palmisano 1978; Dethier and Duggins 1984; Harris et al. 1984; Reed and Foster 1984; Dean et al. 1989; Tegner et al. 1995) few have made this distinction between adults and juveniles. This distinction is important because it emphasizes the ecological importance of kelps (order Laminariales) as providers of habitat and addresses the mechanisms which are critical to the transition of vulnerable juveniles in 68 the understory into members of the community supporting canopy. Finally, this distinction adds to a continuing theme of this thesis, the importance of size. Description of community: Dayton (1975) examined the low intertidal / shallow sub tidal kelp community of which Hedophyllum sessile is a part, and identified three major components: canopy, obligate understory and fugitive algal species. Canopy species appear to dominate the system and were defined as those species growing above the other species and apparently succeeding in domination of the light resource. In the case of H. sessile, a direct physical interaction also occurs between this canopy species and understory species. This kelp lacks a stipe and, consequently, its large blades exert a potentially negative scouring effect upon understory algal and invertebrate species. Removal of canopy forming kelp species directly affects both obligate understory and fugitive algal species. Obligate understory species either die completely or die back to the holdfast due to desiccation, exposure to excessive light intensity or physical battering. Intertidally, the understory group consists primarily of articulated and encrusting coralline algae. Reed and Foster (1984) also describe a kelp understory community dominated by articulated and encrusting coralline algae. These calcareous forms of red algae are known for their inability to withstand drying and their requirement for low light intensity and continual water motion (Johansen 1981). Dayton (1975) describes fugitive algal species as being the 'weeds' of the system which, being intolerant of conditions created by canopy species, quickly respond to any canopy removing disturbances by rapidly colonizing, conforming to the definition of 'opportunistic forms' of Littler and Littler (1980). An interaction web previously reported for this community is that of Dethier and Duggins (1984) who describe an 'indirect commensalism'. While Katharina tunicata may graze a variety of algal forms, including macrophytes such as Hedophyllum sessile, and microalgal diatoms, herbivorous limpets are limited to microalgae. Dethier and Duggins (1984) argue that since diatom abundance is inversely related to the abundance of light 69 dominating canopy species, removal of H. sessile by K. tunicata indirectly benefits limpet species abundance and diversity. METHODS Study site. The study area was the rocky low intertidal zone at Prasiola Point located in Barkley Sound on the west coast of Vancouver Island, British Columbia, Canada (48° 49' N, 125° 10' W). Two distinct areas of Prasiola Point were used for these experiments. The first area was that described for the Chapter 2 chiton density manipulation experiment and will now be referred to as the low wave exposure site. The second area will be referred to as the high wave exposure site. This area is located less than 25 m west of the low wave exposure site, separated by a large rock outcropping and is more exposed to waves. Lessoniopsis littoralis, an indicator species of consistent wave exposure (Dayton 1975), is present at this site but absent from the less wave-exposed site. However, this site does receive some relief from the most westward portion of Prasiola Point and would also be classified as semi-exposed to waves from the northwest. Slope of the substratum in the high wave exposure site is greater than that of the low wave exposure site and could be described as moderate to acute (45°-65° from horizontal). As in the low wave exposure site, experiments were performed in the low intertidal zone (0.5 to 1.0 m above mean lower low tides; Canadian Chart Datum). Experimental Design (1) Herbivore density manipulation experiment (July 1995-July 1996): This experiment was described in detail in Chapter 2. Sampling bimonthly, the understory community was monitored in the permanent plots of the low, control, and high chiton density treatment areas. The understory was classified into functional groups as articulated 70 coralline algae, crustose algae (coralline and fleshy), rock, fugitive algae (Dayton 1975), holdfasts of canopy species, and invertebrates. Percent cover of understory groups was estimated using a random point-intercept method. Of 81 available intercepts created by the monofilament grid (used for mapping and estimating percent cover of canopy species in Chapter 2), 15 points were randomly selected each sampling date and the understory group directly below each point recorded. Starting February 1996 the number of random points was increased to 20 to increase accuracy. The encounter frequency of each group was then used to extrapolate a measure of relative percent cover. These data were analyzed as described in Chapter 2. For all experiments, equality of variances were tested using Fmax- tests and normality was assessed by constructing probability plots. (2) Descriptive study of the high Hedophyllum sessile vs. mid H. sessile zones of the high wave exposure site: July 1995. The greater slope and uniformity of the high wave exposure site results in distinct and recognizable patterns of zonation. In July 1995 there appeared to be a relatively high number of Hedophyllum sessile sporophyte recruits near the upper limit of this kelp's distribution (hereafter this area is referred to as the high H. sessile zone, and compared to the mid H. sessile zone). This high area was dominated by a variety of fugitive-type algae and articulated coralline algae and appeared to support a locally low density of Katharina tunicata. To test the hypothesis that this area may have been acting as a spatial refuge for these young recruits three 10m transects were run through the high H. sessile and mid H. sessile zones. The transect consisted of a lead line with 10 random points marked on it. At each of the 30 random points for each zone a 25 x 25 cm quadrat was placed and the following measures recorded: the number of H. sessile juveniles (<1 cm maximum holdfast diameter) and adults (>1 cm maximum holdfast diameter), the number of K. tunicata, and the substrata on which juvenile H. sessile and K tunicata were found. 71 Measures of adult and juvenile Hedophyllum sessile density and Katharina tunicata density were compared between the two zones using 2-tailed unpaired t-tests. Substrata on which juvenile H sessile and K. tunicata were found were examined using the number of individuals of H sessile or K. tunicatalm found on either articulated coralline algae, crustose algae, or bare rock. The substrata occupied by K. tunicata could not accurately be distinguished between rock and crustose algae and therefore these groups were treated as one substratum. These data were compared using Kruskal-Wallis and Mann-Whitney U- tests. The frequency of substrata occupied by juvenile H. sessile and K. tunicata was compared to the relative abundance of these groups in the control plots of the chiton density manipulation experiment. February 1996. In February 1996, seven months later, the high wave exposure site was re-visited to re-evaluate the status of the high and mid Hedophyllum sessile zones. At this time, winter storms had battered most of the adult individuals down to little more than holdfasts but many juveniles were still present and intact in the high H. sessile zone. The zonation was obvious as there appeared to be a higher percent cover H. sessile in the high H. sessile zone than the mid H. sessile zone. To quantify this zonation the high H. sessile zone was systematically sampled in the following way: (1) a transect line with random points marked on it was placed horizontally at the uppermost extent of the H. sessile zone (2) the first six random points on the transect were used as starting points for six vertical transects (3) at each starting point a 25 x 25 cm quadrat was placed, data recorded, and then 'flipped' down by 25 cm increments to 3 additional positions (thus, each vertical transect consisted of four systematically placed quadrats covering one vertical meter) (4) for each quadrat I recorded H. sessile percent cover, density, maximum holdfast diameter and the length of the two longest blades of each individual (5) quadrats composing one vertical transect were labeled as A,B,C, or D from high to low vertical height. 