Influence of intra‐ and interspecific variation in predator–prey body size ratios on trophic interaction strengths

1. Predation is a pervasive force that structures food webs and directly influences ecosystem functioning. The relative body sizes of predators and prey may be an important determinant of interaction strengths. However, studies quantifying the combined influence of intra‐ and interspecific variation in predator–prey body size ratios are lacking.
2. We use a comparative functional response approach to examine interaction strengths between three size classes of invasive bluegill and largemouth bass toward three scaled size classes of their tilapia prey. We then quantify the influence of intra‐ and interspecific predator–prey body mass ratios on the scaling of attack rates and handling times.
3. Type II functional responses were displayed by both predators across all predator and prey size classes. Largemouth bass consumed more than bluegill at small and intermediate predator size classes, while large predators of both species were more similar. Small prey were most vulnerable overall; however, differential attack rates among prey were emergent across predator sizes. For both bluegill and largemouth bass, small predators exhibited higher attack rates toward small and intermediate prey sizes, while larger predators exhibited greater attack rates toward large prey. Conversely, handling times increased with prey size, with small bluegill exhibiting particularly low feeding rates toward medium–large prey types. Attack rates for both predators peaked unimodally at intermediate predator–prey body mass ratios, while handling times generally shortened across increasing body mass ratios.
4. We thus demonstrate effects of body size ratios on predator–prey interaction strengths between key fish species, with attack rates and handling times dependent on the relative sizes of predator–prey participants.
5. Considerations for intra‐ and interspecific body size ratio effects are critical for predicting the strengths of interactions within ecosystems and may drive differential ecological impacts among invasive species as size ratios shift.

     

Temperature and prey density jointly influence trophic and non‐trophic interactions in multiple predator communities

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Beyond ecological opportunity: Prey diversity rather than abundance shapes predator niche variation

1. Ecological opportunity (i.e. the diversity of available resources) has a pivotal role in shaping niche variation and trophic specialisation of animals. However, ecological opportunity can be described with regard to both diversity and abundance of resources. The degree to which these two components contribute to niche variation remains unexplored.
2. To address this, we used an extensive dataset on fish diet and benthic invertebrate diversity and density from 73 sampling events in three Norwegian rivers in order to explore realised trophic niches and the response of dietary niche variation along gradients of resource diversity (potential trophic niches), resource density (as a proxy of resource abundance) and fish density (as a proxy of inter‐ and intra‐specific competition) in a freshwater top predator (the brown trout, Salmo trutta L.).
3. Linear models indicated that individual and population niche variation increased with increasing ecological opportunity in terms of prey diversity. However, no simple cause‐and‐effect associations between niche indices and prey abundance were found. Our multiple regression analyses indicated that the abundance of certain resources (e.g. Chironomidae) can interact with prey diversity to determine individual and population realised trophic niches. Niche variation (within‐individual component and inter‐individual diet variation) decreased with increasing inter‐ and intra‐specific competition.
4. This study extends prevailing trophic ecology theory by identifying diversity, rather than density, of available prey resources as a primary driver of niche variation in fish of temperate riverine systems with no extensive resource limitation. The study also shows that ecological opportunity may mask the direction of the effect (compression or expansion) of competition on niche variation when food resources are diverse.
5. Our study provides novel empirical insight to the driving forces behind niche variation and reveals that diversity, rather than density, of available prey resources may be a primary driver of niche variation in freshwater fish. Our study supports the view that a broader potential trophic niche promotes broader realised trophic niche variation by individuals, which leads to individual niche diversification by opening access to alternatives resources, resulting in a concomitant rise in the realised trophic niche width of the population.

     

Size‐based interactions across trophic levels in food webs of shallow Mediterranean lakes

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Body mass relationships affect the age structure of predation across carnivore–ungulate systems: a review and synthesis

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Different groups of ground-dwelling spiders share similar trophic niches in temperate forests

1. Generalistic interactions between predator and prey may vary with ecosystem type, predator traits, and prey traits, but the interplay of these factors has not been assessed in ground food webs.
2. We investigated trophic interactions of ground-dwelling spiders across eight forests in European Russia associated with body size, hunting strategy, microhabitat specialization, potential prey type, potential prey population density, and forest type (coniferous vs. broadleaved). We analyzed 128 individual spiders, including juveniles, all identified to the family level with two complementary methods: molecular gut content analysis, and stable isotope analysis of carbon and nitrogen.
3. The results suggest that feeding frequency of spiders is affected by predator body size and by selection of certain prey type. Stable isotope analysis showed similar trophic niches among spider families, varying moderately with forest type. Larger spiders had higher Δ¹³C values than smaller ones, but similar Δ¹⁵N values, suggesting that different size classes of spiders belong to different food chains. Results based on stable isotope and molecular gut content analyses were weakly linked, indicating them targeting different trophic niche dimensions.
4. At least for the group-level interactions, family identity and hunting strategy of predator has little predictive power while predator body size and prey traits affected trophic niche dimensions calling for future studies in this direction. Large spiders feed more and rely on different basal resources than small spiders, suggesting that including small species and juveniles provides a more comprehensive picture of food web organization.

