Effects of Size-Dependent Predation and Competition on Population and Community Dynamics

Effects of size-dependent predation and competition on population and community dynamics

Karin A. Nilsson

Department of Ecology and Environmental Science Umeå 2010

ISBN: 978-91-7459-070-8

List of papers

I. Nilsson, K. A., Persson, L. and van Kooten, T. (2010). Complete compensation in Daphnia fecundity and stage-specific biomass in response to size-independent mortality Journal of Animal Ecology 79: 871-878.

II. Huss M. and Nilsson, K. A. Experimental evidence for emergent facilitation: Promoting the existence of an invertebrate predator by killing its prey Submitted manuscript.

III. Nilsson, K. A., Lundbäck, S., Postavnicheva-Harri A. and Persson, L. Guppy populations differ in cannibalistic degree and adaption to structural environments Submitted manuscript.

IV. Nilsson, K. A. and Persson L. Cannibalism and resource competition determine population structure and regulation in experimental guppy populations Manuscript.

V. Nilsson, K. A. and Persson L. Refuge availability and within species differences in cannibalism determine population variability and dynamics Manuscript.

VI. Schröder, A., Nilsson, K. A., Persson L., van Kooten T. and Reichstein B. (2009) Invasion success depends on invader body size in a size- structured mixed predation-competition community Journal of Animal Ecology 78: 1152-1162.

Papers I and VI are reproduced with the kind permission from the publishers.

Table of Contents

Table of Contents 5 Abstract 6 Introduction 7 Background 7 Resource competition within populations and overcompensation 7 Predation shapes the size-distribution of prey 8 Cannibalism – intraspecific predation 9 Intraguild predation 11 Questions asked 12 Overview Figure 1 13 Complete compensation (paper I) 14 Evidence for emergent facilitation (paper II) 15 Guppies differ in cannibalistic degree (paper III) 16 Life history characteristics of Poecilia reticulata 16 Cannibalism in Poecilia reticulata 16 Regulation in guppies (paper IV) 17 Experimental setup 17 Testing for density dependence 17 Population differences in biomass and per capita cannibalism 18 Density dependent cannibalism and resource competition 19 Population dynamics in guppies (paper V) 20 Cannibal-driven cycles 20 Cohort competition and the stabilising effect of refuges 21 Mixed interactions and size-dependent invasion success (paper VI) 22 Concluding remarks 23 Shifting bottlenecks 23 Refuge use and cannibalism 23 Evolution of cannibalism 24 Size-dependence 25 Acknowledgements 25 References 25 Thanks 32

Abstract

Most animals grow substantially during their lifetime and change in competitive ability, predatory capacity and their susceptibility to predation as they grow. This thesis addresses the implications of this on regulation and dynamics within populations as well as between population interactions. In size-structured populations either reproduction or maturation may be more limiting. If juveniles are competitively superior, the competitive bottleneck will be in the adults and reproduction will be limiting. Mortality will in this case result in overcompensation in juvenile biomass through increased reproduction. Compensation in biomass was demonstrated in Daphnia pulex populations subjected to size-independent mortality, where juvenile biomass did not decrease when a substantial harvest was imposed due to increase per capita fecundity. This supported that juveniles were superior competitors and that population cycles seen in Daphnia are juvenile-driven. Compensatory responses in biomass may lead to that predators facilitate eachothers existence by feeding on a common prey, a phenomenon coined emergent facilitation. In an experimental test of the mechanism behind emergent facilitation it was demonstrated that the invertebrate predator Bythotrephes longimanus was favoured by thinning of its prey Holopedium gibberum. The thinning mimicked fish predation and targeted large individuals while Bythotrephes preferrs small prey. Size dependent predation also occurs within populations, i.e. cannibalism, were large individuals feed on smaller conspecifics. Two populations of the common guppy (Poecilia reticulata) originating from different environments were demonstrated to differ in cannibalistic degree. Cannibalism was also affected by the presence of refuges and females and juveniles from one population were better adapted to structural complexity than the other. The effects of these differences in cannibalism on population regulation and dynamics were studied in long term population experiments. Both populations were regulated by cannibalism in the absence of refuges, and displayed cannibal-driven cycles with suppression of recruitment and high population variability. The presence of refuges decreased density dependence and population variability and harvesting of large females in the absence of refuges led to population extinctions in the more cannibalistic population. The less cannibalistic population had higher population biomass and stronger density-dependence in the presence of refuges. When refuges were present, cohort competition increased and cycles with short periodicity were seen. Large individuals were not only cannibals, but could successfully prey on other species. Small and large guppies were allowed to invade resident populations of Heterandria formosa. Small invaders failed while large invaders succeeded as predation from large invaders broke up the competitive bottleneck that the resident population imposed on juveniles of the invader.

Keywords: size-structure, cannibalism, resource competition, predation, emergent facilitation, population regulation, population dynamics, overcompensation, density- dependence, cycles

Introduction

Background Most animal taxa grow substantially during their lifetime (Peters 1983, Calder 1984, Werner 1988) and as the size of an individual increases, its competitive ability, susceptibility to predation and efficiency as a predator also changes (Paine 1976, Werner and Gilliam 1984, Ebenman and Persson 1988). These changes have major implications for regulation and dynamics within populations and for interactions with other species. This is becoming increasingly acknowledged throughout the scientific community and with my thesis I hope to contribute to our understanding of the consequences of size- dependent processes in populations.