72 Measures were compared by using quadrats of the same vertical position (same letter) as replicate samples (n=6). Measures were compared using one-way ANOVA and Tukey test for post hoc comparisons. May and July 1996. In May and July 1996 the high and mid high Hedophyllum sessile zones of the high wave exposure site were again compared. A single 10m horizontal transect with 10 random points marked on it was placed in each zone. At each random point a 25 x 25 cm quadrat was placed and the following measures were recorded: percent cover of H. sessile, density of Katharina tunicata, percent cover of understory groups. Sampling techniques were the same as those described for the chiton density manipulation experiment. Measures were compared between the high and low H. sessile zones using 2-tailed unpaired t-tests. (3) May 1996 descriptive survey of the low wave exposure zone: To gain more insight into the relationship between Hedophyllum sessile canopy cover and the resulting understory, the low wave exposure site was thoroughly sampled. Four 10m transects with 10 random points marked on each were placed within this site. Transect points which fell within chiton density manipulation areas were excluded, resulting in a sample size of 32. Data recorded were the same as for the previous section. These data were examined in two ways. First, simple correlation analysis was used to examine the relationships between the different measures. Second, assuming that the effects of the Hedophyllum sessile canopy were most pronounced near the extremes of the canopy cover continuum, I compared Katharina tunicata density and the percent cover of understory components between quadrats that had >75% and <25% (N=12 for each) H. sessile cover. These comparisons were tested using 2-tailed unpaired t-tests. 73 RESULTS (1) Herbivore density manipulation experiment (July 1995-July 1996): No significant differences in the percent cover of articulated coralline algae, crustose algae and rock were found at any sampling dates among low, control and high chiton density areas (Figures 4.1-4.3; Tables 4.1-4.3). The percent cover of fugitive algae was highly variable and often did not approach normality. Friedman's test was used to compare treatment groups on all sampling dates. Significant differences were found only in April 1996 when fugitive algae percent cover was higher in control than low (p<0.05) and high (p<0.005) chiton density areas (Figure 4.4). (2) Descriptive study of the high Hedophyllum sessile vs. mid H. sessile zones of the high wave exposure site: July 1995. In July 1995 there were more Hedophyllum sessile juveniles, fewer Katharina tunicata and an equal number of H. sessile adults in the high H. sessile zone than the mid H. sessile zone of the high wave exposure site (Table 4.4). There was a significantly negative correlation between juvenile H. sessile density and K. tunicata density. However, the correlation between adult H. sessile density and K. tunicata density was significantly positive (Table 4.5). The data of substrata on which juvenile Hedophyllum sessile and Katharina tunicata were found 'grossly violated' (Underwood 1981) the assumption of normality, therefore non-parametric analyses were used. Juvenile H. sessile were found significantly more often on articulated coralline algae, and rarely upon fugitive algae forms, than they were on either crustose algae or bare rock. K. tunicata was found significantly more often on crustose algae or bare rock than on articulated coralline algae. In contrast, there was no difference in the relative abundance of these substratum groups in the control plots of the chiton density manipulation experiment (Tables 4.6 and 4.7). This pattern was observed 74 again in April 1996 when H. sessile was recruiting heavily and compared between chiton density manipulation areas (Table 4.8). February 1996. In February 1996 the percent cover and density of Hedophyllum sessile was higher in the high zone (A) that the mid zone (D; Tables 4.9 and 10.). Due to the high variability of H. sessile percent cover within 'A' quadrats they were not included in this analysis. Individuals did not differ significantly in their maximum holdfast diameter but did in blade length (Table 4.10). Figure 4.5 shows the gradation of these measure between the high and mid H. sessile zones. May and July 1996. Table 4.11 summarizes the May 1996 comparison of the high and mid Hedophyllum sessile zones. The percent cover of H. sessile continued to be higher in the high zone than the low zone and, opposite to July 1995, the density of Katharina tunicata was now higher in the high zone than the mid zone. Articulated coralline algae were more abundant in the mid H. sessile zone (p=0.061) while crustose and fugitive algae did not differ between the zones. In July 1996 the cover of Hedophyllum sessile was still higher in the high H. sessile zone than the low H. sessile zone but Katharina tunicata density no longer differed. With the exception of the appearance of the kelp Alaria marginata in the high H. sessile zone, no other differences between these zones were found on this date (Table 4.12). (3) May 1996 descriptive survey of the low wave exposure zone. In May 1996 several significant correlations were found between measured variables in the low wave exposure site (Table 4.13). Hedophyllum sessile canopy cover was positively correlated with Katharina tunicata density and crustose coralline algae negatively correlated 75 with articulated coralline algae and algal bleaching1 Katharina tunicata density had similar, but not significant, correlations with crustose and articulated coralline algae. Articulated coralline algae were strongly correlated with bleaching. Comparison of areas with >75% and <25% cover Hedophyllum sessile showed that chiton densities and the percent understory cover of crustose algae were higher, and the percent cover of articulated coralline algae and the frequency of bleaching lower, in plots with >75% cover H. sessile (Table 4.14). DISCUSSION The results presented in this chapter provide evidence for the importance of several forms of direct and indirect interactions which differ between juvenile and adult forms of Hedophyllum sessile and influence the structure of this low intertidal community. Examined collectively and presented as interaction webs, the complexity of these interactions typifies biological systems (Figure 4.6). Whereas food webs focus on all consumer-prey (trophic) interactions and may be purely observational in nature (Menge and Sutherland 1987; Menge 1995), interaction webs include both trophic and nontrophic interactions for which their strengths have been identified experimentally (Paine 1980). The interactions constituting the interaction webs in Figure 4.6 are based on both experimentally and observationally derived data from this study, and the experimental results of previous investigations by other researchers of this or similar communities. Thus, these models are intended to identify both those interactions which have been experimentally determined and those which require further experimental examination. In order to assess critically, generate predictions as to their relative importance, and design future experiments to test their validity, these interactions must be examined in a 1 Many species of Red algae exhibit characteristic summer 'bleaching', described as loss of pigmentation related to environmental conditions: high temperature, low nutrients and high irradiances (Rico and Fernandez 1996). 76 piecemeal fashion. Accordingly, these models are interpreted here by examining each interaction in isolation, considering first direct, then indirect interactions operating to determine the structure of this low intertidal kelp community. To do this I have primarily followed the definitions used by Menge (1995), with some exceptions which have been noted. Notation of the nature of interactions follows that of Kim (In press) who differentiates between positive and negative interactions by using circles and arrowheads, respectively, rather than more typical 'positive' and 'negative' signs. Direct interactions Predation (p): "a direct trophic interaction which has a negative effect only on the species being consumed." In this system Katharina tunicata is responsible for all types of predation. The susceptibility of juvenile kelps to herbivory is well known (Dayton 1975; Harris et al. 1984; Dean et al. 1984,1989). In this community, the work of Dayton (1975) and Duggins and Dethier (1985) strongly suggest, and the results of Chapter 2 show, that juvenile Hedophyllum sessile is highly vulnerable to grazing by Katharina tunicata. The effect of K. tunicata on adult H. sessile, and the mechanisms responsible for this effect have also been thoroughly discussed in Chapter 2. Direct evidence of coralline algae being grazed by Katharina tunicata comes in two forms: (1) stomach content analysis of K. tunicata (Dethier and Duggins 1988) revealed that articulated corallines can make up a large portion of its diet, while encrusting corallines were rare or absent (Piercy 1987). In addition, while in captivity, fecal pellets of K. tunicata from Prasiola Point were often full of articulated corallines {pers. obs.). More description of the potential effects of K. tunicata on articulated and encrusting coralline algae is given in the next section. 