     

Testing the validity of functional response models using molecular gut content analysis for prey choice in soil predators

Analysis of predator–prey interactions is a core concept of animal ecology, explaining structure and dynamics of animal food webs. Measuring the functional response, i.e. the intake rate of a consumer as a function of prey density, is a powerful method to predict the strength of trophic links and assess motives of prey choice, particularly in arthropod communities. However, due to their reductionist set‐up, functional responses, which are based on laboratory feeding experiments, may not display field conditions, possibly leading to skewed results. Here, we tested the validity of functional responses of centipede predators and their prey by comparing them with empirical gut content data from field‐collected predators. Our predator–prey system included lithobiid and geophilomorph centipedes, abundant and widespread predators of forest soils and their soil‐dwelling prey. First, we calculated the body size‐dependent functional responses of centipedes using a published functional response model in which we included natural prey abundances and animal body masses. This allowed us to calculate relative proportions of specific prey taxa in the centipede diet. In a second step, we screened field‐collected centipedes for DNA of eight abundant soil‐living prey taxa and estimated their body size‐dependent proportion of feeding events. We subsequently compared empirical data for each of the eight prey taxa, on proportional feeding events with functional response‐derived data on prey proportions expected in the gut, showing that both approaches significantly correlate in five out of eight predator–prey links for lithobiid centipedes but only in one case for geophilomorph centipedes. Our findings suggest that purely allometric functional response models, which are based on predator–prey body size ratios are too simple to explain predator–prey interactions in a complex system such as soil. We therefore stress that specific prey traits, such as defence mechanisms, must be considered for accurate predictions.     

Hunting‐mediated predator facilitation and superadditive mortality in a European ungulate

Predator‐prey theory predicts that in the presence of multiple types of predators using a common prey, predator facilitation may result as a consequence of contrasting prey defense mechanisms, where reducing the risk from one predator increases the risk from the other. While predator facilitation is well established in natural predator‐prey systems, little attention has been paid to situations where human hunters compete with natural predators for the same prey. Here, we investigate hunting‐mediated predator facilitation in a hunter‐predator‐prey system. We found that hunter avoidance by roe deer (Capreolus capreolus) exposed them to increase predation risk by Eurasian lynx (Lynx lynx). Lynx responded by increasing their activity and predation on deer, providing evidence that superadditive hunting mortality may be occurring through predator facilitation. Our results reveal a new pathway through which human hunters, in their role as top predators, may affect species interactions at lower trophic levels and thus drive ecosystem processes.     

Migratory herds of wildebeests and zebras indirectly affect calf survival of giraffes

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EVOLUTION OF SPRINT SPEED IN AFRICAN SAVANNAH HERBIVORES IN RELATION TO PREDATION

Predator–prey arms races are widely speculated to underlie fast speed in terrestrial mammals. However, due to lack of empirical testing, both the specificity of any evolutionary coupling between particular predator and prey species, and the relevance of alternative food‐based hypotheses of speed evolution, remain obscure. Here I examine the ecological links between the sprint speed of African savannah herbivores, their vulnerability to predators, and their diet. I show that sprint speed is strongly predicted by the vulnerability of prey to their main predators; however, the direction of the link depends on the hunting style of the predator. Speed increases with vulnerability to pursuit predators, whereas vulnerability to ambush predators is associated with particularly slow speed. These findings suggest that differential vulnerability to specific predators can indeed drive interspecific variation in speed within prey communities, but that predator hunting style influences the intensity and consistency with which selection on speed is coupled between particular species.     

Small stonefly predators affect microbenthic and meiobenthic communities in stream leaf packs

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Prey‐specific foraging tactics in a web‐building spider

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Density-dependent prey behaviours and mutable predator foraging modes induce Allee effects and over-prediction of prey mortality rates

1. Predator–prey models are often used to represent consumptive interactions between species but, typically, are derived using simple experimental systems with little plasticity in prey or predator behaviours. However, many prey and predators exhibit a broad suite of behaviours. Here, we experimentally tested the effect of density-dependent prey and predator behaviours on per capita relative mortality rates using Florida bass (Micropterus floridanus) consuming juvenile Bluegill (Lepomis macrochirus).
2. Experimental ponds were stocked with a factorial design of low, medium, and high prey and predator densities. Prey mortality, prey–predator behaviours, and predator stomach contents were recorded over or after 7 days. We assumed the mortality dynamics followed foraging arena theory. This pathologically flexible predator–prey model separates prey into invulnerable and vulnerable pools where predators can consume prey in the latter. As this approach can represent classic Lotka–Volterra and ratio-dependent dynamics, we fit a foraging arena predator–prey model to the number of surviving prey.
3. We found that prey exhibited density-dependent prey behaviours, hiding at low densities, shoaling at medium densities, and using a provided refuge at high densities. Predators exhibited ratio-dependent behaviours, using an ambush foraging mode when one predator was present, hiding in the shadows at low prey–high predator densities, and shoaling at medium and high prey–high predator densities. The foraging arena model predicted the mortality rates well until the high prey–high predator treatment where group vigilance prey behaviours occurred and predators probably interfered with one another resulting in the model predicting higher mortality than observed.
4. This is concerning given the ubiquity of predator–prey models in ecology and natural resource management. Furthermore, as Allee effects engender instability in population regulation, it could lead to inaccurate predictions of conservation status, population rebuilding or harvest rates.

