Predator Body Size Research Articles

Associations between Non‐Lethal Injury, Body Size, and Foraging Ecology in an Amphibian Intraguild Predator

AbstractTheoretical treatments of intraguild predation and its effects on behavioral interactions regard the phenomenon as a size‐structured binary response wherein predation among competitors is completely successful or completely unsuccessful. However, intermediate outcomes occur when individuals escape intraguild (IG) interactions with non‐lethal injuries. While the effects of wounds for prey include compromised mobility and increased predation risk, the consequences of similar injuries among top predators are not well understood, despite the implications for species interactions. Using an amphibian IG predator, Ambystoma opacum (Caudata: Ambystomatidae), we examined associations between non‐lethal injuries and predator body size, foraging strategy, microhabitat selection, and intraspecific agonistic interactions. Wounds were common among IG predators, generally increasing in frequency throughout larval ontogeny. Non‐lethal injuries were associated with differences in predator body size and behavior, with injured predators exhibiting smaller body sizes, increased use of benthic microhabitats, reduced agonistic displays, and increased risk of intraspecific aggression. While such effects were not ultimately associated with reduced foraging success, non‐lethal injury could contribute to niche partitioning between injured and healthy predators via habitat selection, but injured predators likely continue to exert predatory pressure on IG and basal prey populations. Our results indicate that studies of top‐down population regulation should incorporate injury‐related modifications to both prey and predator behavior and size structure.

Vulnerability of age-0 pallid sturgeon Scaphirhynchus albus to predation; effects of predator type, turbidity, body size, and prey density

Predation can play an important role in the recruitment dynamics of fishes with intensity regulated by behavioral (i.e., prey selectivity) and/or environmental conditions that may be especially important for rare or endangered fishes. We conducted laboratory experiments to quantify prey selection and capture efficiency by three predators employing distinct foraging strategies: pelagic piscivore (walleye Sander vitreus); benthic piscivore (flathead catfish Pylodictis olivaris) and generalist predator (smallmouth bass Micropterus dolomieu) foraging on two size classes of age-0 pallid sturgeon: large (75–100 mm fork length [FL]) and small (40–50 mm FL). Experiments at high (> 70 nephalometric turbidity units [NTU]) and low (< 5 NTU) turbidity for each predator were conducted with high and low densities of pallid sturgeon and contrasting densities of an alternative prey, fathead minnow Pimephales promelas. Predator behaviors (strikes, captures, and consumed prey) were also quantified for each prey type. Walleye and smallmouth bass negatively selected pallid sturgeon (Chesson’s α = 0.04–0.1) across all treatments, indicating low relative vulnerability to predation. Relative vulnerability to predation by flathead catfish was moderate for small pallid sturgeon (α = 0.44, neutral selection), but low for large pallid sturgeon (α = 0.11, negative selection). Turbidity (up to 100 NTU) did not affect pallid sturgeon vulnerability, even at low density of alternative prey. Age-0 pallid sturgeon were easily captured by all predators, but were rarely consumed, suggesting mechanisms other than predator capture efficiency govern sturgeon predation vulnerability.

The Geography and Body-Size Dependence of Top-Down Forcing in New England's Lobster-Groundfish Interaction

We examined changes in the body-size dependence of the groundfish-American lobster interaction across the steep environmental and biogeographic gradient from the Gulf of Maine to southern New England, USA. Lobster (Homarus americanus H. Milne-Edwards, 1837) abundance has increased dramatically in the Gulf of Maine since the 1980s. A parallel upsurge in southern New England ended in 1997 with the onset of a disease epizootic. To characterize the geography and body-size scaling of the predator-prey interaction, and to determine whether changes in the predator assemblage could explain the increases in lobster abundance, we drew on field predation experiments and surveys on nurseries where lobsters first settle, as well as trawl-survey time series of fish and lobster abundance on habitats exploited by older lobsters. Our results suggested that the fish predators frequenting lobster nurseries in the Gulf of Maine are less diverse or abundant than those in southern New England and that predation risk in the Gulf of Maine is correspondingly lower. Trawl-survey time series suggested that the expansion of the lobster population was more strongly correlated with declines in average body mass of fish predators in the Gulf of Maine than in southern New England. The resultant relaxation of predation pressure on the American lobster in the Gulf of Maine reflects a functional change in a key link of the food web induced by a change in predator body size. A closer examination of size-specific interactions of individual species and functional groups in the groundfish assemblage is therefore warranted.