77 Interference competition (c): "the direct and mutually negative interaction of two species." Common to the interactions web of both juvenile and adult Hedophyllum sessile is the interaction between articulated and encrusting coralline alga (Figure 4.6). The ecology of coralline algae is largely unknown and the nature of interactions between articulated and encrusting forms in this study remains circumstantial. The fact that encrusting corallines are most abundant only when canopy cover and herbivore density is high suggests that interference competition exists between these two forms. The ecology of encrusting coralline algae is reviewed by Steneck (1986). Encrusting corallines are reported to be least abundant in intertidal zones, except under dense and persistent canopies of algae or in areas of constant wave wash because of their susceptibility to desiccation. The abundance of encrusting corallines appears to be dependent upon biotic and abiotic sources of disturbance which prevent fouling by algae and invertebrates. Wave action, sand scour and algal whiplash (a potentially important but uninvestigated implication of Hedophyllum sessile morphology) are among the physical sources of disturbance reported to prevent fouling, however, herbivory is most often reported as the primary source of disturbance which keeps encrusting corallines clean and healthy. Steneck (1986) notes that positive correlations between herbivores and dominance of encrusting corallines are well documented and that these algae likely benefit primarily as the result of removal of competing algal forms by grazers. In this community, high densities of Katharina tunicata and the potential whiplash effects of Hedophyllum sessile appear to be responsible for high encrusting coralline abundance in areas of high H. sessile cover (Table 4.14). That this pattern was not induced experimentally (Figure 4.2), accordingly, is likely due to the rapid decline of H. sessile cover and the slow growing nature of encrusting corallines (Johansen 1981). Even though high K. tunicata densities persisted in the high chiton densities treatment areas (Chapter 2), once the canopy was removed encrusting corallines became susceptible to conditions of high light, desiccation and fouling by species more tolerant of these 78 conditions. However, as Paine (1980) notes from an investigation of this community, articulated coralline genera appear more immune to this source of mortality. Inhibition of recruitment (ir): "a direct negative effect wherein an established occupant of a habitat reduces the rate of successful invasion of the habitat by recruiting stages of another species." The differential importance of articulated and encrusting coralline algae in this system are intriguing. Although roughly equal in abundance, sporelings of Hedophyllum sessile rarely appear on encrusting coralline algae and nearly always appear on articulated corallines. Inhibition of//, sessile recruitment by encrusting corallines in this community may, at least in part, be responsible for this pattern as encrusting coralline algae have been found to inhibit kelp recruitment by continually sloughing epithelial cells (Breitburg 1984; Johnson and Mann 1986). However, additional mechanisms may be involved. Inhibition of feeding (if): "a direct negative effect in which an interactor reduces the feeding activity of another, usually mobile, species." The physical nature of articulated corallines may interact direcdy with Katharina tunicata to inhibit mobility and feeding. The wave swept environment in which Katharina tunicata lives demands that its large muscular foot is firmly attached to the substratum at all times. Stebbins (1988) suggests that K. tunicata body size varies between sites differing in wave exposure due to the increased hydrodynamic stress and decreased survival of large individuals. Similarly, Hay (1981) and Steneck and Watling (1982) also describe the increased risk of dislodgment to herbivores caught attempting to traverse unstable substrata. In view of this evidence, it is not surprising that field observations made in July 1995 found that K. tunicata was always found on either bare rock or encrusting coralline algae but never on the turf-like articulated coralline algae. Inhibition of K. tunicata feeding by articulated corallines may contribute significantly to the observed pattern of H. sessile recruitment and has been indicated as such in Figure 4.6a. 79 Enhancement of recruitment (er): "a direct positive effect when a prior occupant of a habitat increases the rate of successful invasion of the habitat by offering favored attachment sites or otherwise raising the probability of colonization." In the scenario described here, enhancement of recruitment is closely tied to inhibition of feeding. While the physical nature of articulated corallines likely inhibit K. tunicata feeding, the same physical properties most probably also provide superior attachment sites to H. sessile recruits as compared to encrusting corallines. Therefore, the implications of this direct interaction have been incorporated into Figure 4.6a. However, in a different community, Reed and Foster (1984) report that articulated coralline algae may inhibit, not facilitate, kelp recruitment under the canopy of Macrocystis pyrifera. Reduction of light by the branches of articulated corallines was suspected as the mechanism underlying this pattern. However, this is unlikely to occur in an intertidal habitat. Provision of habitat of shelter (hs): "a direct positive effect wherein an organism increases the survival of associates, or attracts migrants to itself, because it offers a more attractive microhabitat than exists away from the organism." Attaining a size at which it is significantly less susceptible to grazing, a processes facilitated by refuges potentially created by articulated corallines, allows juvenile Hedophyllum sessile to make the transition to the adult canopy. The primary change associated with this transition is the habitat which these new canopy members now provide. The canopy created by adult H. sessile is a primary determinant of structure in this community (Dayton 1975; Duggins and Dethier 1985). The difficulty of maintaining chiton densities in the high chiton density treatment areas of the manipulative field experiment following canopy removal, and the significantly positive correlations between kelp canopy cover and chiton density, show that Katharina tunicata relies on this kelp for protection from biotic and abiotic factors. Dayton 1975, Paine 1984, and Duggins and Dethier 1985 also found that K. tunicata rapidly emigrates when the H. sessile canopy is removed. 80 Comparison of the relative degree of bleaching between areas of high and low canopy cover emphasized the importance of cover to understory species and the relationship between articulated and encrusting coralline algae. Bleaching was most common, and chiton density much lower, in areas with <25% cover and positively correlated with articulated corallines (Tables 4.13 and 4.14). These results are consistent with patterns previously described. In May 1996, areas with <25% cover and consequentiy low Katharina tunicata density permitted the persistence of articulated corallines, however these forms quickly became bleached by the high light intensity accompanying spring. Together, these results justify the habitat-facilitating role of adult Hedophyllum sessile identified in Figure 4.6b. Indirect interactions Recent interest in indirect interactions, both positive and negative (Dethier and Duggins 1984; Van Tamelen 1987; Wooton 1993; Menge 1995), has revealed their ubiquity, and importance for the manner in which they amplify the interconnectedness of biological systems. Figure 4.7 shows in isolation the indirect interactions potentially operating in this low intertidal kelp community as indicated in Figure 4.6. Habitat facilitation: when "one organism indirectly improves the habitat of a second by altering the abundance of a third interactor." Potential inhibition of feeding (if) of Katharina tunicata by articulated corallines, and the fact that Hedophyllum sessile recruits are found almost exclusively on articulated corallines, suggests that articulated corallines indirectly improves the habitat of juvenile