     

Effects of prey abundance,distribution, visual contrast and morphology on selection by a pelagic piscivore

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Dietary complexity and hidden costs of prey switching in a generalist top predator

1. Variation in predator diet is a critical aspect of food web stability, health, and population dynamics of predator/ prey communities. Quantifying diet, particularly among cryptic species, is extremely challenging, however, and differentiation between demographic subsets of populations is often overlooked.
2. We used prey remains and data taken postmortem from otter Lutra lutra to determine the extent to which dietary variation in a top predator was associated with biotic, spatial, and temporal factors.
3. Biotic data (e.g., sex, weight, and length) and stomach contents were taken from 610 otters found dead across England and Wales between 1994 and 2010. Prey remains were identified to species where possible, using published keys and reference materials. Multi‐model inference followed by model prediction was applied to test for and visualize the nature of associations.
4. Evidence for widespread decline in the consumption of eels (Anguilla anguilla) reflected known eel population declines. An association between eel consumption and otter body condition suggested negative consequences for otter nutrition. Consumption of Cottus gobio and stickleback spp. increased, but was unlikely to compensate (there was no association with body condition). More otters with empty stomachs were found over time. Otter sex, body length, and age‐class were important biotic predictors of the prey species found, and season, region, and distance from the coast were important abiotic predictors.
5. Our study is unique in its multivariate nature, broad spatial scale, and long‐term dataset. Inclusion of biotic data allowed us to reveal important differences in costs and benefits of different prey types, and differences between demographic subsets of the population, overlaid on spatial and temporal variation. Such complexities in otter diet are likely to be paralleled in other predators, and detailed characterization of diet should not be overlooked in efforts to conserve wild populations.

     

Realized trophic niche driven by apparent competition: an example with marsupials

According to apparent competition theory, the co‐occurrence of two species that share the same predators appears to affect each other's population growth and abundance. However, due to habitat loss and over‐hunting, top predators are being made rare worldwide. Considering that apparent competitors share similar resources, we would expect the absence of top predators to reflect in changes on prey realized trophic niches. To test our hypothesis, we developed a model to predict the abundance ratio of apparent competitor species based on changes in their realized trophic niches. We tested our model against field data on the Neotropical marsupials Didelphis aurita and Metachirus nudicaudatus. Our results revealed that D. aurita and M. nudicaudatus are two species under apparent competition and their realized trophic niche and diet overlap change according to the presence of top predators. The model was able to predict the actual relative abundances of D. aurita and M. nudicaudatus in the three empirical studies analyzed. Our study presents quantitative support to the apparent competition theory; however, the model's applications to other groups still need to be verified. Additionally, our study shows that the lack of top predators has consequences on the realized trophic niche of their prey, and therefore, we reinforce that conservation plans need to focus on the effects of top predator loss on ecosystems.     

Does the strength of cross‐ecosystem trophic cascades vary with ecosystem size? A test using a natural microcosm

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Inter‐specific differences in invader and native fish functional responses illustrate neutral effects on prey but superior invader competitive ability

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Functional responses of three guilds of spiders: Comparing single‐ and multiprey approaches

Spiders are successful natural enemies of pests occurring throughout the different strata of an agroecosystem. The study of their functional responses can provide information related to the potential effectiveness of different species and guilds on reducing a pest population. However, multiple prey availability may change the functional response of a predator. In this study, the functional responses of three species of spiders in single‐prey and multiple‐prey experiments were modelled. The spider species Haplodrassus rufipes, Araniella cucurbitina and Synema globosum were chosen as being representatives of ground runners, orb‐weavers and ambushers, respectively. Three prey species were selected: a target prey, the Mediterranean fruit fly Ceratitis capitata and two alternate prey species, the flour moth Ephestia kuehniella and the house cricket Acheta domesticus. When the selected target prey C. capitata was supplied in the single‐prey experiments, the most and the least efficient spider species were H. rufipes and S. globosum, respectively. However, opposite results were obtained when alternative preys were supplied. Also, A. cucurbitina significantly changed its functional response with the presence of alternative prey species. Considering the prey preference during the multiple‐prey experiments, A. domesticus, used as representative of a heavy, long‐sized and highly motile prey, was avoided whereas C. capitata, the target pest used as representative of light, small‐sized and moderately motile prey was preferred by the three species of spiders. Ephestia kuehniella, used as representative of light, medium‐sized and low motile prey was occasionally consumed. Each guild could include efficient predators against pests according to its hunting strategies and the ecological exploited niches. Orb‐weavers could be efficient predators against flying pests; ambushers such as S. globosum could contribute to the reduction of the populations of flower‐visiting pests, whereas active ground hunters may also play an important role preying on pests that develop a part or all of its life cycle in the ground. However, further research on feeding behaviour such as prey switching is needed for a better understanding of the effectiveness of spiders as natural enemies.     

Flow‐mediated predator–prey dynamics influence fish populations in a tropical river

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