Predator biomass determines the magnitude of non-consumptive effects (NCEs) in both laboratory and field environments

Predator body size often indicates predation risk, but its significance in non-consumptive effects (NCEs) and predator risk assessment has been largely understudied. Although studies often recognize that predator body size can cause differing cascading effects, few directly examine prey foraging behavior in response to individual predator sizes or investigate how predator size is discerned. These mechanisms are important since perception of the risk imposed by predators dictates behavioral responses to predators and subsequent NCEs. Here, we evaluate the role of predator body size and biomass on risk assessment and the magnitude of NCEs by investigating mud crab foraging behavior and oyster survival in response to differing biomasses of blue crab predators using both laboratory and field methods. Cues from high predator biomass treatments including large blue crab predators and multiple small blue crab predators decreased mud crab foraging and increased oyster survival, whereas mud crab foraging in response to a single small blue crab did not differ from controls. Mud crabs also increased refuge use in the presence of large and multiple small, but not single small, blue crab predators. Thus, both predator biomass and aggregation patterns may affect the expression of NCEs. Understanding the impact of predator biomass may therefore be necessary to successfully predict the role of NCEs in shaping community dynamics. Further, the results of our laboratory experiments were consistent with observed NCEs in the field, suggesting that data from mesocosm environments can provide insight into field situations where flow and turbulence levels are moderate.

Season and prey type influence size dependency of predator−prey body mass ratios in a marine fish assemblage

Marine and freshwater food webs are strongly structured by size-dependent preda- torprey interactions. Predatorprey body mass ratios (PPMR) are important parameters in size- based food-web models, but studies evaluating the temporal stability of PPMR or its relationship to predator feeding modes are scant. Using a large data set of predatorprey pairs from a diverse fish community sampled in summer, fall, and winter, we showed that community-level PPMR var- ied with predator mass in a nonlinear (dome-shaped) manner. PPMR was higher in the summer relative to the fall and winter for all predator body size classes regardless of whether prey were fish or invertebrate. Further, the size dependency of PPMR was dome-shaped for invertebrate prey but positive and linear for fish prey. We empirically show that community-level PPMR is dynamic rather than fixed, which is in agreement with general expectations set by simulation studies of biomass spectra. However, we are presently unable to identify the specific processes underlying these patterns. Size-based models of marine ecosystems offer considerable promise over traditional taxa-based approaches, and our analyses provide insight into major patterns of variation in PPMR in a temperate marine system.

Predator size interacts with habitat structure to determine the allometric scaling of the functional response

While both predator body size and prey refuge provided by habitat structure have been established as major factors influencing the functional response (per capita consumption rate as a function of prey density), potential interactions between these factors have rarely been explored. Using a crab predator (Panopeus herbstii) – mussel prey (Brachidontes exustus) system, we examined the allometric scaling of the functional response in oyster (Crassostrea virginica) reef habitat, where crevices within oyster clusters provide mussels refuge from predation. A field survey of mussel distribution showed that mussels attach closer to the cluster periphery at high mussel density, indicating the potential for saturation of the refuge. In functional response experiments, the consumption rate of large crabs was depressed at low prey density relative to small crabs, while at high prey density the reverse was true. Specifically, the attack rate coefficient and handling time both decreased non‐linearly with crab size. An additional manipulation revealed that at low prey densities, the ability of large crabs to maneuver their claws and bodies to extract mussels from crevices was inhibited relative to small crabs by the structured habitat, reducing their attack rate. At high prey densities, crevices were saturated, forcing mussels to the edge of clusters where crabs were only limited by handling time. Our study illuminates a potentially general mechanism where the quality of the prey refuge provided by habitat structure is dependent on the relative size of the predator. Thus anthropogenic influences that alter the natural crab size distribution or degrade reef habitat structure could threaten the long‐term stability of the crab –mussel interaction in reefs.

