Cascading effects from predator removal depend on resource availability in a benthic food web

We tested joint effects of predator loss and increased resource availability on the grazers’ trophic level and the propagation of trophic interactions in a benthic food web by excluding larger predatory fish from cages and manipulating nutrients in the coastal zone of the Baltic Sea. The combination of nutrient enrichment and excluding larger predators induced an increase in medium-sized predatory fish (three-spined stickleback). The meso-predator fish in turn did not change the total abundance of the invertebrate herbivores, but did cause a substantial shift in their community composition towards the dominance of gastropods by reducing amphipods by 40–60%, while gastropods were left unchanged. The shift in grazer composition generated a 23 times higher producer biomass, but only under nutrient enrichment. Our results show that top-predator declines can substantially shift the species composition at the grazers’ level, but that cascading effects on producers by a trophic cascade strongly depend on resource availability.     

The community effects of phenotypic and genetic variation within a predator population

Natural populations are heterogeneous mixtures of individuals differing in physiology, morphology, and behavior. Despite the ubiquity of phenotypic variation within natural populations, its effects on the dynamics of ecological communities are not well understood. Here, we use a quantitative genetics framework to examine how phenotypic variation in a predator affects the outcome of apparent competition between its two prey species. Classical apparent competition theory predicts that prey have reciprocally negative effects on each other. The addition of phenotypic trait variation in predation can marginalize these negative effects, mediate coexistence, or generate positive indirect effects between the prey species. Long-term coexistence or facilitation, however, can be preceded by long transients of extinction risk whenever the heritability of phenotypic variation is low. Greater heritability can circumvent these ecological transients but also can generate oscillatory and chaotic dynamics. These dramatic changes in ecological outcomes, in the sign of indirect effects, and in stability suggest that studies which ignore intraspecific trait variation may reach fundamentally incorrect conclusions regarding ecological dynamics.     

Counterintuitive density-dependent growth in a long-lived vertebrate after removal of nest predators

Examining the phenotypic and genetic underpinnings of life-history variation in long-lived organisms is central to the study of life-history evolution. Juvenile growth and survival are often density dependent in reptiles, and theory predicts the evolution of slow growth in response to low resources (resource-limiting hypothesis), such as under densely populated conditions. However, rapid growth is predicted when exceeding some critical body size reduces the risk of mortality (mortality hypothesis). Here we present results of paired, large-scale, five-year field experiments to identify causes of variation in individual growth and survival rates of an Australian turtle (Emydura macquarii) prior to maturity. To distinguish between these competing hypotheses, we reduced nest predators in two populations and retained a control population to create variation in juvenile density by altering recruitment levels. We also conducted a complementary split-clutch field-transplant experiment to explore the impact of incubation temperature (25 degrees or 30 degrees C), nest predator level (low or high), and clutch size on juvenile growth and survival. Juveniles in high-recruitment (predator removal) populations were not resource limited, growing more rapidly than young turtles in the control populations. Our experiments also revealed a remarkably long-term impact of the thermal conditions experienced during embryonic development on growth of turtles prior to maturity. Moreover, this thermal effect was manifested in turtles approaching maturity, rather than in turtles closer to hatching, and was dependent on population density in the post-hatching rearing environment. This apparent phenotypic plasticity in growth complements our observation of a strong, positive genetic correlation between individual body size in the experimental and control populations over the first five years of life (rG - +0.77). Thus, these Australian pleurodiran turtles have the impressive capacity to acclimate plastically to major demographic perturbations and enjoy the longer-term potential to evolve adaptively to maintain viability.     

Intraspecific variation in a predator affects community structure and cascading trophic interactions