H. sessile by excluding herbivores (Figure 4.7a). This mechanism functionally creates a refuge for juveniles, increasing the probability they will reach an 'escape in size.' A modification of this interaction also occurs, which I have termed 'habitat facilitation, expanded', to describe the scenario where an interactor indirectly improves the 81 habitat of a nonprey species by directly improving the habitat of a herbivore which grazes a competing prey species. This appears to be the case when adult Hedophyllum sessile provides cover for Katharina tunicata, which eventually removes articulated corallines, resulting in an increased abundance of encrusting coralline algae (Figure 4.7c). Habitat degradation: Although not described as such by either Fairweather (1990) or Menge (1995), it is conceivable that the reverse of the definition of 'habitat facilitation' also occurs (i.e. one organism indirectly degrades the habitat of a second by altering the abundance of a third interactor). I propose this to be the case when conditions of high Hedophyllum sessile cover result in an understory dominated by encrusting coralline algae. The attraction of Katharina tunicata to the cover provided by H. sessile indirectly leads to the removal of articulated corallines (Figure 4.7b). Apparent predation: "a predator or herbivore indirectly harms a nonprey species by reducing a prey species upon which the nonprey depends for habitat or shelter or which enhances recruitment." The activities of Katharina tunicata which result in the loss of adult Hedophyllum sessile indirectly negatively affect both encrusting and articulated coralline algae by removing the cover on which they depend (Figures 4.7d and 4.7e). Seasonal loss of adults to hydrodynamic forces will also lead to this result. Keystone predation: "a consumer indirectly increases the abundance of competitors of its prey through consumption of the prey." Katharina tunicata acts as a keystone species by indirectly increasing the abundance of encrusting coralline algae when by consuming articulated coralline algae (Figure 4.7f). Adult /juvenile kelp interactions Recruiting into the understory of adult forms, juvenile kelps are functionally distinct and experience interspecific interactions which differ in number and nature from those of 82 adults. However, whereas the 'goal' of adults is to survive and reproduce, juveniles must first 'escape' from the understory. Although the model presented in Figure 4.6a addresses the kinds of interspecific interactions which may determine if and how juvenile Hedophyllum sessile achieve this transition into adult members of the canopy, it does not address the effects of adult conspecifics. A final proposed interaction web is shown in Figure 4.8 which identifies mechanisms of interaction between adult H. sessile and juvenile sporophytes recruiting into the understory. The left half of this model suggests that the canopy formed by adult H. sessile indirectly degrades the habitat for juvenile Hedophyllum sessile recruitment and survivorship by attracting Katharina tunicata. In turn, K. tunicata grazes juvenile H. sessile directly and indirectly results in an understory dominated by encrusting coralline algae which inhibit H. sessile recruitment. The right half of Figure 4.8 shows how grazing by K. tunicata and the hydrodynamic forces imposed by waves indirectly benefit juvenile H. sessile by reducing the negative indirect and potential direct effects (scouring and shading) of adult H. sessile. Thus, these two forms of physical and biological disturbance act in opposition to the left half of this model. This model (Figure 4.8) predicts that Hedophyllum sessile recruitment and survivorship of juvenile sporophytes will be highest in areas where adult H. sessile cover is lowest. Although no significant differences in recruitment were found between the chiton density manipulation treatment areas (Chapter 2, this thesis), simple correlation analysis indicated a negative interaction between adult H. sessile cover and juvenile recruitment (Table 2.8). High variation of H. sessile percent cover within chiton density treatment areas, potentially slow response rates of coralline algae relative to the growth rates of H. sessile, low sample size, and the effect of artificially maintained densities of Katharina tunicata are likely responsible for the for the insignificance of these correlations. Experiments designed specifically to test these interaction directly are likely to be more insightful. 83 General conclusions Reed and Foster (1984) describe a community wherein low levels of disturbance allow perennial algal species to dominate light, excluding other alga species and inhibiting their own recruitment. In the community described here, adult Hedophyllum sessile also appear to inhibit their own recruitment, but through mechanisms in addition to, or other than, light domination. The mechanisms identified here include attracting herbivores and physical scouring which result in an understory community dominated by encrusting coralline algae which further inhibit recruitment. Previous investigators of this community have eluded to the often puzzling combinations of positive, negative, direct and indirect interactions experienced by some species in this community. For example, Paine (1980) notes the mutualistic relationship between herbivores and encrusting corallines and the enigmatic relationship (Paine 1984) between competitively superior canopy species whose holdfasts easily can overgrow obligate understory encrusting corallines. Similarly, Duggins and Dethier (1985) note the interesting relationship between the chiton Katharina tunicata and Hedophyllum sessile, in which the chiton eats the young plants but relies on the old plants for refuges from predation or physical stress. This study has attempted to identify and summarize the potential interactions which regulate the structure of this low intertidal kelp community. Future experiments should be designed to test these interactions and determine their relative importance. 84 LITERATURE CITED Breitburg, DL (1984) Residual effects of grazing: inhibition of competitor recruitment by encrusting coralline algae. Ecology 65(4): 1136-1143. Dayton PK (1975) Experimental evaluation of ecological dominance in a rocky intertidal algal communtiy. Ecol Monogr 45: 137-159. Dean TA, Schroeter SC, Dixon JD (1984) Effects of grazing by two species of sea urchins (Stronglylcentrotus franciscanus and Lytechinus anamesus) on recruitment and survival of two species of kelp (Macrocystis pyrifera and Pterygophora californica). Mar Biol 78: 301-313. Dean TA, Thies K, Lagos SL (1989) Survival of juvenile giant kelp: the effects of demographic factors, competitors, and grazers. Ecology 70(2): 483-495. Dethier MN, Duggins DO (1984) An "Indirect commensalism" between marine herbivores and the importance of competitive hierarchies. Am Nat 124(2): 205- 219. Dethier MN, Duggins, DO (1988) Variation in strong interactions in the intertidal zone along a geographic gradient: a Washington-Alaska comparison. Mar Ecol Prog Ser 50: 97-105 Duggins DO, Dethier MN (1985) Experimental studies of herbivory and algal competition in a low intertidal habitat. Oecologia 67: 183-191. Dungan, ML (1986) Three-way interactions: barnacles, limpets, and algae in a Sonoran Desert rocky intertidal zone. Am Nat 127(3): 292-316 Estes JA, Palmisano JF (1974) Sea otters: their role in structuring nearshore communities. Science 185: 1058-1060. Fairweather PG (1990) Is predation capable of interacting with other community processes on rocky reefs? Austral J Ecol 15: 453-464. Harris LG, Ebling AW, Laur DR, Rowley RJ (1984) Community recovery after storm damage: a case of facilitation in primary succession. Science 224: 1336-1338. Hay ME (1981) The functional morphology of turf-forming seaweeds: persistence in stressful marine habitats. Ecology 62(3): 739-750. Johansen HW (1981) Coralline Algae, a first synthesis. CRC Press, Florida. Johnson CR, Mann KH (1986) The importance of plant defence abilities to the structure of subtidal seaweed communities: the kelp Laminaria longicruris de la Pylaie survives grazing by the snail Lacuna vincta (Montagu) at high population densities. J Exp Mar Biol Ecol 97: 231-267. Kim JH (In press) The role of herbivory, and direct and indirect interactions, in algal succession. J Exp Mar Biol Ecol 0: 00-00 85 Littler MM, Littler DS (1980) The evolution of thallus form and survival strategies in benthic marine macroalgae: field and laboratory tests of a functional form model. Am Nat 116: 24-44. Menge BA (1995) Indirect effects in marine rocky intertidal interaction webs: patterns and importance. Ecol Monogr 65(1): 21-74 Menge BA, Sutherland JP (1987) Community regulation: variation in