Resource competition within populations and overcompensation The competition for resources between individuals is one of the most fundamental processes in nature. Concerning interspecific resource competition, the species that is able to sustain itself at the lowest resource level is the best competitor (Tilman 1982). The same is true for within population competitiveness (i.e. intraspecific resource competition), where the size or stage that can sustain itself at the lowest resource level is the superior competitor. The level of resources on which an individual can sustain itself depends on how efficient it is at resource intake and how high its metabolic costs are. Usually the resource intake has a hump shaped relation to size, whereas metabolic costs increase with the size of the individual, depending on the relative scaling of these processes different sizes will have the lowest resource demand (Persson et al. 1998, de Roos et al. 2007a). The body size of the most competitively fit individual determines where the competitive bottleneck in a population will be and which process will be the most regulating. In a size or stage structured population there are typically two regulating processes within a population; reproduction and maturation. If juveniles are superior competitors to adults, the competitive bottleneck is during the adult stage of the lifecycle and reproduction is more limiting. In contrast, when adults are superior competitors maturation is more limiting (de Roos et al. 2007a, 2008a). The maturation and reproduction regulation determines compensatory responses in populations. Recent theoretical insights have shown that size- specific biomass overcompensation can take place in size-structured populations (de Roos et al. 2007a, 2008a). In which stage the compensation occurs depends on whether maturation or reproduction is the more limiting process in the population and the relative change in the two rates with increasing mortality. For example, if juveniles are competitively superior and

adults are the bottleneck in the population, mortality on the juvenile stage may actually result in an increase in the density of the juvenile stage (de Roos et al. 2007, 2008a). This was nicely demonstrated early on by Nicholson (1957) who, by experimentally manipulating food availability in a blowfly population, made either the juvenile or adult stage more limited. When juveniles received little food, the adult density increased with adult harvesting in accordance with maturation regulation governed by juvenile competition. In contrast, when adults received little food, juvenile density increased with adult harvest, which is in accordance with reproduction regulation and high adult competition. Other examples of biomass overcompensation includes experiments with poeciliid fish (Schröder et al. 2009), soil mites (Cameron & Benton 2004), more blowflies (Moe et al. 2002) and flour beetles (Watt 1955) Previous studies on compensation have all used size-selective harvesting. In paper I, we however investigate whether compensatory responses are present in Dapnia pulex by imposing size-independent mortality on experimental populations. The population dynamics also depend on relative competiveness between differently sized individuals and on which process that is more limiting (Persson et al. 1998, de Roos and Persson 2003, de Roos et al. 2008a). Large enough differences in competitiveness between differently sized individuals result in large amplitude cycles driven by competition, referred to as single- generation cycles or cohort cycles (de Roos and Persson 2003). When adults are superior competitors the cycles are “adult-driven” and slow maturation of juveniles is a crucial aspect. When juveniles are superior competitors the cycles are “juvenile-driven” and suppression of adult fecundity is crucial (de Roos and Persson 2003). Population dynamics of Daphnia have been extensively studied, and the nature of single-generation cycles seen has been largely debated (Murdoch and McCauley 1985, de Roos et al. 1997, McCauley et al. 1999, McCauley et al. 2008). It is still not certain which size is the most competitive in a Daphnia population. In paper I we also address the nature of cycles in Daphnia and make our own contribution to the discussion on competitive ability in Daphnia populations.

Predation shapes the size-distribution of prey Mortality inducing biomass overcompensation in a population may also come from a predator. When predators are feeding on a prey population that is size-structured, removal of prey changes the size-structure and overcompensation can occur. In fact, a size-selective predator may, by feeding on its prey, create a size-structure that is favourable for its own existence. This can have the effect that when the density of predators is low they may no longer be able to induce a favourable size-structure in their

prey. This may result in an Allé effect which increases the scope for catastrophic collapses and may give rise to alternative stable states (de Roos and Persson 2002a, de Roos et al. 2002). The emergent Allé effect has also been demonstrated in a natural system. In lake Takvatn, large-scale removal of charr enabled the trout population to increase and when the trout population had established it could maintain the changed size-distribution of the charr via predation (Persson et al. 2007). It has been suggested that this may be a scenario facing the Baltic cod where heavy fishing decreased cod stocks so that they can no longer change the size-structure of their prey in a favourable way (van Leeuwen et al. 2008). By modifying the size structure of its prey, a predator may inadvertently positively affect other predators feeding on different size-classes of prey. This phenomenon, that predator species may help each other to persist by size-selective foraging, inducing a compensatory response in another stage, has been coined emergent facilitation (de Roos et al. 2008b). In paper II we perform an experimental test of the mechanism behind emergent facilitation using the spiny water flea Bythotrephes longimanus feeding primarily on the zooplankton Holopedium gibberum. We mimicked fish predation by selective removal of large zooplankton and followed the response in Bythotrephes populations asking the following question: Will Bythotrephes be more successful when we harvest its prey?

Cannibalism - intraspecific predation Size-dependent predation can also take place within a population through cannibalism. In this case, predator and prey may not only share resources as in an intraguild predation system, but as they belong to the same population, they are also linked through reproduction and maturation. This makes cannibalism a complex interaction that allows for several feedbacks in the system (Claessen et al. 2000, 2004). Three aspects are crucial for the dynamics and regulation of a cannibalistic population; the cannibalistic voracity, the size of offspring and the predation window (Claessen et al. 2004, van Kooten et al. 2010). The predation window describes which sizes of prey that a predator can consume. The larger the size difference is between cannibal and victim the higher the voracity is. However, very small victims can be difficult to handle for a cannibal, creating a lower size limit of predation (Claessen et al. 2004). The lower size limit of the predation window and how it changes as the cannibal grows has been shown to be an important factor when explaining differences in dynamics and regulation between species (Persson et al. 2004). The common guppy, Poecilia reticulata is rather well studied and populations from different predation environments have been shown to differ in a number of life-history characteristics. We investigate whether

guppy populations originating from two different environments differ in cannibalistic voracity in paper III. We also manipulated the availability of refuge for juveniles to hide in and allowed females to feed on juveniles from their own population as well as the other (paper III). In addition we examined which sizes of females that were efficient cannibals and which sizes of juveniles were vulnerable to predation in order to get an estimation of the predation window.