Intraspecific phenotypic variation in ecologically important traits is widespread and important for evolutionary processes, but its effects on community and ecosystem processes are poorly understood. We use life history differences among populations of alewives, Alosa pseudoharengus, to test the effects of intraspecific phenotypic variation in a predator on pelagic zooplankton community structure and the strength of cascading trophic interactions. We focus on the effects of differences in (1) the duration of residence in fresh water (either seasonal or year-round) and (2) differences in foraging morphology, both of which may strongly influence interactions between alewives and their prey. We measured zooplankton community structure, algal biomass, and spring total phosphorus in lakes that contained landlocked, anadromous, or no alewives. Both the duration of residence and the intraspecific variation in foraging morphology strongly influenced zooplankton community structure. Lakes with landlocked alewives had small-bodied zooplankton year-round, and lakes with no alewives had large-bodied zooplankton year-round. In contrast, zooplankton communities in lakes with anadromous alewives cycled between large-bodied zooplankton in the winter and spring and small-bodied zooplankton in the summer. In summer, differences in feeding morphology of alewives caused zooplankton biomass to be lower and body size to be smaller in lakes with anadromous alewives than in lakes with landlocked alewives. Furthermore, intraspecific variation altered the strength of the trophic cascade caused by alewives. Our results demonstrate that intraspecific phenotypic variation of predators can regulate community structure and ecosystem processes by modifying the form and strength of complex trophic interactions.     

Variable effects of a generalist parasitoid on a biocontrol seed predator and its target weed

Biological control (the importation of enemies from an invader's native range) is often considered our best chance of controlling the most widespread invaders. Ideally, the agent reduces invader abundance to some acceptably low level, and the two coexist at low density with the agent providing continuous control over the long-term. But the outcome may be complicated when the agent is attacked by native predators and parasites. We used a spatially explicit, discrete-time, individual-based, coupled plant-seed predator-parasitoid model to estimate the impact of the biocontrol agent Eustenopus villosus (a seed predator) on the invasive, annual weed Centaurea solstitialis, both with and without the generalist parasitoid Pyemotes tritici. We estimated the agent's ability to reduce plant density, spread rate, and population growth rate over 50 years. We used long-term demographic data from two sites in central California, USA, to parameterize the model and assess how populations in different climatic zones might respond differently to the agent and the parasitoid. We found that the biocontrol agent reduced plant density (relative to predictions for an uncontrolled invasion), but its impact on the invader's spread rate was modest and inconsistent. The agent had no long-term impact on population growth rate (lambda). Parasitism caused a trophic cascade, the strength of which varied between sites. At our coastal site, the parasitoid entirely eliminated the impact of the agent on the plant. At our Central Valley site, even when parasitized, the agent significantly reduced plant density and spread rate over several decades (although to a lesser degree than when it was not parasitized), but not invader lambda. Surprisingly, we also found that the length of time the invader was allowed to spread across the landscape prior to introducing the agent (5, 25, or 50 years) had little influence over its ability to control the weed in the long-term. This is encouraging news for land managers attempting to control invasive plants that have already established widespread, high-density populations. Unfortunately, our results also show that attack by the native generalist parasitoid had a larger influence over how effectively the agent reduced invader performance.     

Effect of canopy removal on a rocky shore community metabolism and structure

The effects of the permanent removal of the canopy-forming alga Fucus serratus was studied in terms of both functioning and diversity on a mid–low rocky shore ecosystem in the south-west English Channel (48°N 43.686′, 3°W 59.282′). Ecosystem functioning was examined as net or gross primary productivity (NPP or GPP) and respiration (Resp) measured through CO₂ fluxes. Diversity was examined as number and composition of species. Measurements were performed in situ, during emersion times, without altering target assemblages. The experiment was designed with two treatments [control (C) or canopy removed (CR)] and five replicates, and was conducted over an 18-month period (from February 2006 to August 2007) to integrate the seasonal variability. The mean GPP and Resp were severely reduced in CR treatment compared to control throughout the survey. The mean NPP was not affected at first, due to the development of opportunistic green algae, but was drastically reduced after 9 months of experiment. The canopy removal affected neither the number of species nor their distribution among trophic groups, and the algal community was only slightly affected. The abundance and biomass of mobile invertebrates, however, were greatly reduced in the absence of canopy. This indicates an important effect of the dominant alga on the higher trophic levels of the community. At this tidal level, the canopy did not seem to affect the community by dampening the environmental stress but by providing food, habitat or both.     