disturbance, competition, and predation in relation to environmental stress and recruitment. Am Nat 130(5): 730-757. Paine RT (1980) The third Tansely lecture. Food webs: linkage, interaction strength and community infrastructure. J Anim Ecol 49: 667-685. Paine RT (1984) Ecological determinism in the competition for space. Ecology 65(5): 1339-1348. Paine RT (1994) Marine rocky shores and community ecology: An experimentalist's perspective. In Excellence in Ecology, (ed. O. Kinne) Ecology Institue, Norbunte, Germany. Piercy RD (1987) Habitat and food preferences in six Eastern Pacific chiton species (Mollusca: Polyplacophora). Veliger 29(4): 388-393. Reed DC, Foster MS (1984) The effects of canopy shading on algal recruitment and growth in a giant kelp forest. Ecology 65: 937-948. Rico JM, Fernandez C (1996) Seasonal nitrogen metabolism in an intertidal population of Gelidium latifolium (Gelidiaceae, Rhodophyta). Eur J Phycol 31: 149-155. Stebbins TD (1988) Variable population structure and tenacity in the intertidal chiton Katharina tunicata (Mollusca: Polyplacophora) in Norther California. Veliger 30(4): 351-357. Steneck RS, Watling L (1982) Feeding capabilities and limitation of herbivorous molluscs: a functional group approach. Mar Biol 68: 299-319. Steneck RS (1986) The ecology of coralline algal crusts: Convergent patterns and adaptative strategies. Ann Rev Ecol Syst 17: 273-303. Tegner MJ, Dayton PK, Edwards PB, Riser KL (1995) Sea urchin cavitation of giant kelp (Macrocystis pyrifera C Agardh) holdfasts and its effects on kelp mortality across a large California forest. J Exp Mar Biol Ecol 191: 83-99. Underwood AJ (1981) Techniques of analysis of variance in experimental marine biology and ecology. Ocenogr Mar Biol 19: 513-605. Van Tamelen PG (1987) Early successional mechanisms in the rocky intertidal: the role of direct and indirect interactions. J Exp Mar Biol Ecol 112: 39-48. Wooton JT (1993) Indirect effects and habitat use in an intertidal community: interactin chains and interaction modifications. Am Nat 141(1): 71-89. 86 • Low JL95 S95 095 D95 F96 A96 JU96 JL96 SAMPLING DATE (MONTH/YEAR) Figure 4.1 Articulated coralline algae percent understory cover within low, control and high chiton density manipulation areas. Data are the mean+SE of 6 replicates for density manipulations on each sampling date (see Figure 2.1 for abbreviations). No significant differences (p<0.05) were found within any sampling dates. 87 • Low • Control • High JL95 S95 095 D95 F96 A96 JU96 JL96 SAMPLING DATE (MONTH/YEAR) Figure 4.2 Crustose algae percent understory cover within low, control and high chiton density manipulation areas. Data are the mean+SE of 6 replicates for density manipulations at each sampling date (see Figure 2.1 for abbreviations). No significant differences (p<0.05) were found within any sampling dates. 88 JL95 S95 095 D95 F96 A96 JU96 JL96 SAMPLING DATE (MONTH/YEAR) Figure 4.3 Bare rock percent understory cover within low, control and high chiton density manipulation areas. Data are the mean+SE of 6 replicates for density manipulations at each sampling date (see Figure 2.1 for abbreviations). No significant differences (p<0.05) were found within any sampling date. 89 50 • El Low 45 • Control 40 • High cr 35 LLI > 30 o o 25 LHC Z 111 o 20 in Ol 15 X a. 10 5 I 0 I JL95 S95 095 D95 F96 A96 JU96 JL96 SAMPLING DATE (MONTH/ YEAR) Figure 4.4 Fugitive algae percent understory cover within low, control and high chiton density manipulation areas. Data are the mean+SE of 6 replicates for density manipulations on each sampling date (see Figure 2.1 for abbreviations). Results of multiple comparison tests for significant Friedmans's tests (Zar 1984;p. 230) are shown. Letters sharing an underline were not significantly different (p>0.05). 90 H.sessile Density Holdfast Blade % cover (#/sq.m) diameter length (cm) (cm) Figure 4.5 February 1996 comparison of Hedophyllum sessile percent cover, density, holdfast diameter, and blade length between the high and mid H. sessile zones. Letters represent the transition from the high H. sessile zone (A) to the mid H. sessile zone (D). N=6 for replicate quadrats of the same letter. Results of Tukey tests for significant ANOVA are labeled. Letters sharing an underline were not significantly different (p>0.05). 91 Figure 4.6 Comparison of interaction webs involving juvenile (A) and adult (B) Hedophyllum sessUe. A partial refuge in size by H. sessile from herbivory distinguished the two webs. Solid lines indicate direct interactions and dashed line indirect ones. Arrows indicate negative interactions and circles positive ones. The type of indirect interactions are indicated and codes for direct interactions follow those of Menge (1995): predation (p), inhibition of feeding (if), interference competition (c), provision of habitat or shelter (hs), inhibition of recruitment (ir), enhancement of recruitment (er). 92 Chiton Juvenile B. Chiton O hs _ Adult ^QKelp /Kelp . / Habitat y habitat Degradation / Facilitation AC 1* AC hs Adult Adult C. Chiton O D. Chiton " Kelp Kelp , Habitat I ' Facilitation Apparent Predation t ^Expanded AC • EC Chiton —L_^Adult F. Chiton \ Kelp Keystone \ Predation p Apparent^ ^ hs Predation t AC o EC0 EC Figure 4.7 Types of indirect interactions occurring between the chiton Katharina tunicata, adult and juvenile Hedophyllum sessile and articulated (AC) and encrusting (EC) coralline algae. Solid lines indicate direct interactions and dashed line indirect ones. Arrows indicate negative interactions and circles positive ones. Codes for direct interactions follow those of Menge (1995): predation (p), inhibition of feeding (if), interference competition (c), provision of habitat or shelter (hs). 93 Chiton O—- Adult «hdf/p Waves / Chitons Habitat Kelp 'Keystone I ^degradation i predation | Habitat facilitation AC —•EC Juvenile CX" (Fugitives) ir Kelp (AC / Fugitives) Figure 4.8 Direct and indirect interactions occurring between juvenile and adult Hedophyllum sessUe, articulated (AC) and encrusting (EC) coralline algae, fugitive algae, Katharina tunicata (Chiton) and wave force. Solid lines indicate direct interactions and dashed lines indirect ones. Arrows indicate negative interactions and circles positive ones. Labels for indirect interactions and codes for direct interactions follow those of Menge (1995): predation (p), interference competition (c), provision of habitat or shelter (hs), and inhibition of recruitment (ir). The hydrodynamic forces imposed by waves are denoted as (hdf). 94 Table 4.1 Results of ANOVA on the treatment effect of Katharina tunicata density on the understory percent cover of articulated coralline algae. Abbreviations indicate degrees of freedom (df), mean square (MS), F ratio (F), and the probability of a Type 1 error (p). Source of variation df MS A. July 1995 Friedman's test B. Sept. 1995 Treatment 2 266.667 0.989 0.406 Block 5 300.741 1.115 0.411 Error 10 269.630 C. Oct. 1995 Treatment 2 255.556 0.868 0.449 Block 5 592.593 3.306 0.051 Error 10 179.259 D. Dec. 1995 Treatment 2 0.337 0.084 0.920 Block 5 22.227 1.201 0.375 Error 10 318.519 E. Feb 1996 Treatment 2 101.389 0.298 0.748 Block 5 391.389 1.152 0.395 Error 10 339.722 F. April 1996 Treatment 2 634.722 2.006 0.185 Block 5 435.556 1.377 0.311 Error 10 316.389 G. June 1996 Treatment 2 429.067 0.989 0.405 Block 5 323.894 0.747 0.607 Error 10 433.727 H. July 1996 Treatment 2 254.167 0.532 0.603 Block 5 563.333 1.180 0.384 Error 10 477.5 00 95 Table 4.2 Results of ANOVA on the treatment effect of Katharina tunicata density on the understory percent cover of encrusting coralline algae. Abbreviations indicate degrees of freedom (df), mean square (MS), F ratio (F), and the probability of a Type 1 error (p). Source of variation df MS A. July 1995 Treatment 2 585.170 3.434 0.073 Block 5 296.281 1.739 0.213 Error 10 170.385 B. Sept. 1995 Treatment 2 1172.840 7.787 0.009 Block 5 519.506 3.449 0.045 Error 10 150.617 C. Oct. 1995 Treatment 2 600.000 2.166 0.165 Block 5 401.481 1.449 0.288 Error 10 277.037 D. Dec. 1995 (Log transformed) Treatment 2 0.428 1.270 0.323 Block 5 0.267 0.792 0.579 Error 10 0.337 E. Feb 1996 Treatment 2 434.722 0.967 0.413 Block 5 408.056 0.907 0.513 Error 10 449.722 F. April 1996 Friedman's test G. June 1996 Treatment 2 442.137 1.429 0.285 Block 5 874.454 2.826 0.076 Error 10 309.448 H. July 1996 (Log transformed) Treatment 2 1.715 0.391 0.603 Block 5 1.661 0.379 0.384 Error 10 4.383 96 Table 4.3 Results of ANOVA on the treatment effect of Katharina tunicata density on the understory percent cover of rock. Abbreviations indicate degrees of freedom (df), mean square (MS), F ratio (F), and the probability of a Type 1 error (p). Source of variation df MS A. July 1995 (Log transformed) Treatment 2 1.241 0.343 0.718 Block 