Resource competition and cannibalism, separately or in combination, may regulate populations (Leibold 1996, Holt et al. 1994, Chase et al. 2002). The regulating capacity of cannibalism has been shown in theoretical studies (Ricker 1954, Diekmann et al. 1986, Hastings and Costantino 1991, Cushings 1992). That cannibalism can be regulating or contribute to regulation has also been demonstrated in empirical studies in several taxa, including fish (Fox 1975a, Polis 1981, Claessen et al. 2004, Persson et al. 2003, Byström 2006), salamanders (Wissinger et al. 2010) and insects (Lloyd 1968, Benoit et al. 1998, Fox 1975b, Orr et al. 1990, Leonardsson 1991, Wise 2006). We wanted to investigate the regulating processes in guppy populations and whether the relative contribution of resource competition and cannibalism depended on the cannibalistic voracity of the populations (using the same populations as in paper III). We manipulated the density- dependence and the bottleneck in the population by addition of refuges and by harvesting of large females in paper IV.

Cannibalism can have both a stabilising and a destabilising effect on population dynamics. Populations that display high amplitude cycles driven by competition in the absence of cannibalism may become more stable when cannibalism decreases resource competition. Strong cannibalism on the other hand may induce a different kind of cyclicity based on severe suppression of recruitment (Claessen et al. 2004). Cannibalism is a classical example of density-dependence that creates delayed feedback cycles (Ricker 1957). The scope for cannibalism to affect recruitment is large in the two guppy populations (see above) due to the relatively large offspring size and the low per capita fecundity compared to other species. We predicted there to be a large scope for cannibal driven cycles with strong cannibalism especially in the more cannibalistic population that also has smaller offspring. We analyse the effects of different cannibalistic voracity on population dynamics in paper V. Harvest of large females may have a destabilising effect in a population regulated by cannibalism due to the fact that it will allow pulses of recruits to break through the cannibal regulation (van Kooten et al. 2007, 2010). We

also address the question of whether or not harvesting destabilizes population dynamics in paper V. The presence of refuges in predator-prey systems is generally considered to stabilise population dynamics and decrease interspecific predator efficiency (Rosensweig and Mac Arthur 1963, Stenseth 1980, Krivan 1998). Refuge availability will also reduce cannibalism (Yamagishi 1976, Persson and Eklöv 1996, Benoit et al. 2000). In the case where the cannibalism in a population is so strong that it induces cycles in the absence of refuges, addition of refuges can be hypothesised to stabilise dynamics. In contrast, the presence of refuges for juveniles may be destabilising if it decreases juvenile mortality to such an extent that juveniles may deplete resources and high amplitude cycles driven by competition may arise (de Roos et al. 2002b). We address the question of whether or not refuge availability for juveniles acts stabilising or destabilising on population dynamics in paper V.

Intraguild predation The size of an individual determines its performance both as a cannibal and as a predator on other species. Before a predator is large enough to successfully forage on its prey it may experience strong competition from the prey, creating a recruitment bottleneck in the predator population (Werner and Gilliam 1984, Byström et al. 1998). Predation from large individuals may release predator offspring from competition and enable recruitment to the predator adult stage (Neill 1988, Walters and Kitchell 2001). In systems with intraguild predation (IGP), i.e. in systems when predator and prey are feeding on a common resource (Holt and Polis 1997), the prey must be competitively superior in resource consumption in order for coexistence to take place. In IGP systems alternative states are possible due to the mixed interaction of competition and predation. Under certain conditions, predators are incapable of invading a system with consumers due to severe resource competition. On the other hand, with the same environmental preconditions, prey species might be unable to invade resident predator populations due to high mortality risk caused by high predator densities. Coexistence of prey and predator is possible at intermediate productivity levels, whereas high productivity excludes the prey (Mylius et al. 2001). Most studies on IGP systems have not taken into account size-dependence and food-dependent growth. Size-dependent interactions will further decrease the possibilities for coexistence of IGP predators and prey (van de Wolfshaar 2006). In paper VI we investigate whether invasion success depends on the body size of the invaders. We also vary the productivity of the system. We used small and large guppies invading resident least killifish (Heterandria formosa) populations.

Questions asked

(1) Is overcompensation in biomass possible in a Daphnia population when imposing size-independent mortality? Which stage, if any, will display a compensatory response? Which stage is the most competitive? Are patterns of biomass overcompensation in models affected by including two juvenile stages instead of one? (Paper I)

(2) Can we facilitate the existence of an invertebrate predator by removing its prey? (Paper II)

(3) Do guppies from different populations differ in cannibalistic voracity and how is this affected by the presence of refuges? Which sizes are vulnerable to predation and which sizes are efficient cannibals? (Paper III)

(4) What regulates guppy populations with different cannibalistic voracity? How is regulation affected by the presence of refuges and harvesting of large adults in a cannibalistic population? (Paper IV)

(5) What population dynamics do guppies with different voracity display? What effect does the presence of refuges for juveniles to hide in and harvesting of large females have on dynamics? (Paper V)