Competitive exclusion within the predator community influences the distribution of a threatened prey species

While much effort has been made to quantify how landscape composition influences the distribution of species, the possibility that geographical differences in species interactions might affect species distributions has received less attention. Investigating a predator-prey setting in a boreal forest ecosystem, we empirically show that large-scale differences in the predator community structure and small-scale competitive exclusion among predators affect the local distribution of a threatened forest specialist more than does landscape composition. Consequently, even though the landscape parameters affecting Siberian flying squirrel (Pteromys volans) distribution (prey) did not differ between nest sites of the predators Northern Goshawks (Accipiter gentilis) and Ural Owls (Strix uralensis), flying squirrels were heterospecifically attracted by goshawks in a region where both predator species were present. No such effect was found in another region where Ural Owls were absent. These results provide evidence that differences in species interactions over large spatial scales may be a major force influencing the distribution and abundance patterns of species. On the basis of these findings, we suspect that subtle species interactions might be a central reason why landscape models constructed to predict species distributions often fail when applied to wider geographical scales.     

Non-consumptive effects of predator presence on copepod reproduction: insights from a mesocosm experiment

Reproduction in planktonic animals depends on numerous biotic and abiotic factors. One of them is predation pressure, which can have both direct consumptive effects on population density and sex ratio, and non-consumptive effects, for example on mating and migration behaviour. In copepods, predator vulnerability depends on their sex, motility pattern and mating behaviour. Therefore, copepods can be affected at multiple stages during the mating process. We investigated the reproductive dynamics of the estuarine copepod Eurytemora affinis in the presence and absence of its predator the mysid Neomysis integer in a mesocosm experiment. We found that the proportion of ovigerous females decreased in the presence of predators. This shift was not caused by differential predation as the absolute number of females was unaffected by mysid presence. Presence of predators reduced the ratio of males to non-ovigerous females, but not by predation of males. Our combined results suggest that the shift from ovigerous to non-ovigerous females under the presence of predators was caused by either actively delayed egg production or by shedding of egg sacs. Nauplii production was initially suppressed in the predation treatment, but increased towards the end of the experiment. The proportion of fertilized females was similar in both treatments, but constantly fell behind model predictions using a random mating model. Our results highlight the importance of non-consumptive effects of predators on copepod reproduction and hence on population dynamics.     

A review of predator diet effects on prey defensive responses

Numerous studies have examined how predator diets influence prey responses to predation risk, but the role predator diet plays in modulating prey responses remains equivocal. We reviewed 405 predator–prey studies in 109 published articles that investigated changes in prey responses when predators consumed different prey items. In 54 % of reviewed studies, prey responses were influenced by predator diet. The value of responding based on a predator’s recent diet increased when predators specialized more strongly on particular prey species, which may create patterns in diet cue use among prey depending upon whether they are preyed upon by generalist or specialist predators. Further, prey can alleviate costs or accrue greater benefits using diet cues as secondary sources of information to fine tune responses to predators and to learn novel risk cues from exotic predators or alarm cues from sympatric prey species. However, the ability to draw broad conclusions regarding use of predator diet cues by prey was limited by a lack of research identifying molecular structures of the chemicals that mediate these interactions. Conclusions are also limited by a narrow research focus. Seventy percent of reviewed studies were performed in freshwater systems, with a limited range of model predator–prey systems, and 98 % of reviewed studies were performed in laboratory settings. Besides identifying the molecules prey use to detect predators, future studies should strive to manipulate different aspects of prey responses to predator diet across a broader range of predator–prey species, particularly in marine and terrestrial systems, and to expand studies into the field.     

Indirect effects of an exploited predator on recruitment of coral-reef fishes

The more ecologists examine the role of trait-mediated indirect interactions (TMIIs), especially in regulating predator-prey interactions, the more we recognize their fundamental role in structuring food webs. However, most empirical evidence for TMIIs comes from studies that are either conducted in laboratory or mesocosm venues or are restricted to simple food webs involving lower trophic-level animals. Here, I quantified the direct and indirect effects of interactions between high-level vertebrate predators on their vertebrate prey using a field experiment. Specifically, I tested how varying densities of a large-bodied, top predator (Nassau grouper; Epinephelus striatus) affected persistence, growth, and behavior of two smaller-bodied, intermediate predators (coney and graysby groupers; Cephalopholis fulva and C. cruentata) on 20 isolated patch reefs in the Bahamas. Large-bodied groupers are capable of consuming their smaller-bodied counterparts, and previous observational studies have indicated that local abundances of these groupers are negatively correlated. I measured the effects of interactions among groupers on lower trophic-level prey by quantifying recruitment of coral-reef fishes to the reefs. The field experiment demonstrated a strong trophic cascade that was entirely mediated by modified behavior of the intermediate predators. These results indicate that indirect, nonlethal interactions in natural systems can have strong cascading effects even at high trophic levels and in high-diversity food webs. Incorporating the complexity of such indirect effects into fisheries management may improve the sustainability of fished populations and strengthen marine conservation efforts; however these results also indicate that the effects of fishing are complex and difficult to predict.     