5 2.195 0.607 0.697 Error 10 3.618 B. Sept. 1995 Treatment 2 155.556 21.981 0.188 Block 5 260.741 3.321 0.050 Error 10 78.519 C. Oct. 1995 (Log transformed) Treatment ' 2 0.372 0.128 0.881 Block 5 2.224 0.766 0.595 Error 10 2.902 D. Dec. 1995 Treatment 2 96.296 0.867 0.450 Block 5 59.259 0.533 0.747 Error 10 111.111 E. Feb 1996 (Log transformed) Treatment 2 2.747 1.777 0.219 Block 5 2.324 1.503 0.513 Error 10 1.546 F. April 1996 Treatment 2 134.722 0.871 0.448 Block 5 208.889 1.350 0.320 Error 10 154.722 G. June 1996 Treatment 2 28.624 0.126 0.883 Block 5 200.588 0.833 0.527 Error 10 227.267 H. July 1996 Treatment 2 109.722 0.835 0.462 Block 5 28.056 0.214 0.949 Error 10 131.389 97 Table 4.4 July 1995 comparison of juvenile and adult Hedophyllum sessile and Katharina tunicata density between the High and Mid H sessile zones. Abbreviations indicate sample size (N), mean±standard error (MEAN±SE), and the probability of a Type 1 error (p). High zone Mid zone Variable N (MEAN+SE) (MEAN+SE) p Juvenile H. sessile 30 66.67±12.16 17.07±6.94 0.001 density (#/m2) Adult H. sessile 30 20.27+3.42 25.60±2.61 0.221 density (#/m2) K. tunicata 30 26.13±3.79 42.67±5.90 0.023 density (#/m2) Table 4.5 Simple correlation analysis of July 1995 densities of juvenile and adult Hedophyllum sessile and Katharina tunicata. Abbreviations indicate sample size (N), correlation coefficient (r), and the probability of a Type 1 error (p) Variables compared (N=60) r P Juvenile H. sessile density -0.388 0.002 vs. K. tunicata density Adult H. sessile density 0.340 0.008 vs. K. tunicata density Juvenile H. sessile density -0.162 0.218 vs. Adult H. Sessile 98 Table 4.6 July 1995 comparison of the substrata on which Katharina tunicata and juvenile Hedophyllum sessile were found and the relative abundances of those substrata. Abbreviations indicate mean±standard error (X±SE) and the probability of a Type 1 error (p). The substrata occupied by K. tunicata could not be accurately distinguished between rock and crustose coralline algae and therefore was treated as a single substratum. ROCK CRUSTOSE ARTICULATED CORALLINES CORALLINES Variables N (X+SE) (X±SE) (X±SE) P Relative abundance 6 16.67+4.47 14.44±2.68 24.44±8.19 0.436 (% cover) K. tunicata 60 33.6±3.7 0.27+0.27 <0.0001 (#/m2) Juvenile 60 2.13±0.90 4.53±0.90 34.93+6.90 <0.0001 H. sessile (#/m2) Table 4.7 Results of ANOVA for the July 1995 comparison of the relative abundance of rock, crustose algae, and articulated coralline algae in control plots of the chiton density manipulation experiment. Abbreviations indicate degrees of freedom (df), mean square (MS), F ratio (F), and the probability of a Type 1 error (p). Source of variation df MS F A. July 1995 Treatment 2 165.46 0.877 0.436 Block 5 188.61 Error 10 99 Table 4.8 Comparison of the substrata on which the April 1996 cohort of Hedophyllum sessile recruits were found and the relative abundance of those substrata, between low, control and high chiton density treatment areas. Abbreviations indicate the pecentage (%) of juvenile H. sessile found on the different substrata pooled over six replicates for each treatment group and the resulting sample size (N). Relative abundances of substrata are reported as the mean±standard error (mean±SE). ARTICULATED CRUSTOSE BARE H. SESSILE CORALLINE ALGAE ROCK HOLDFASTS ALGAE A. Low chiton density treatment areas Relative abundance 34.17+9.78 35±15.55 14.0+6.0 5.0±2.235 (N=6; mean+SE) % Juvenile recruits(N=106) 69.81% 12.26% 5.66% 11.32% B. Control chiton density treatment areas Relative abundance 40.0+6.33 9.165±1.54 16.67±6. 6.65±2.47 (N=6; mean±SE) 01 % Juvenile recruits (N=67) 82.09% 2.99% 4.48% 10.45% C. High chiton density treatment areas Relative abundance 54.165±6.51 26.65±6.15 7.5±3.36 0.84±0.84 (N=6; mean±SE) % Juvenile recruits (N=75) 86.67% 6.67% 4.0% 3.08% 100 Table 4.9 February 1996 comparison of Hedophyllum sessile percent cover, holdfast diameter, and blade length and Katharina tunicata density between the High and Mid H sessile zones. Abbreviations indicate sample size (N), mean±standard error (MEAN±SE), the probability of a Type 1 error (p). High zone Mid zone Variables N (MEAN±SE) (MEAN±SE) P H. sessile Percent cover 6 26.50±7.919 6.33+0.882 0.042 H. sessile density (#/m2) 6 74.72±0.12.16 24+3.584. 0.033 Maximum holdfast diameter (cm) 6 2.94+0.51 4.48±0.96 0.295 Blade length/ plant/quadrat (cm) 6 18.04±2.11 11.08±2.08 0.040 Table 4.10 Results of ANOVA of February 1996 comparison of Hedophyllum sessile percent cover, density, holdfast diameter and blade length between four vertical intertidal levels (A,B,C and D) from the high to low H. sessile zones. Abbreviations indicate degrees of freedom (df), mean square (MS), F ratio (F), and the probability of a Type 1 error (p). Source of variation df MS A. H. sessile percent cover (Log transformation; 'A' not included in analysis ) Tidal height 2 2.004 9.821 0.002 Error 15 0.204 B. H. sessile density (Log transformation) Tidal height 3 0.904 4.788 0.011 Error 20 0.189 C. H. sessile holdfast diamter Tidal height 3 4.601 1.322 0.295 Error 20 3.481 D. H. sessile blade length Tidal height 3 64.005 3.333 0.040 Error 20 19.204 101 Table 4.11 May 1996 comparison of Hedophyllum sessile canopy cover, Katharina tunicata density, and understory components between the High and Mid H. sessile zones. Abbreviations indicate sample size (N), mean±standard error (X±SE), and the probability of a Type 1 error (p). Upper H. sessile Mid H. sessile Zone Zone Variables N (MEAN±SE) (MEAN+SE) P H. sessile 10 74.20±8.23 7.80±1.54 <0.0001 (percent cover) Chiton density (#/m2) 10 43.20+8.94 20.80+8.94 0.093 Articulated corallines 10 42.463+3.12 52.99+4.207 0.061 (percent cover) Crustose corallines 10 26.358±6.60 38.02±4.40 0.161 (percent cover) Fugitive algae 10 12.512±5.76 5.00±3.16 0.272 (percent cover) Table 4.12 July 1996 comparison of Hedophyllum sessile canopy cover, Katharina tunicata density, and understory components between the High and Mid H. sessile zones. Abbreviations indicate sample size (N), mean±standard error (X+SE), and the probability of a Type 1 error (P). Upper H. sessile Mid H. sessile Zone Zone Variables N (MEAN±SE) (MEAN+SE) P H. sessile 10 69.90±8.85 35.00±9.16 0.013 (percent cover) Chiton density (#/m2) 10 20.80±5.36 24.00±5.96 0.695 Articulated corallines 10 29.00±3.86 34.50±4.62 0.373 (percent cover) Crustose corallines 10 17.00±4.10 24.00±6.05 0.352 (percent cover) Fugitive algae 10 43.50±6.84 31.50+6.50 0.219 (percent cover) 102 Table 4.13 Simple correlation analysis of May 1996 Hedophyllum sessile canopy cover, Katharina tunicata density, and understory components. Abbreviations indicate sample size (N), correlation coefficient (r), and the probability of a Type 1 error (p). Variables compared (N=32) r P H. sessile canopy vs. 0.623 <0.0001 chiton density H. sessile canopy vs. -0.370 0.037 articulated corallines H. sessile canopy vs. 0.474 0.006 crustose corallines H. sessile canopy vs. -0.678 <0.0001 bleaching Chiton density vs. -0.276 0.127 articulated corallines Chiton density vs. 0.330 0.065 crustose corallines Articulated corallines vs. -0.565 0.0006 crustose corallines Articulated corallines vs. 0.662 <0.0001 bleaching Crustose corallines vs. -0.388 0.027 bleaching Table 4.14 May 1996 comparison of Katharina tunicata density and understory components between areas with >75% and <25% cover Hedophyllum sessile. Abbreviations indicate sample size (N), meantstandard error (MEAN±SE), and the probability of a Type 1 error (p). > 75% cover < 25% cover H. sessile H. sessile Variables N (MEAN±SE) (MEAN±SE) P Chiton density (#/m2) 12 34.67±8.16 2.67±1.79 <0.0001 Articulated corallines 12 24.41±3.33 46.562±10.53 0.041 Crustose corallines 12 50.61+6.78 25.05±5.28 0.006 Bleaching 12 7.39+3.00 50.74+8.50 <0.0001 103 CHAPTER 5 General conclusions and summary: A successional model of the mechanisms and processes regulating a low intertidal kelp community. This thesis has examined and determined specific mechanisms which regulate the demography and population structure of the intertidal kelp Hedophyllum sessile. The chemical ecology of H. sessile was examined in the context of both present-day implications that polyphenolics hold for influencing the effect of the primary herbivore in this community, Katharina tunicata, and proposed evolutionary frameworks for polyphenolics. In Chapter 4, several mechanisms which determine the nature of interactions found in this community have been identified and presented as two models of community regulation which emphasize the functional and interactive differences between juvenile and adult forms of kelp. The conclusions of these investigations will not be repeated here. Instead, I have used this final chapter to present a somewhat informal model of our understanding of