Limitations of Neoseiulus baraki and Proctolaelaps bickleyi as control agents of Aceria guerreronis

Several predatory mites have been found in association with the coconut mite, Aceria guerreronis Keifer, in northeast Brazil. However, the latter still causes damage to coconut in that region. The objectives of this work were to compare the frequencies of occurrence of Neoseiulus (Phytoseiidae) and Proctolaelaps (Melicharidae) species on standing and aborted coconuts in coastal Pernambuco State, northeast Brazil and to analyze their possible limitations as control agents of the coconut mite, based on evaluations of the restrictions they may have to access the microhabitat inhabited by the pest and their functional and reproductive responses to increasing densities of the latter. Neoseiulus baraki (Athias-Henriot) was found mostly on standing coconuts whereas Proctolaelaps bickleyi (Bram) was found mostly on aborted coconuts. Measurements of the entrance to the microhabitat occupied by the coconut mite, between the bracts and the subjacent fruit surface, showed that this different pattern of predator prevalence could be related to predator sizes, although other environmental factors could not be disregarded. Progressively higher predation rate of N. baraki was observed up to an experimental density that corresponded to 1,200 coconut mites per fruit, which is close to the average number determined in northeast Brazil, reducing slightly afterwards. Predation rate of P. bickleyi reduced consistently but slightly with increasing prey densities, but in absolute values, rates were always much higher than determined for N. baraki. The excessively high killing capacity of P. bickleyi, probably related to its high feeding requirement, may be detrimental in terms of stability. In fact, such high requirement for food suggests that P. bickleyi might not have a strong relation with the coconut mite and that the latter may not be its main food source under natural conditions. It is concluded that body sizes of both predators and the exceedingly high feeding requirement of P. bickleyi may limit their performance as control agents of the coconut mite.

Seasonal shifts in predator body size diversity and trophic interactions in size‐structured predator–prey systems

1. Theory suggests that the relationship between predator diversity and prey suppression should depend on variation in predator traits such as body size, which strongly influences the type and strength of species interactions. Prey species often face a range of different sized predators, and the composition of body sizes of predators can vary between communities and within communities across seasons. 2. Here, I test how variation in size structure of predator communities influences prey survival using seasonal changes in the size structure of a cannibalistic population as a model system. Laboratory and field experiments showed that although the per-capita consumption rates increased at higher predator-prey size ratios, mortality rates did not consistently increase with average size of cannibalistic predators. Instead, prey mortality peaked at the highest level of predator body size diversity. 3. Furthermore, observed prey mortality was significantly higher than predictions from the null model that assumed no indirect interactions between predator size classes, indicating that different sized predators were not substitutable but had more than additive effects. Higher predator body size diversity therefore increased prey mortality, despite the increased potential for behavioural interference and predation among predators demonstrated in additional laboratory experiments. 4. Thus, seasonal changes in the distribution of predator body sizes altered the strength of prey suppression not only through changes in mean predator size but also through changes in the size distribution of predators. In general, this indicates that variation (i.e. diversity) within a single trait, body size, can influence the strength of trophic interactions and emphasizes the importance of seasonal shifts in size structure of natural food webs for community dynamics.

Flow regime alters body size but not the use of aquatic subsidies in a riparian predatory arthropod

AbstractAlterations to river flow conditions have wide impacts on riparian organisms in terms of behavior and biomass. However, little is known about natural flood impacts on prey use and individual growth of riparian predators. Using stable carbon isotope analysis, we investigated flood impacts on aquatic‐prey use and the size structure of an orb‐web spider, Nephila clavata, during 3 years under different flood conditions in a black locust forest in the middle reaches of the Chikuma River. Large floods depressed aquatic‐prey abundance, but did not affect terrestrial‐prey abundance in the riparian forest. Consequently, spider growth was stunted after large floods. Spider body size was positively correlated with the body sizes of both aquatic and terrestrial insects in spider webs, where terrestrial insects were significantly larger than aquatic insects. The δ13C of aquatic insects was about 8‰ higher than that of terrestrial insects, and the δ13C of both insect groups did not vary significantly between months or among years. A negative relationship was found between body size and δ13C in spiders under different subsidies levels. Our results showed that flow regime altered spider growth through changes in aquatic subsidies level, but not aquatic‐prey use by the spiders due to relative body sizes of predators and prey. Changes in relative body sizes of predator and prey may be an important factor in understanding nutrients, materials, and energy flows in aquatic and terrestrial linkages in the context of flow regime.