(6) Is invasion success dependent on the size of the invader in a intraguild predation system? Does invasion success depend on the productivity of the system? (Paper VI)

Figure 1. Overview

Paper I: Complete compensation

We used an experimental setup of 12 aquaria with populations of Daphnia pulex that were feeding on the algae Scenedesmus obtusiusculus. The algae were continuously flowing into the aquaria generating semi-chemostatic resource dynamics. We imposed a size-independent mortality, equivalent of 20% per day, in 6 of the Daphnia populations to asses the compensatory responses. We sampled the populations twice a week over 13 weeks, to obtain population numbers, size-structure and per capita fecundity. We found complete compensation in juvenile biomass in response to harvesting and the total population fecundity was as high in the mortality treatments as in the controls. The compensation in fecundity was caused by both a higher proportion of fecund females and a larger clutch size under increased mortality. From this we could conclude that the regulating process within the population was reproduction and that juveniles were superior competitors. There were no differences in resource levels between treatments, which is in line with juveniles being superior competitors and setting the resource level. We also developed a stage-structured biomass model with two juvenile stages and one adult stage. We found that both stages of juveniles have to be superior to adults in terms of resource competition for the compensatory response to take place in juvenile biomass. Hence, the worst, not the best, competitor decides were the bottleneck in the population is. The result that juveniles were superior competitors to adults has implications for population dynamics and supports that the cohort cycles seen in Daphnia populations are juvenile-driven. There has been an ongoing debate concerning the nature of cohort cycles in Daphnia. Early papers suggest juveniles to be superior competitors (McCauley et al. 1987, de Roos et al. 1990, McCauley et al. 1999, Murdoch et al. 2003) and juveniles suppressing adult fecundity and their own growth by reducing resource levels was viewed as the major mechanism behind cohort cycles. This corresponds to a reproduction regulated system. Energetic models on the other hand have suggested large juveniles to be superior competitors (Gurney et al. 1990, de Roos et al. 1997), and recent studies and models have suggested the dynamics to be adult-driven (McCauley et al. 2008). Our results, however, support the findings that juveniles are superior competitors and cycles juvenile-driven. Since Daphnia is a key grazer in many aquatic systems, it is important to understand how it responds to increased mortality and the potential effects that this may have on food webs. The potential for emergent facilitation is one scenario to be considered.

Paper II: Evidence for emergent facilitation

To test the mechanism behind emergent facilitation we used plastic bag enclosures in a lake (9 m deep, ø 1.6 m) filled with lake water that was filtered to contain only phytoplankton. The bags were inoculated with herbivorous zooplankton from the lake and after 2 weeks we performed selective removal of large zooplankton in the enclosures with the help of a large net. This was to mimic fish predation, as planktivorous fish are known to selectively feed on large zooplankton (mainly adult Holopedium) (Brooks and Dodson 1965, Zaret 1980). We then introduced 200 individuals of the spiny water flea Bythotrephes longimanus into each enclosure and we subsequently followed the population response of Bythotrephes. We sampled the zooplankton and rotifer community as well as chlorophyll levels over the following 40 days. The density of the juvenile stage of Holopedium increased as a response to harvesting. We argue that this was a result of increased per capita fecundity of adult Holopedium, which in turn was the result of competitive release following harvest. By netting out large zooplankton we induced a compensatory response in the Holopedium that was the dominating zooplankton in the enclosures and constituted 85% of the average zooplankton biomass. This had a positive effect on the Bythotrephes populations that had higher densities in the harvest treatment. Chlorophyll levels were not affected by harvesting, which is in line with that small Holopedium would be superior competitors (as for Daphnia in paper I). Rotifers also responded positively to the harvesting, likely due to competitive release from large Holopedium. Rotifers are not preferred food items for Bythotrephes (Hovius et al. 2006), and were hence not the reason for higher Bythotrephes densities in the harvest treatment. These results suggest that fish predation may be facilitating the existence of Bythotrephes. To our knowledge, this is the first experimental test of the mechanism behind emergent facilitation (although the thinning experiment in Lake Takvatn in Persson et al. 2007 is somewhat related). Emergent facilitation may be important in the light of predator extinctions since it explains how the extinction of one predator can have a negative effect on other predators feeding on the same prey.

Paper III: Guppies differ in cannibalistic degree

Life-history characteristics of Poecilia reticulata The common guppy is a small poecilid fish that lives in freshwater ponds and streams. In streams in Trinidad (West Indies) guppies can be found in high predation or low predation environments. In high predation environments the guppies coexist with predatory cichlids and migration barriers such as waterfalls prevent these predators from moving upstream creating low predation environments (Rodd and Reznick 1997). Guppy life-history traits are affected by predation regime and guppies from high predation localities mature at younger ages and smaller sizes, they produce smaller offspring and have a higher rate of investment in reproduction than low predation guppies (Reznick and Endler 1982, Reznick et al. 1996). Guppies in high predation environments also experience higher growth rates than guppies in low predation environments reflecting differences in resource limitation (Reznick et al. 2001). The life-history of guppies has been shown to differ in a number of aspects, but any differences in cannibalism are yet to be explored.