Foraging tactics of an ambush predator: the effects of substrate attributes on prey availability and predator feeding success

The foraging sites selected by an ambush forager can strongly affect its feeding opportunities. Foraging cane toads (Rhinella marina) typically select open areas, often under artificial lights that attract insects. We conducted experimental trials in the field, using rubber mats placed under lights, to explore the influence of substrate color and rugosity on prey availability (numbers, sizes, and types of insects) and toad foraging success. A mat's color (black vs. white) and rugosity (smooth vs. rough) did not influence the numbers, sizes, or kinds of insects that were attracted to it, but toads actively preferred to feed on rugose white mats (50% of prey-capture events, vs. a null of 25%). White backgrounds provided better visual contrast of the (mostly dark) insects, and manipulations of prey color in the laboratory showed that contrast was critical in toad foraging success. Insects landing on rugose backgrounds were slower to leave, again increasing capture opportunities for toads. Thus, cane toads actively select backgrounds that maximize prey-capture opportunities, a bias driven by the ways that substrate attributes influence ease of prey detection and capture rather than by absolute prey densities.     

Effects of combining an intraguild predator with a cannibalistic intermediate predator on a species-level trophic cascade

A greater diversity of natural enemies can in some cases disrupt prey suppression, particularly when natural enemies engage in intraguild predation, where natural enemies compete with and prey upon each other. However, empirical studies have often demonstrated enhanced prey suppression despite intraguild predation. A recent theoretical study proposed the hypothesis that, when the intermediate predator is cannibalistic, intraguild predation can reduce cannibalism within the intermediate predator population, leading to little change in intermediate predator mortality and thus enhanced prey suppression. The goal of this study was to examine this hypothesis empirically. Two summer-long field enclosure experiments were conducted in cotton fields. We investigated the effects of adding an intraguild predator, Zelus renardii, on (1) the abundance of a cannibalistic intermediate predator, Geocoris pallens, (2) the abundance of a herbivore, Lygus hesperus, and (3) cotton plant performance. G. pallens adult abundance did not increase, even when food availability was high and natural enemies were absent, suggesting that density-dependent cannibalism imposes an upper limit on its densities. Furthermore, although Z. renardii is an intraguild predator of G. pallens, G. pallens long-term densities were unaffected by Z. renardii. In the presence of the intermediate predator, the addition of the intraguild predator Z. renardii enhanced suppression of L. hesperus, and there were suggestions that Z. renardii and G. pallens partitioned the L. hesperus population. Effects of herbivore suppression cascaded to the plant level, improving plant performance. In conclusion, we provide empirical support for the hypothesis that the addition of an intraguild predator may enhance prey suppression if the intermediate predator expresses density-dependent cannibalism. Intraguild predation and cannibalism co-occur in many communities; thus their joint effects may be broadly important in shaping predator effects on herbivores and plant performance.     

不同干扰下阿拉善荒漠啮齿动物群落格局的变动特征

干扰广泛被认为是影响物种多样性变化的最主要的因素。近年来,物种多样性对干扰的响应是生态学家关注和研究的热点问题之一。2002-2011年,在内蒙古阿拉善荒漠区对人为不同利用（干扰）方式形成的禁牧、轮牧、过牧和开垦4种不同区域中,通过对啮齿动物群落的物种数、物种总捕获率、Shannon-Wiener多样性指数和Pielou均匀性指数等4个变量的时间序列资料的比较,分析了这些变量的年间变动特点及在不同干扰条件下的变动规律,并对不同干扰区各变量的趋势进行分析,探讨啮齿动物群落对不同干扰过程的响应。结果表明：4个变量的最高值均出现在受中等干扰的轮牧区中,物种总捕获率2003年出现了各区的第1个高峰,总捕获率分别为过牧区10.75%、轮牧区19.60%、禁牧区11.20%、开垦区10.55%;2008年出现第2个高峰,4个区物种总捕获率均达到了各区的最高峰,捕获率分别为过牧区29.60%、轮牧区34.15%、禁牧区14.20%、开垦区27.10%。过牧区与轮牧区群落物种总捕获率、Shannon-Wiener指数和Pielou均匀性指数显著相关（p〈0.05）,变动规律相似。过牧区与轮牧区、禁牧区与过牧区、轮牧区与禁牧区之间的物种总捕获率呈极显著正相关（p〈0.01）。过牧区与轮牧区相似度较高（I=0.8568）;过牧区Shannon-Wiener指数表现为有显著的下降趋势（Y=-0.142 lnt＋1.487,F=6.320,p＜0.05）,表明随着过牧时间的积累,对生境的干扰逐步加重,啮齿动物群落的多样性会逐渐降低。     