Nutrient enrichment differentially affects body sizes of primary consumers and predators in a detritus‐based stream

We assessed how a 5‐yr nutrient enrichment affected the responses of different size classes of primary consumers and predators in a detritus‐based headwater stream. We hypothesized that alterations in detritus availability because of enrichment would decrease the abundance and biomass of large‐bodied consumers. In contrast, we found that 2 yr of enrichment increased the biomass and abundance of all consumers regardless of body size. Furthermore, during the fourth and fifth year of enrichment, the abundance and biomass of largebodied primary consumers continued to increase, while small‐bodied primary consumers returned to pretreatment levels. The size structure of a dominant primary consumer (Pycnopsyche spp.) also shifted during the 5‐yr enrichment: its average and maximum individual body size increased in the treatment stream compared with the reference stream. Positive enrichment effects also occurred on small‐bodied predators, but not on large‐bodied predators. Thus, enrichment increased prey body size, but these positive effects on large prey did not propagate up to higher trophic levels to affect large predators. Because consumer body size can be an important species‐specific trait determining population dynamics and ecosystem processes, these observed shifts in consumer size distributions suggest a potentially important pathway for global increases in nutrient enrichment to alter stream structure and function.

Predation intensifies parasite exposure in a salmonid food chain

Summary 1. Parasites can influence ecosystem structure, function and dynamics by mediating predator–prey interactions. Recurrent infestations of the salmon louse Lepeophtheirus salmonis associated with salmon aquaculture may mediate interactions between juvenile salmonids. Louse infection increases pink salmon Oncorhynchus gorbuscha susceptibility to predation, resulting in the trophic transmission of lice (with an adult male bias) to coho salmon O. kisutch predators. While experimental evidence is accumulating, the extent to which trophic transmission structures the distribution of lice among juvenile salmon in the wild is unknown. 2. We used a hierarchical modelling approach to examine the abundance and sex ratio of salmon lice on juvenile pink and coho salmon, collected from a region of salmon aquaculture during sea louse infestations, to test the hypothesis that trophic transmission of salmon lice increases infection on coho that feed upon infected pink salmon prey. 3. As predicted, coho had higher adult and pre‐adult louse abundance than their pink salmon prey, and louse abundance was more adult male biased on predators than sympatric prey. We estimate that trophic transmission accounts for 53–67% of pre‐adult and adult louse infection on coho. 4. Synthesis and applications. These results suggest that, by evading predation, salmon lice can accumulate up juvenile salmon food webs. Predators, such as coho, can experience a two‐ to threefold increase in parasite exposure through predation on infected prey than would otherwise occur through passive exposure to infective larvae. Thus, predation may intensify parasite exposure and undermine the protection to ectoparasites conferred by the larger body size of predators. For larger predatory wild juvenile salmon, the risk of louse transmission from farmed salmon may therefore be greater than previously appreciated. These findings argue for an ecosystem perspective in monitoring and managing the marine environment in areas of intensive salmon aquaculture that includes the productivity and ecological interactions of all salmonid species.

Disentangling the effects of predator body size and prey density on prey consumption in a lizard

Summary 1. Understanding proximate determinants of predation rates is a central question in ecology. Studies often use functional response (density dependent) or allometric (mass dependent) models but approaches that consider multiple factors are critical to capture the complexity in predator–prey interactions. We present a novel comprehensive approach to understand predation rates based on field data obtained from a vertebrate predator. 2. Estimates of food consumption and prey abundance were obtained from 21 semi‐natural populations of the lizard Zootoca vivipara. We identified the most parsimonious feeding rate function exploring allometric, simple functional response and allometric functional response models. Each group included effects of sex and weather conditions. 3. Allometric models reveal the importance of predator mass and sex: larger females have the highest natural feeding rates. Functional response models show that the effect of prey density is best represented by a Holling type II response model with a mass, sex and weather dependent attack rate and a constant handling time. However, the best functional response model only received moderate support compared to simpler allometric models based only on predator mass and sex. 4. Despite this limited effect of prey densities on feeding rates, we detected a significant negative relationship between an index of preferred prey biomass and lizard density. 5. Functional response models that ignore individual variation are likely to misrepresent trophic interactions. However, simpler models based on individual traits may be best supported by some data than complex allometric functional responses. These results illustrate the importance of considering individual, population and environmental effects while also exploring simple models.