this low intertidal community. This model describes the annual successional trajectory of this system as it is currently understood and incorporates most aspects of the thesis. I end this chapter with a few thoughts on future investigations and experimental design. Model of Succession Models of successional sequences (Connell and Slayter 1977; Van Tamelen 1987; Farrell 1991) play keys roles when interpreting seasonal and intra-seasonal variation in community structure so often encountered by ecologists. The recognition of the mid and high Hedophyllum sessile zones of the high wave exposure site presented a unique opportunity to perform a natural experiment following seasonal changes within these two zones. These data, viewed in combination with the results of chapter 2, 3, and 4, and the 104 results of previous studies of this community, permit a synthesis, presented as a model, of the mechanisms and processes which determine the successional trajectory of this rocky low intertidal community (Figure 5.1). As a starting point for the model, I consider a typical scenario which characterizes this community between June and September of any given year. In this scenario the maximum wave force has been low in recent months and the air temperature and light intensity relatively high. Although extreme low tides usually occur early in the morning, these conditions occasionally result in episodes of high UV light exposure and desiccation. Hedophyllum sessile forms a dominant monospecific canopy, accounting for 75-100% cover. Gaps in the canopy do exist and support the temporary existence of a variety of fugitive algal species. Katharina tunicata is rare in these gaps due to the way these species inhibit mobility and the lack of protective cover they provide. Under the H. sessile canopy articulated and encrusting coralline algae dominate, with encrusting forms significantly more abundant than articulated forms, attributable to the conspicuous presence of Katharina tunicata. This generalist herbivore seeks out the H. sessile canopy for protection from waves, desiccation, UV light, and perhaps avian and echinoid predators, and grazes on the kelp's blades while they are accessible during low tides. In addition to K tunicata, the understory supports a phylogenetically diverse collection of invertebrates and a few species of fish. The canopy formed by Hedophyllum sessile is a dynamic factor in this community which is regulated primarily by the seasonal hydrodynamic forces placed upon it. As individual plants are lost to waves, available cover for Katharina tunicata changes and may become locally limited. Limitation of the canopy resource leads to locally high densities of K. tunicata in areas where cover remains. In these areas any fugitive algal forms present are quickly grazed, then articulated corallines begin to decline as well. Eventually, and as long as H sessile cover remains, high densities of K. tunicata lead to an understory dominated by encrusting coralline algae. 105 Hedophyllum sessile recruitment is low and sporelings rich in polyphenolics that do appear do not survive long under these conditions. The nature of the trade-offs Katharina tunicata faces when grazing chemically defended kelps versus the morphologically resistant coralline algal forms is unclear. However, the significant decline in H. sessile survivorship at high chiton densities, and the known resistance of encrusting coralline algae to intense herbivory, suggests that polyphenolics are less effective than calcification. H. sessile sporelings are removed, and adult blades are grazed, during low tides and adult holdfasts are grazed and burrowed into when emerged. The physical effects of the stipeless kelp, H. sessile, on understory algal forms, including H. sessile sporelings, remains untested. Starting in September wave force increases and continues throughout the fall and winter. Survivorship of all sizes of Hedophyllum sessile is similar in areas with low or moderate chiton densities. However, in areas where chiton densities are or have been high, H. sessile mortality is generally high but decreases with increasing holdfast size. Decline in canopy cover in these areas is accelerated by the indirect effect of chitons weakening the holdfasts of adult plants. In October, Hedophyllum sessile becomes reproductive and continues to release spores through January. Spores persist in the water column a short time (less than 24 hours), contact the substratum and begin to germinate into gametophytes. Although little is known of gametophyte survivorship, presumably areas which provide protection from waves and herbivores are advantageous and articulated coralline algae perhaps meet these requirements. By December the canopy in high chiton density areas had been reduced to less than 10%. Surviving large adults (>4 cm holdfasts) constituting the remaining cover were able to reproduce and release spores, however, many young adults (1-4 cm holdfasts) likely had not reached reproductive maturity before being torn from the substratum. During the winter the activity of Katharina tunicata is low and the Hedophyllum sessile canopy is of less importance due to the combined effects of high wave exposure which may inhibit feeding and limit desiccation, and the seasonal weather which severely 106 limits UV intensity. K. tunicata is a generalist grazer and thus allows this chiton to graze the algal forms that are present. During March and April wave force declines and desiccation and light intensity again increase. The activity of Katharina tunicata increases and rapid emigration of the chiton occurs from areas where the canopy cover of Hedophyllum sessile is low, to avoid the effects of the changing environmental conditions. It is also during this time that new sporophytes of H. sessile begin to appear in large numbers, particularly upon articulated coralline algae. Because recruitment of H. sessile is strongly associated with articulated coralline algae, and articulated corallines are adversely affected by chiton density and perhaps scouring by the H sessile canopy, sporelings are most abundant where canopy cover is low. These areas are characterized by a mixture of articulated coralline algae and fugitive algae; such habitats act as spatial refuges for the new sporelings from K. tunicata. As the sporelings rapidly grow throughout the spring and summer they reach a partial escape in size from K. tunicata. By the following spring some will have made the transition from being understory components to contributing to the H. sessile canopy. In concert with the appearance of Hedophyllum sessile sporelings in the spring is the vegetative growth of new blades from holdfasts that have managed to survive through the winter. These new blades begin as half circles emerging from holdfasts which eventually split to form multiple blades. Together with recruitment, recovery of the H. sessile canopy is rapid, returning to 75-100% cover within one to two years in areas where the entire canopy was removed. 107 Future Research As much as has been learned of this community since it was first critically examined by Dayton in 1975, an extensive list remains, and continues to grow, of questions which need to be answered to fully understand the dynamics of this community. Many of these questions in themselves constitute significant ecological subjects for which this community may act as an ideal model. During the time I have spent over the past two years observing and thinking about processes which may be at work in this low intertidal mind-game, several questions and observations came to mind which I will discuss here. The unique morphology of Hedophyllum sessile, lacking a stipe to raise its blades from the substratum, generates numerous questions. Is scouring by adult blades a significant factor in shaping the structure of the understory community? What are the precise differences in juvenile survivorship and growth under and away from the H. sessile canopy? Many times I observed juvenile H. sessile recruiting into or near adult holdfasts which eventually coalesced with the adult. Are there critical distances close to or away from adults H. sessile which allow juveniles to avoid the potential physical effects of adults? The whole question of substrata on which sporelings appear is a topic that Kristen Milligan (a Ph.D. student currently in our laboratory) and I have spend much time discussing. We have had sporelings appear on nearly every substratum available, including the bolts used to mark our permanent quadrats, but nine times out of ten, they are on articulated corallines or within turfs of fugitive algal forms. Kristen and I hope to combine our data and examine this question of differential substratum occupation. The predominance of