Large nonlethal effects of an invasive invertebrate predator on zooplankton population growth rate

We conducted a study to determine the contribution of lethal and nonlethal effects to a predator's net effect on a prey's population growth rate in a natural setting. We focused on the effects of an invasive invertebrate predator, Bythotrephes longimanus, on zooplankton prey populations in Lakes Michigan and Erie. Field data taken at multiple dates and locations in both systems indicated that the prey species Daphnia mendotae, Daphnia retrocurva, and Bosmina longirostris inhabited deeper portions of the water column as Bythotrephes biomass increased, possibly as an avoidance response to predation. This induced migration reduces predation risk but also can reduce birth rate due to exposure to cooler temperatures. We estimated the nonlethal (i.e., resulting from reduced birth rate) and lethal (i.e., consumptive) effects of Bythotrephes on D. mendotae and Bosmina longirostris. These estimates used diel field survey data of the vertical gradient of zooplankton prey density, Bythotrephes density, light intensity, and temperature with growth and predation rate models derived from laboratory studies. Results indicate that nonlethal effects played a substantial role in the net effect of Bythotrephes on several prey population growth rates in the field, with nonlethal effects on the same order of magnitude as or greater (up to 10-fold) than lethal effects. Our results further indicate that invasive species can have strong nonlethal, behaviorally based effects, despite short evolutionary coexistence with prey species.     

Negative effects of vertebrate herbivores on invertebrates in a coastal dune community

Although competition has been a major focus in ecology for the past century, most empirical and theoretical studies in this area have emphasized interactions between closely related species. However, there is growing evidence that negative interactions among distantly related taxa also occur and may be far more important than previously thought. In this study, we took advantage of an 11-year-old replicated vertebrate-exclosure experiment in a coastal dune community in northern California, USA, to examine the effects of the two most common vertebrate herbivores (jackrabbits and black-tailed deer) on the abundance of the three most visible invertebrate herbivores (two snail, a moth, and a grasshopper species). Our results indicate that four of the six possible pairwise interactions were significantly negative for the invertebrates. Jackrabbits reduced the abundances of snails by 44-75%, tiger moth caterpillars by 36%, and grasshoppers by 62%. Deer reduced the abundances of snails by 32%, increased the abundances of caterpillars by 31%, and had no measurable effect on grasshopper abundance. Our data also revealed that jackrabbits significantly decreased the volume of forbs and common shrubs and the flowering by grasses in our study plots. We were unable to detect an effect of deer on these measures of vegetation. These results suggest that by changing vegetation, jackrabbits may reduce invertebrate populations that are limited by food, protective structures, or microclimate provided by plants. Of these three mechanisms, only shade was strongly supported as limiting snail numbers in smaller-scale manipulations. In most systems, as in this one, the number of pairs of distantly related herbivores far exceeds the number of pairs of congeners. Since interactions among distantly related herbivores may be common in many cases, these interactions are likely to be important and should receive far more attention from ecologists.     

Plant facilitation of a belowground predator

Interest in facilitative predator plant interactions has focused upon above-ground systems. Underground physical conditions are distinctive, however, and we provide evidence that bush lupine, Lupinus arboreus, facilitates the survival of the predatory nematode Heterorhabditis marelatus. Because H. marelatus is prone to desiccation and lupines maintain a zone of moist soil around their taproots even during dry periods, we hypothesized that dry-season nematode survival under lupines might be higher than in the surrounding grasslands. We performed field surveys and measured nematode survival in lupine and grassland rhizospheres under wet- and dry-season conditions. Nematodes survived the crucial summer period better under lupines than in grasslands; however, this advantage disappeared in wet, winter soils. Modeling the probability of nematode population extinction showed that, while even large nematode cohorts were likely to go extinct in grasslands, even small cohorts in lupine rhizospheres were likely to survive until the arrival of the next prey generation. Because this nematode predator has a strong top-down effect on lupine survival via its effect on root-boring larvae of the ghost moth Hepialus californicus, this facilitative interaction may enable a belowground trophic cascade. Similar cases of predator facilitation in seasonally stressful environments are probably common in nature.     