Cannibalism in Poecilia reticulata We used one population originating from a high predation environment (Turure) and one population from a low predation environment (Quare) to investigate if guppies originating from different environments differed in their degree of cannibalism. We put large females together with small juveniles in aquaria and assessed the number of juveniles at regular intervals over a 48 h period. We varied the availability of refuges for the juveniles to hide in and we allowed females from both populations to feed on juveniles from their own population as well as from the other. Low predation (Quare) females were more efficient cannibals and low predation juveniles were better at avoiding cannibalism compared to high predation guppies when no refuges were present. However, the population specific cannibalism was higher in the Turure population. The addition of refuges caused a decrease in cannibalism when females from the low predation population were feeding on juveniles from the high predation population. In contrast, cannibalism increased with the addition of refuges for all other combinations. This is in contrast to most studies were predation decreases with the presence of refuges (Stenseth 1980, Diehl 1988, Krivan 1998). The high predation females were also superior cannibals and the high predation juveniles were better at escaping cannibalism than were the low predation guppies with refuges present. The high predation guppies (Turure) may be more adapted to structural complexity due to the predation pressure they experience in their natural environment. Accordingly, guppies

are more abundant close to margins (were there should be more cover) in high predation streams and positioned more in the centre of streams in low predation environments (Reznick et al. 2001). Refuge use did not differ between juvenile populations and juveniles seemed to be able to respond to if a female was cannibalistic. We also assessed the juvenile’s refuge use in the absence of females and found that juveniles hardly used the refuges at all when no females were present. There was a strong size-dependence in the interaction, the smaller the victims and the larger the cannibal, the higher the voracity. Estimations of the predation window were used in paper V. Note that the within population cannibalism was higher in the Turure population when no refuges were present and higher in Quare when refuges were present. However, the fact that the Turure population has smaller offspring size will likely increase the cannibalism in Turure relative to Quare in breeding populations.

Paper IV: Regulation in guppies

Experimental setup We used the same two guppy populations as were used in paper III in a long term population experiment where 40 aquaria, each 132 L, attached to a circulation system were used. 82 guppies were stocked in each aquaria and automatic feeding of guppies took place 8 times per day when fish food was dropped on the water surface. We varied the availability of refuges for juveniles and harvesting of large females. There were 5 replicates of each treatment in a full factorial design as follows: controls, harvesting, refuge, refuge and harvesting. In the treatments with refuges, 4 refuges of green plastic fibre were each spread over a 5 L volume. Harvesting was performed on females larger than 25 mm, shown to be efficient cannibals (paper III). Sampling took place every fifth week by removing all individuals, sorting them and photographing them. The experiment lasted for 812 days (the population dynamics are analysed in paper V). Juveniles were divided into a vulnerable and invulnerable stages based on paper III. We also calculated a potential cannibalistic pressure (non population specific) based on the size-specific cannibalism in paper III.

Testing for density dependence We used the Ricker function (y= a*x (-b*x)) to relate reproductive biomass and estimated cannibalistic pressure at time t (x) to the density of non-vulnerable juveniles (i.e. recruits) at time t+1 (y). The parameter b reflects the density

dependent and parameter a the density independent aspect of the relationship. The relationship between vulnerable and invulnerable juveniles was investigated by fitting a linear function (y = c * x) and a quadratic polynomial function (y = c * x + d * x2) to vulnerable juveniles at time t (x) and non-vulnerable juveniles at time t+1 (y). We compared fits for the two models for each treatment using the Akaike criteria to asses if the quadratic function provided a better fit, which we related to the amount of density dependence in the relationship.

Population differences in biomass and per capita cannibalism The Quare population had a higher total biomass and total density than the Turure population. Harvesting resulted in lower total biomass in both populations while the presence of refuges resulted in higher biomass. The biomass of reproductive females was also higher in the Quare populations and when refuges were present and lower when the populations were harvested. The density of vulnerable juveniles and non-vulnerable juveniles was higher in the Quare populations and when refuges were present. This suggests that cannibalism was overall more prominent in the Turure populations. The estimated cannibalistic pressure was higher in the Quare populations than in the Turure populations and decreased with harvest. However, the actual cannibalism also depends on the population specific cannibal voracity which was higher in the Turure populations. Parameter a in the Ricker function is a reflection of both per capita fecundity and per capita cannibalism without any density dependent components. The parameter a was higher for the Quare populations than for the Turure populations. Considering the higher intrinsic reproductive capacity in the Turure population this supports that the per capita fecundity was higher in the Turure population. The presence of refuges resulted in an overall increase in parameter a, which supports that cannibalism decreased with the presence of refuges.

In general harvesting tended to result in higher values of the parameter a which could be related to increased reproduction through competitive release, although this was not the case for the Turure populations without refuges. All 5 replicates of the Turure harvest treatment went extinct. We suggest that this was because harvesting reduced reproduction more than it reduced cannibalism. This could be related to the size-specific scaling of fecundity in the Turure population. When refuges were present harvesting had only a small effect as few females grew large enough to be harvested.