H. sessile recruits on articulated coralline algae appears to be the result of differential survivorship, but which mechanism is most important, the refuge provided from herbivores or an increased holdfast attachment ability? Recruitment is also very high on old H sessile holdfasts. Is this the result of the morphology of holdfasts which may be 108 a place which favors gametophyte survivorship? The whole question of gametophyte and early post settlement stage survivorship and the effect of herbivory remains a black box. Katharina tunicata may influence sporophyte survivorship but what are the effects of all the other herbivores on these stages? What is the relative importance of survivorship of these stages to community structure? The ecology of Egregia menziesii often came to mind as I dug through it to get to some of my permanent plots. The apparent increase in E. menziesii abundance in recent years has provoked many hypotheses as to how it is able establish and displace more abundant kelps. From my experience, although recruitment of E. menziesii seems relatively very low, sporelings that do appear nearly always manage to persist. The holdfasts of E. menziesii are amazing structures which are very hard and quickly overgrow anything in the immediate vicinity. Finally, as with Hedophyllum sessile, the potential scouring effects of the 'feather-boa' kelp E. menziesii are most probably large and need experimental examination. How populations of the chiton Katharina tunicata are regulated has also been a popular topic of thought and conversation throughout this study. As mentioned in Chapter 2, what is the mechanism which permits higher densities of K. tunicata to persist in high versus low wave exposed sites? I suspect that differences in desiccation, and possible differences in Hedophyllum sessile morphology and survivorship (now being investigated by Kristen Milligan), are responsible. As with all previous investigations of this community, no real insight has been gained as to predators which may limit K. tunicata populations. Similarly, little is known of recruitment patterns of this chiton (Dethier and Duggins 1984). K tunicata exhibits a large range of body sizes in the field. How does body size influence feeding preferences and what are the combined effects of differences in body size and density on algal community structure? What are the behavioural traits of K. tunicata which influence feeding and how do they change with body size and changes in 109 density? The list goes on and on and I hope that some of these questions may stimulate future work. A Note on Experimental Design In general, the design I eventually settled upon for manipulation of chiton densities and monitoring of population structure and demography proved to be both efficient and informative. However, when it came to monitoring responses of the understory community, considerable variation may have masked important results. In part, the difficulty in interpreting these data may have resulted from not having concise a priori hypotheses as to what I might have expected to find. However, given the complexity of interactions that were revealed, making such hypotheses remains a difficult task. Increasing the number of replicates of chiton density treatment areas is the obvious remedy for this problem, however, logistical constraints often force field ecologists to make compromises in their experimental designs. Specifically, trade-offs may have to be made between acceptable levels of variation, the likelihood that a treatment effect will be observed, and the ability to generalize the resulting conclusions. In this study, permanent quadrat locations were randomly determined within the intertidal distribution of Hedophyllum sessile. Thus, the six replicates samples for each treatment were intended to represent the overall state of this community at any given time. However, on the scale being considered, at any given time these six replicate samples represented several points along a successional continuum of community states. For this reason, within the three chiton density treatment groups, replicate samples ranged from those with 100% cover H. sessile and extensive understory communities, to those with <10% cover H. sessile and comparatively barren understories consisting primarily of bare rock. Due to this variation, the likelihood that treatment effects would be detected in the understory community was low. 110 Given the difficulties described above, more careful site matching may be necessary to test some of the predictions of understory species interactions posed in Chapter 4, or other small-scale ecological experiments in general. For example, assuming that areas with similar percent covers of H. sessile will have similar understory communities, it may be acceptable to randomly assign permanent plot locations from only areas within a defined range of canopy cover (i.e. areas with 75-100% cover), thereby ensuring somewhat similar starting points for all samples. For similar reasons, Paine (1994) argues that unmanipulated control areas do not represent conceptually insightful baseline states, and suggests that the use of reference states (controls) that arise from a coherent theoretical view, or are generated in the field by an experimental protocol, will enhance both comparison and understanding. While selecting well matched replicate samples may decrease ones ability to generalize about the resulting conclusions, it may provide more insight as to the mechanisms of community regulation being examined and allow for the design of subsequent experiments from which it may be more easy to generalize from. Limited by the experiments they can design and accomplish, ecologists inevitably face trade-offs as they strive for predictive ability in identifying the mechanisms which determine the nature and magnitude of interactions which regulate community structure. The ability to generalize, and therefore to predict, may require that components of communities are initially examined in isolation or in their most basic forms. Ill LITERATURE CITED Connell JH, Slatyer RO (1977) Mechanisms of succession in natural communities and their role in community stability and organization. Am Nat 111: 1119-1143. Dayton PK (1975) Experimental evaluation of ecological dominance in a rocky intertidal algal community. Ecol Monogr 45: 137-159. Dethier MN, Duggins DO (1984) An "Indirect commensalism" between marine herbivores and the importance of competitive hierarchies. Am Nat 124(2): 205- 219. Farrell TM (1991) Models and mechanisms of succession: an example from a rocky intertidal community. Ecol Monogr 61(1): 95-113. Paine RT (1994) Marine rocky shores and community ecology: An experimentalist's perspective. In Excellence in Ecology, (ed. O. Kinne) Ecology Institute, Norbunte, Germany. Van Tamelen PG (1987) Early successional mechanisms in the rocky intertidal: the role of direct and indirect interactions. J Exp Mar Biol Ecol 112: 39-48. 112 JUNE - SEPTEMBER H. sessile canopy K. tunicata density 75-100% cover high in areas of high Wave force low, H. sessile cover air temp and UV Understory dominated by intensity high articulated and encrusting coralline algae Understory modified by K. tunicata \ -1-2 yrs following and scouring by H. sessile decline in cover Articulated coralline algae / \ Juvenile H. sessile survivorship Phenolic rich sporelings Blade number and blade undergo rapid growth length increase Encrusting coralline algae and attain a partial refuge H. sessile holdfast cavitation in size Vegetative re-growth H. sessile recruitment from surviving adults Articulated coralline and holdfasts algae act as spatial refuge OCTOBER -FEBRUARY Areas of low H. sessile cover H. sessile adult sporophytes ^ Fugitive algal species become reproductive and release spores. Reproductive and ^ Articulated coralline algae vegetative tissues do not differ in phenolic content ^ Encrusting coralline algae ^Wave force with seasonal storms 4 Chiton density in areas of low cover primarily due to rapid ^ H. sessile adult survivorship emigration but potentially also (Individuals with 1-4 cm due to increased predation holdfasts removed first due to weakened holdfasts) MARCH-JUNE K. tunicata and understory Wave force begins to decrease less dependant upon canopy as air temp., light, and desiccation due to conditions of low light, increases again temperature and desiccation Figure 5.1 Summary model of annual successional trajectory of the temperate low intertidal community dominated by the kelp Hedophyllum sessile and the chiton Katharina tunicata. These communities are typical of wave exposed and semi-exposed coasts of British Columbia, Canada. 'Up' arrows indicate an increase of the associated factor and 'down' arrows indicate a decrease.