Sentinel systems on the razor's edge: effects of warming on Arctic geothermal stream ecosystems

AbstractThe Earth is experiencing historically unprecedented rates of warming, with surface temperatures projected to increase by 3–5 °C globally, and up to 7.5 °C in high latitudes, within the next century. Knowledge of how this will affect biological systems is still largely restricted to the lower levels of organization (e.g. species range shifts), rather than at the community, food web or ecosystem level, where responses cannot be predicted from studying single species in isolation. Further, many correlational studies are confounded with time and/or space, whereas experiments have been mostly confined to laboratory microcosms that cannot capture the true complexity of natural ecosystems. We used a ‘natural experiment’ in an attempt to circumvent these shortcomings, by characterizing community structure and trophic interactions in 15 geothermal Icelandic streams ranging in temperature from 5 °C to 45 °C. Even modest temperature increases had dramatic effects across multiple levels of organization, from changes in the mean body size of the top predators, to unimodal responses of species populations, turnover in community composition, and lengthening of food chains. Our results reveal that the rates of warming predicted for the next century have serious implications for the structure and functioning of these fragile ‘sentinel’ ecosystems across multiple levels of organization.

Food of the Little Swift Apus affinis and African Black Swift Apus barbatus in South Africa

The food habits of the Little Swift Apus affinis and African Black Swift Apus barbatus were quantified at Kimberley, Northern Cape province and Makapansgat, Limpopo province, South Africa. As previously documented for other species, both of these swifts took a wide variety of aerial arthropods including spiders as well as 10 orders and 64 families of insects in the combined samples. Little Swift prey items at Kimberley averaged 3.2 mm in body length (SD = 1.6, n = 2 178) and ranged from 1.2 to 16.0 mm. Prey items of African Black Swifts at Kimberley averaged 4.4 mm (SD = 4.1, n = 185) with a range of 0.9 to 15.9 mm, and 2.8 mm (SD = 0.8, n = 2 099) at Makapansgat with a range of 1.2 to 10.8 mm. At Kimberley, African Black Swifts took more (8.3%) larger prey items (>8 mm), such as termites, than Little Swifts (2.3%). As also true of other swifts, both African Black Swifts and Little Swifts took a majority (>90%) of smaller prey items that are abundant in the air column. Mean prey size is significantly correlated with predator body size in Apus and Tachymarptis swifts.

Decline in top predator body size and changing climate alter trophic structure in an oceanic ecosystem

Globally, overfishing large-bodied groundfish populations has resulted in substantial increases in their prey populations. Where it has been examined, the effects of overfishing have cascaded down the food chain. In an intensively fished area on the western Scotian Shelf, Northwest Atlantic, the biomass of prey species increased exponentially (doubling time of 11 years) even though the aggregate biomass of their predators remained stable over 38 years. Concomitant reductions in herbivorous zooplankton and increases in phytoplankton were also evident. This anomalous trophic pattern led us to examine how declines in predator body size (approx. 60% in body mass since the early 1970s) and climatic regime influenced lower trophic levels. The increase in prey biomass was associated primarily with declines in predator body size and secondarily to an increase in stratification. Sea surface temperature and predator biomass had no influence. A regression model explained 65 per cent of prey biomass variability. Trait-mediated effects, namely a reduction in predator size, resulted in a weakening of top predation pressure. Increased stratification may have enhanced growing conditions for prey fish. Size-selective harvesting under changing climatic conditions initiated a trophic restructuring of the food chain, the effects of which may have influenced three trophic levels.

Predator size, prey size, and dietary niche breadth relationships in marine predators

Based on geographical and home range sizes, physiology, and gape limitation, a positive relationship between predator size and diet breadth is expected. Alternatively, larger predators might avoid smaller prey; in this case no relationship would be found. Here, I used a large data set on the diets of marine predators to describe and identify mechanisms responsible for the relationships among predator body size, diet breadth, and the mean, minimum, maximum, and variance of prey size. I found no relationship between predator size and diet breadth. Mean, minimum, maximum, and variance of prey size were all positively associated with predator size. I found that larger predators increase their minimum and maximum prey size with similar slopes, which explains the lack of relationship between predator size and diet breadth. The results support predictions of the hypothesis that optimal foraging is the main factor constraining the shape of the relationships among predator size, prey size, and diet breadth. Future research should focus on examining the relationship between body size and the breadth of different niche axis across different groups of organisms to assess whether a positive relationship between body size and niche breadth is a general rule in macroecology.