Density dependent cannibalism and resource competition With increasing density of large females both cannibalism and reproduction increases and results in negative density-dependence in the reproductive biomass- vulnerable juvenile relationship, captured in the parameter b in the Ricker function. In the absence of refuges, the Turure population showed stronger density dependence (higher b values) than the Quare population. We suggest that cannibalism was the main factor causing this strong density dependence and that cannibalism was limiting the inflow of individuals into the adult stage. Even if the density-dependence was not as strong in the Quare control populations, we suggest that cannibalism was the main regulating process also in this treatment. There were however strong indications of resource competition as well (see also paper V). For the Turure population, density-dependence became weaker with the presence of refuges, while the Quare population showed stronger density dependence when refuges were present. We suggest that the presence of refuges decreased cannibalism in both populations but that cannibalism was still keeping the biomasses and the resource competition in the Turure population at a lower level. The stronger resource competition in the refuge treatments lead to stronger density dependence, likely through decreased fecundity of females. Additionally, it affected the juvenile growth and survival. In the vulnerable - non-vulnerable juvenile relationship there was strong negative density-dependence in all refuge treatments but no density dependence without refuges present. This difference between refuge treatments and no refuge treatments indicates that it was resource competition that affected juvenile growth and survival. The fact that very few females became large enough to be harvested in the presence of refuges suggests substantial resource limitation among adults which most likely reduced their per capita reproduction. Hence, both adults and juveniles were affected by density-dependent resource competition in the refuge treatments. Empirical data on scaling of vital rates suggests that small fish individuals are superior exploitative competitors to large fish and that in populations where adults and juveniles compete for a common resource, adults are the competitive bottleneck (de Roos et al. 2007). This would lead to reproduction regulation in populations. Cannibalism is one process that can alter the bottleneck in a population. Cannibalism per se, releases the cannibalistic stage from severe resource competition by decreasing the number of juveniles and can also be a significant energy gain for the cannibal (Claessen et al. 2004, Claessen and de Roos 2003). An indirect effect of cannibalism is that the risk of being eaten may affect juvenile behaviour.

The juveniles in our study most likely reduced their activity and spent more time in refuges which decreased their intake rate of the common resource. Hence, the adults did not experience as much competition as could have been expected. Theoretical studies on size-structured populations have shown that small juveniles can accumulate in refuges and experience strong competition leading to death by starvation (de Roos et al. 2002b, Persson and de Roos 2003). Additionally, reduced growth in the vulnerable stage will cause the juveniles to spend a longer time at sizes where they are susceptible to cannibalism (Rice et al. 1997). Hence the competitive bottleneck may have been shifted (at least partly) to the juvenile stage due to the behaviour of juveniles.

Paper V: Population dynamics in guppies

This paper is based on the same experiment as in paper IV but is focusing on the population dynamics. We used coefficient of variation (standard deviation/mean density) as a measure of population variability. Time-series data was also analysed with autocorrelations and cross-correlation for different stages.

Cannibal-driven cycles There was high population variability in the control treatments. Cycles with a periodicity of approximately 75 weeks appeared in total density for the Quare populations, while the periodicity in the Turure populations was longer, 90 weeks. High juvenile densities did not co-occur with high cannibal densities in control treatments suggesting that cannibalism had a strong effect on recruitment. In cannibalistic cycles, cannibals depress recruitment, and it is not until the old cannibals die off that a new recruitment pulse is possible. This causes the cycle period to be substantially longer than the generation time. The periodicity of the cycles in the guppy control treatments was in the order of 2 generations which is in accordance with what is expected for cannibalistic cycles. Empirical examples of cannibalistic cycles with a periodicity substantially exceeding the generation time includes salamander and leech populations (Wissinger et al. 2010, Elliott 2004). We suggest that the difference in cycle period between populations is related to the overall lower cannibalism in the Quare control populations. In addition, Quare juveniles are larger at birth and will hence spend a shorter time in the predation window, which may contribute to a lower level of cannibalism.

Harvesting induced a higher variability in total population density in both populations when no refuges were present. As mentioned, several harvested Turure population without refuges present went extinct which could be

related to that harvest removed a large part of the reproductive potential compared to the decrease in cannibalism it caused. The Quare harvest treatment also showed a higher variation in population density than control treatments. Models including size-structure and resource competition have shown that harvest of large individuals in cannibalistic populations may lead to a destabilisation of population dynamics (van Kooten et al. 2007, 2010). That harvest of adults can allow for recruitment in a cannibalistic system has been shown in pike (Alm 1951, Sharma and Borgstrøm 2008). There was some support for this in the Quare harvest treatment, although our time- series are too short for us to be conclusive concerning the dynamics. Few large cannibals were harvested in the refuge and harvest treatments. This can be related to increased resource competition because of higher total densities made it more difficult for cannibals to grow as large as the harvesting limit. As a consequence, harvest in the refuge treatments had only minor effects. Cross-correlations with juveniles and intermediately sized individuals indicated fast growth in control treatments, supporting that resource competition was lower in the control treatments than in refuge treatments. (see also paper IV).

Cohort competition and the stabilising effect of refuges Refuge availability clearly had a stabilising effect on dynamics and decreased population variability. Refuge addition lead to a decrease in cannibalism, and stabilised the cannibal-driven cycles. However, there were also indications of a periodicity with a relatively shorter period length when refuges were present and densities of vulnerable juveniles and cannibals were highly synchronised. This, in combination with overall high densities in refuge treatments points towards a cyclicity induced by resource competition among refuging juveniles in the Quare refuge treatments. In the absence of cannibalism, size-structured consumer-resource systems have been shown to often be characterized by high amplitude generation cycles driven by competition (de Roos 1997, Persson et al. 1998, de Roos and Persson 2003, as discussed in the Daphnia case, paper I). The dynamics did not share all characteristics of a typical single- generation cycle and our results suggest that cannibal thinning of recruits may prevent juvenile cohorts from depressing the shared resource level to such an extent that generation cycles emerge. In addition, juvenile utilisation of the refuges may have lead to intensified resource competition within the refuge and low juvenile survival, further decreasing the possibility for juveniles to induce starvation in the adult stage (also discussed in paper IV).