Lethal and nonlethal predator effects on an herbivore guild mediated by system productivity

Indirect effects propagated through intervening species in a food web have important effects on community properties. Traditionally, these indirect effects have been conceptualized as mediated through density changes of the intervening species, but it is becoming increasingly apparent that those mediated through trait (phenotypic) responses also can be very important. Because density- and trait-mediated indirect effects have different properties, it is critical that we understand the mechanisms of transmission in order to predict how they will interact, and when or where they will be important. In this study, we examined the mechanisms and consequences of the lethal (density-mediated) and nonlethal (trait-mediated) effects of a larval odonate predator on a guild of four herbivore species (a larval anuran and three species of snails) and their resources. We also manipulated system productivity in order to explore the effects of environmental context on the transmission of these two types of indirect effects. We show that trait-mediated effects arising from the predator can be very strong relative to density-mediated effects on both the competing herbivores and the species composition and production of their resources. A number of these indirect effects are shown to be contingent on productivity of the system. We further present evidence that trait- and density-mediated indirect effects originating from a predator may be transmitted independently through different routes in a food web, particularly when spatial responses of the transmitting prey are involved. Finally, effects on prey growth due to trait responses to the predator varied from negative to positive in predictable ways as a function of time and indirect effects on the larger food web. These results indicate the important role that trait-mediated indirect effects can play in trophic cascades and keystone predator interactions, and we discuss how the mechanisms involved can be incorporated in theory.     

Stable isotope ratios of a tropical marine predator: confounding effects of nutritional status during growth

Stable isotope analysis of carbon and nitrogen is frequently used to study the diets and foraging ecology of marine predators. However, isotopic values may also be affected by an individual’s nutritional status and associated physiological processes. Here, we use C and N stable isotopes in blood and feathers of blue-footed booby chicks at the Galápagos Islands to examine how isotopic values are related to body condition and growth rate, and to assess the consistency in the isotope ratios of individuals during growth. Size dimorphism in blue-footed boobies provided an additional opportunity to examine how isotope ratios differ between sexes in relation to body size and growth rate. There was no significant difference between sexes but both C and N stable isotopes were significantly negatively related to the body condition of chicks. These data were consistent with individual variation in physiological processes affecting fractionation, although we cannot rule out the possibility that they were also influenced to some extent by population-level variation in the stable isotope ratios of prey fed to chicks, for instance related to prey size, depth or lipid content. Our results highlight the need for methods that take proper account of confounding physiological factors in isotopic studies of foraging ecology and diet.     

Predator identity and additive effects in a treehole community

Multiple predator species can interact as well as strongly affect lower trophic levels, resulting in complex, nonadditive effects on prey populations and community structure. Studies of aquatic systems have shown that interactive effects of predators on prey are not necessarily predictable from the direct effects of each species alone. To test for complex interactions, the individual and combined effects of a top and intermediate predator on larvae of native and invasive mosquito prey were examined in artificial analogues of water-filled treeholes. The combined effects of the two predators were accurately predicted from single predator treatments by a multiplicative risk model, indicating additivity. Overall survivorship of both prey species decreased greatly in the presence of the top predator Toxorhynchites rutilus. By itself, the intermediate predator Corethrella appendiculata increased survivorship of the native prey species Ochlerotatus triseriatus and decreased survivorship of the invasive prey species Aedes albopictus relative to treatments without predators. Intraguild predation did not occur until alternative prey numbers had been reduced by approximately one-half. Owing to changes in size structure accompanying its growth, T. rutilus consumed more prey as time progressed, whereas C. appendiculata consumed less. The intermediate predator, C. appendiculata, changed species composition by preferentially consuming A. albopictus, while the top predator, T. rutilus, reduced prey density, regardless of species. Although species interactions were in most cases predicted from pairwise interactions, risk reduction from predator interference occurred when C. appendiculata densities were increased and when the predators were similarly sized.