Disease in prey population and body size of intermediate predator reduce the prevalence of chaos-conclusion drawn from Hastings–Powell model

In ecology the disease in the prey population plays an important role in controlling the dynamical behaviour of the system. We modify Hastings and Powell’s (HP) [Hastings, A., Powell, T., 1991. Chaos in three-species food chain. Ecology 72 (3), 896–903] model by introducing disease in the prey population. The conditions for which the modified HP model system represents extinction, permanence or impermanence of population are worked out. The modified model is analyzed to obtain different conditions for which the system exhibits stability around the biologically feasible equilibria. Through numerical simulations we display that the modified system enters into stable solutions depending upon the force of infection in prey population as well as body size of intermediate predator. Our results demonstrate that disease in prey population and body size of intermediate predator are the key parameters for controlling the chaotic dynamics observed in original HP model.

Optimal foraging constrains macroecological patterns: body size and dietary niche breadth in lizards

ABSTRACTAim To explore and identify probable mechanisms contributing to the relationships among body size, dietary niche breadth and mean, minimum, maximum and range of prey size in predaceous lizards.Location Our data set includes species from tropical rainforests, semi‐arid regions of Brazil, and from deserts of the south‐western United States, Australia and the Kalahari of Africa.Methods We calculated phylogenetic and non‐phylogenetic regressions among predator body size, dietary breath and various prey size measures.Results We found a negative association between body size and dietary niche breadth in 159 lizard species sampled across most evolutionary lineages of squamate reptiles and across major continents and habitats. We also show that mean, minimum, maximum and range of prey size were positively associated with body size.Main conclusions Our results suggest not only that larger lizards tend to eat larger prey, but in doing so offset their use of smaller prey. Reduction of dietary niche breadth with increased body size in these lizards suggests that large predators target large and more profitable prey. Consequently, the negative association between body size and niche breadth in predators most likely results from optimal foraging. Though this result may appear paradoxical and runs counter to previous studies, resources for predators may be predictably more limited than resources for herbivores, thus driving selection for more profitable prey.

Body size and trophic position in a temperate estuarine food web

Abstract We used stomach contents and stable isotope ratios of fishes and macroinvertebrates, collected bi-monthly over 18 months from Mad Island Marsh, a small tidal estuary on the northwestern Gulf of Mexico coast, to examine potential body size–trophic position relationships. Mean body size (length) of predator taxa yielding measurable prey items were significantly correlated with body size (length) of their prey and mean volume of their stomach contents, however, the strength of the correlation was greater when two larger detrivores (i.e., striped mullet and gizzard shad) were excluded from the analysis. Similarly, trophic positions estimated by volumetric stomach contents were also significantly related to predator body size but not related to mean volume of stomach contents, but again excluding those detrivores from the analyses increased the strength of the relationship. Trophic positions estimated from stable isotopes and δ 15 N as an index of trophic position were also unrelated to predator body length, but significantly related to predator body mass. Although estimates of trophic positions in this tidal estuary using both methods were largely concordant, there were some exceptional zooplanktivorous and detritivorous species that had higher trophic levels according to nitrogen isotope ratios. Excluding those species from the analyses increased the strength of relationships between size and trophic positions of predators. A significant relationship between body sizes of consumers and their prey supports the view that body size is a key variable influencing trophic interactions and the structure of aquatic food webs. Our results also suggest that body size (especially consumer mass) is a good predictor of trophic levels estimated by stable isotopes, whereas consumer length is an important trait predicting the trophic level estimated from stomach contents in this tidal estuarine system.

Joint evolution of predator body size and prey-size preference

We studied the joint evolution of predator body size and prey-size preference based on dynamic energy budget theory. The predators' demography and their functional response are based on general eco-physiological principles involving the size of both predator and prey. While our model can account for qualitatively different predator types by adjusting parameter values, we mainly focused on 'true' predators that kill their prey. The resulting model explains various empirical observations, such as the triangular distribution of predator-prey size combinations, the island rule, and the difference in predator-prey size ratios between filter feeders and raptorial feeders. The model also reveals key factors for the evolution of predator-prey size ratios. Capture mechanisms turned out to have a large effect on this ratio, while prey-size availability and competition for resources only help explain variation in predator size, not variation in predator-prey size ratio. Predation among predators is identified as an important factor for deviations from the optimal predator-prey size ratio.