Paper VI: Mixed interactions and size-dependent invasion success

In a laboratory experiment we allowed either small or large guppies (from the Turure population) to invade resident populations of another poecilid fish, the least killifish Heterandria formosa. 2 large females and 2 males or 5 juveniles were introduced into each Heterandria population. We used 3 productivity levels in addition to the small and large invader treatment, with 4 replicates each resulting in a total of 24 aquaria (80 L). In addition we also allowed guppies to invade aquaria without Heterandria to ensure that they could successfully reside in the system at the lowest productivity level (small and large invaders, 8 additional replicates). In separate experiments we estimated attack rates when large guppies were feeding on small Hetrandria and when large Heterandria were feeding on small guppies. Attack rates were used to assess the scope for predation in the mixed systems. Small guppies failed to invade the Heterandria populations. This was most likely due to the competitive recruitment bottleneck imposed on them by the resident population. Large guppies, on the other hand, were successful invaders and drove the resident populations to extinction. They were preying on the Heterandria juveniles and decreased densities of the resident population which reduced the competitive bottleneck experienced by their offspring and enabled recruitment. The outcome of the invasions was not dependent of productivity level which agrees with theoretical studies concerning intraguild predation where size-dependent growth has been taken into account (van der Wolfshaar 2006). Our results strongly suggest that the size-structure of invaders affects their ability to invade. This aspect of invasions has so far been neglected in intraguild predation systems and is poorly studied in general. It may however be important in, for example, explaining the lack of recovery in exploited fish stocks.

Concluding remarks

Shifting bottlenecks As I hope is evident by now, it is crucial for a population at which size individuals experience a competitive bottleneck. The scaling of vital rates is important in determining which stage is the most competitive (or rather the inferior competitor, see paper I) which in turn determines the compensatory biomass responses, as the ones demonstrated in paper I-II. However, the bottleneck in a population can be changed by several factors, such as cannibalism, interference, and diet differences between juveniles and adults. It is not a requirement that juveniles and adults share the same resource for overcompensation to occur. In the cases when juveniles and adults do not share the same resources, the relative supply of juvenile and adult resources will also affect in which stage the competitive bottleneck in the population will be present (de Roos et al. 2007, Schreiber and Rudolf 2008, Schellekens 2010).

Refuge use and cannibalism It is quite likely that small guppies indeed are superior competitors to large guppies in the sense that they can sustain themselves on a lower resource level, even though strong cannibalism was able to change the bottleneck in the experimental populations (paper IV-V). For the treatments where no refuges were present we suggested that maturation was the limiting process, not in the sense that strong resource competition made it difficult for small juveniles to grow to adulthood (as expected in a population with inferior juveniles and resource competition only), but rather that cannibalism kept down the inflow of individuals to the adult stage. When refuges were present we believe that resource competition affected fecundity of adults, a result which is in line with the study by Schröder et al. (2009) where biomass overcompensation in the juvenile stages of Heterandria was observed. This suggests that these populations were regulated by reproduction. However, there was reduced growth and survival of juveniles in the refuge treatments in the guppy experiment, a pattern that is likely due to the risk of high mortality forcing juveniles to spend time inside the refuges where food availability was low. Hence, the bottleneck in the guppy populations was affected by cannibalism and resource competition acting on both the adult and the juvenile stage, and was furthermore affected by population origin and refuge availability. We are still not certain what the main limiting process in the refuge treatments was, however, experimental manipulations could be used to investigate this further. Another aspect of the cannibalistic interaction is the energy gain for cannibals. Decreased levels of cannibalism due to refuges may actually

increase the energy gain for the cannibal. If the average victim can grow to a larger size before it is consumed, the energy gain may be so substantial that it results in increased reproduction. Theoretical studies have shown that energy gain in cannibalistic populations can give rise to alternative stable states (Claessen et al. 2003). However, overcompensation in cannibalistic population has not been fully analysed, this also concerns the effects of the availability of refuges for juveniles to hide in. In the study by de Roos et al. (2002b), mortality in different habitats was only modelled as different background mortality rates and did not include any dynamic predator or cannibal.

There was a discrepancy between the results in paper III and IV-V concerning the effect of refuges on cannibalism. In the experiment in paper III there was no food available for juveniles at all, and it is likely that hunger affected their behaviour and made them spend a larger fraction of their time outside the refuges (compared to in the population experiment). Moreover, the cannibal-victim interaction may change when we increase the scale of the experiment, resulting in lower cannibal voracity in larger aquaria. The effects we see from refuge addition (and perhaps from increasing size of the aquaria) may be applicable in a more general sense and comparable to when increasing the scale of a system, for example when increasing lake size.

Evolution of cannibalism In their natural environment in Trinidad guppies are a prey species and they experience predation that has shaped their life-history (Magurran 1998, Reznick et al. 2001). Cannibalistic voracity may be yet another example of a life-history characteristic that has evolved in relation to the predation environment. We however, only investigate two populations; one population (Turure) originating from a high predation environment and the other population (Quare) originating from a low predation environment, and cannot make any strong conclusions about this. In fact, the difference in cannibalism between populations may reflect differences between streams and more populations need to be investigated to resolve this matter (Nilsson et al. in prep). The questions why and how cannibalism has evolved in relation to predation environment are also worth addressing. It could be so that cannibalistic voracity is just a secondary property and an adaptation to the food availability in an environment (other than conspecific prey). We have shown that differences in cannibalistic voracity within the same species can have major effects. However, we do not know what relevance cannibalism and the population dynamics seen in this study has in natural systems, but if cannibalism is indeed acting in natural populations, there is potential for interplay between evolution and population dynamics.

Not only cannibalism but also evolutionary aspects of both facilitation and overcompensation, for example in relation to apparent competition, should be worth addressing.

Size-dependence As demonstrated in this thesis, size-dependent responses can have major effect on populations and species interactions that at first may seem counterintuitive. It needs to be investigated how common these kinds of responses are in natural systems, for which organisms they are present and under what conditions they occur. Most organisms grow substantially during their life-time and experience food-dependent development and growth (Werner and Gilliam 1984, Ebenman and Persson 1988) so there is certainly a large scope for compensatory responses in biomass and other size- dependent phenomenon to be present in many systems and also have profound effects on management practices.

Acknowledgements I would like to thank my dear friends Jenny Ask, Melanie Monroe and Gunnar Öhlund for commenting on the thesis summary. The work in this thesis received financial support from the Swedish Research Council, the Knut and Alice Wallenberg Foundation and the Kempe Foundation.

References

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Thanks

Lennart, first and foremost I would like to thank you for being precisely Lennart Persson, with all its consequences. I have really learnt a lot from you and I am truly happy that you took me on as a PhD student. Thank you for not telling me what to do (most of the time), and for being right when you do tell me what to do. Your clarity of vision has left imprints in my head, some people may mistake it for a brain damage, but I am very grateful. Another person that made a great impression is my friend and assistant supervisor Tobias van Kooten, I miss you (and Irina!), our discussion and your sense of humour, but see you soon. Magnus Huss, we have had so many good laughs and violent discussions ever since we started to study here in Umeå, you are not away enough for me to miss that much you yet, I hope it stays that way, thanks for all your help! ….Per Ask, for the much needed trips to Abisko, for your companionship and your safe and Norrlandic character and you Jenny Ask for being such a lovely person and supporting me in so many things. ….. Arne Shröder for your positive German attitude, for dinners and good times, for introducing Esther (!) and for being my predecessor in the poecilid business and Birte Reichstein for taking over the torch and being such a sweet personality. …..Pelle Byström and the old-timers Jens Andersson and Johan Lövgren for being good examples and very agreeable people in general. Mårten Söderquist, now in retrospect I can admit that it is not that important to clean away all the guppy poo, I have realised that now. Thanks for help with building up the guppy experiment and everything else. ….other people that helped out in the basement, Blair Daniels, Daniel Lusetti, Frans Olofsson, Krister Mattson and especially Emma Göthe and Sara Rundqvist for additional moral support. … Sofi Lundbäck and Alexandra Postavnicheva-Harri, good work with the guppies! …Therese and other students for showing me the joy of teaching. ….William Larsson, for saving me in various messes at various hours and Lars and Gunnar Lundmark for making the feeders work. … David Reznick, for always being friendly and helpful when I ask more or less relevant questions about guppies, for sending me fish and helping out with my trip to Trinidad. Mandy Banet, you are truly a good companion in a rainforest stream, I would like to thank you, Jon Svendsen, Christel Thunell and the lovely Ramlal family for all your help and for making the time in Trinidad wonderful. ….Andre de Roos for being a constant source of inspiration (and annoyance), Tim Schellekens for you company and sense of humour, Annike van Leuwen for your amazing abilities in sorting zooplankton, it has always been really nice when you have visited.

….scientists from all over visiting our department and LEREC workshops, the Uppsala people, Anja Wensel, Christoph Jaeger, Toni Liess, Mats Jansson, Seb Diehl and other LEREC addicts. I have had the great fortune to share office with two of my favourite people on this planet, Carolyn Faithful and Maria Lundgren, thank you for all the good talks we had and all the fun, I would happily run or roll down a mountain with you any day. Two of my other favourite people are Gunnar Öhlund and Lotta Ström, I have so much to thank you for, but here I will restrict myself to thanking you for feeding me jam-cookies and taking me on fishing trips when I need it the most. Gunnar also gets an extra star for being willing to discuss life-history evolution day and night. ….Melanie Monroe for teaching me about floaties and various things, Kristin Dahlgren for being such a good friend (and Anders!), Martin Lind for your abilities in describing science-art-wine-carex, Åsa Berglund and David Hall for always making me in a good mood, Patrik Blomberg for being my ski-pimp, Anna Dietrich (and Javier!) for fiestas, Micke J, Maano A, Peter M, Henrik S, Carolina T, Emma Å, Martin F, Pieter D, Fredrik L, Åsa G, Viktora P, Anna JC, Ullis, Jugga, Darius and many more… ….ALL new and old PhD students for all the good wine, German beer, barbeques, dinners, parties, dancing, painting, pottery, ski-trips, jazz, punk and opera we shared (well, there were some scientific discussions too). ……Tommy Olsson, Anders Nilsson, the rest of the teachers and Birgitta Nordin for guiding me (back in the days) and all other students. ….Lauri for being truly inspirational, and the rest of the Oksanen family for their generosity. ….Björn Malmqvist and Brendan McKie for the time I spent in the stream ecology group (is smashing up ice with a sledge hammer singing hearts on fire a good inspiration for doing science? -yes). ….Frank Johansson and Johan Olofsson for being great guys in general and for providing advice on such things as statistic, how to defend yourself against knifing, tiny mountain flowers and my study plan. …all personalities at EMG, Philosophy-Göran, Empathy-Bent, chief Kicki, Ulla C-G, Agneta A, Stefan E, our flower girl Katarina S, Per H, Johanna Ö, the guppy personality Tomas B, our singing administrators, charming post- docs, enduring computer fixers and the whole of EMG for being so great!

Tack mamma, pappa, alla mina underbara syskon, annan familj och vänner som påminner mig om att det finns tjäderskogar att vandra i, älvar att paddla i, gotländska stränder, jämtländska fjällhedar, våldsam träning, obskyra filmer, bastubadning, dykning och allt annat. Tack Mathias, du är bäst! /Karin