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Natural enemy ecology strives to unify predator–prey and host–pathogen interactions under a common framework to gain insights into community- and ecosystem-level processes. To address this goal, ecologists need a greater emphasis on: (1) quantifying pathogen-mediated effects on community structure to enable comparisons with predator-mediated effects and (2) determining the interactive effects of combined natural enemies on communities. We conducted a mesocosm experiment to assess the individual and combined effects of predators (dragonfly larvae and adult water bugs) and a pathogen (ranavirus) on the abundance and composition of a larval amphibian assemblage. We found that our three natural enemies structured victim assemblages in unique ways, producing distinct assemblages. Additionally, we found that in combination treatments, predators mainly drove assemblage structure such that the assemblages most closely resembled their respective predator treatments. We also found that predators reduced infection prevalence in combination treatments, and that the magnitude of this effect was dependent on predator identity. Compared to virus-alone treatments, the presence of dragonflies and water bugs reduced infection prevalence by 79% and 63%, respectively. Additionally, the presence of dragonflies eliminated ranavirus infection in two species, which demonstrates the prominent role of predators in disease dynamics in this system. Overall, this work demonstrates the importance of considering natural enemies in community ecology, as each enemy can elicit a unique structural change. Additionally, this study provides a unique empirical test of the healthy herds hypothesis for multi-species assemblages and underscores the importance of advancing our understanding of multi-enemy interactions within communities. [ABSTRACT FROM AUTHOR]

We manipulated the diversity of top predators in a three trophic level marine food web. The food web included four top benthic marine fish predators (black goby, rock goby, sea scorpion and shore rockling), an intermediate trophic level of small fish, and a lower trophic level of benthic invertebrates. We kept predator density constant and monitored the response of the lower trophic levels. As top predator diversity increased, secondary production increased. We also observed that in the presence of the manipulated fish predators, the density of small gobiid fish (intermediate consumers) was suppressed, releasing certain groups of benthic invertebrates (caprellid amphipods, copepods, nematodes and spirorbid worms) from heavy intermediate predation pressure. We attribute the mechanism responsible for this trophic cascade to a trait-mediated indirect interaction, with the small gobiid fish changing their use of space in response to altered predator diversity. In the absence of top fish predators, a full-blown trophic cascade occurs. Therefore the diversity of predators reduces the likelihood of trophic cascades occurring and hence provides insurance against the loss of an important ecosystem function (i.e. secondary production). [ABSTRACT FROM AUTHOR]

Many organisms undergo ontogenetic niche shifts due to considerable changes in size during their development. These ontogenetic shifts can alter the trophic position of individuals, the type and strength of ecological interactions across species, and allow for cannibalism within species. In this study we ask if and how the interaction of a size refuge and cannibalism in the prey alters the dynamics of intraguild predation (IGP) systems. By manipulating the composition of large cannibalistic ( Aeshna umbrosa) and predatory ( Anax junius) dragonfly larvae in mesocosms we show that the interaction of cannibals and predators was non-linear and increased the survival of prey. The structure of the final resource community shared by prey and predator differed between small and large dragonfly treatments but not within size classes across species. In general, the small prey stage showed similar shifts in microhabitat use and refuge use when exposed to either conspecific cannibals or predators, while large cannibals showed no clear anti-predator response. However, further behavioral experiments revealed that specific behavioral components, such as distances between individuals or number of movements, differed when individuals were exposed to either cannibals or predators. This indicates that individuals discriminated between conspecific or heterospecific predators. Furthermore, in similar experiments large cannibals and predators showed different behaviors when exposed to conspecifics rather than to each other. These changes in behavior are consistent with the observed increase in prey survival. In general, the results indicate that cannibalism and ontogenetic niche shifts can result in behavior-mediated indirect interactions that reduce the impact of the predator on the mortality of its prey and alter the interactions of IGP systems. However, they also indicate that size is not the sole determinant and that we also need to account for the species identity when predicting the dynamics of communities. [ABSTRACT FROM AUTHOR]

Interactions between multiple predator species are frequent in natural communities and can have important implications for shared prey survival. Predator density may be an important component of these interactions between predator species, as the frequency of interactions between species is largely determined by species density. Here we experimentally examine the importance of predator density for interactions between predator species and subsequent impacts on prey. We show that aggressive interactions between the predatory shore crabs Carcinus maenas and Hemigrapsus sanguineus increased with predator density, yet did not increase as fast as negative interactions between conspecifics. At low density, interactions between conspecific and heterospecific predators had similar inhibitory impacts on predator function, whereas conspecific interference was greater than interference from heterospecifics at high predator density. Thus the impact of conspecific interference at high predator density was sufficient in itself that interactions with a second predator species had no additional impact on per capita predation. Spatial and temporal variability in predator density is a ubiquitous characteristic of natural systems that should be considered in studies of multiple predator species. [ABSTRACT FROM AUTHOR]

Animals in social aggregations often spend more time foraging than solitary conspecifics. This may be a product of the relative safety afforded by aggregations: group members can devote more time to foraging and less time to antipredator behaviors than solitary animals (the “risk reduction” effect). All else being equal, risk reduction should result in higher food intake for grouped animals. However, intragroup competition may force group members to spend more time foraging in order to obtain the same food ration as solitary individuals (the “resource competition” effect). We compared these opposing explanations of foraging time allocation in a coral reef fish, bluehead wrasse ( Thalassoma bifasciatum). Aggregations of juvenile bluehead wrasse experience safety-in-numbers, and preliminary observations suggested that juveniles in aggregations spent more time foraging for copepods in the water column than solitary juveniles. However, the risk reduction and resource competition hypotheses are indistinguishable on the basis of behavioral observations alone. Therefore, we collected behavioral, dietary, and growth data (using otolith growth rings) for bluehead wrasse at multiple reefs around a Caribbean island. Despite spending more time foraging in the water column, grouped fish did not capture more prey items and had slower growth rates than solitary fish. Thus, the increased foraging time of grouped fish appears to reflect resource competition, not risk reduction. This competition may limit the size and frequency of aggregations among juvenile bluehead wrasse, which have been shown to experience reduced mortality rates in larger groups. Bluehead wrasse recruits also spent less time foraging but grew faster at sites where planktonic copepod prey were more abundant. This suggests the possibility that large-scale spatiotemporal variability in the abundance of planktonic copepods over coral reefs may produce corresponding variability in the dynamics of reef fish populations. [ABSTRACT FROM AUTHOR]

When foraging together, multiple predator species that share a single prey often cause prey mortality that cannot be predicted based on knowledge of predation by each species separately. Modeling and managing the effects of multiple predator species depend on accurately assessing these combined effects. Two methods are currently used to experimentally examine combined predation by multiple predator species: the additive and substitutive experimental designs. I simultaneously employed both experimental designs to examine predation by two crab species on shared mussel prey. I show that the two methods yield results that disagree both quantitatively and qualitatively, leading to very different conclusions about the way that predator species combine to affect prey mortality. This discrepancy occurred because the two methods examine complimentary, but not interchangeable questions. I advocate using an experimental design that incorporates both additive and substitutive designs to achieve a more complete understanding of the combined effects of multiple predator species. [ABSTRACT FROM AUTHOR]

Prey are often consumed by multiple predator species. Predation rates on shared prey species measured in isolation often do not combine additively due to interference or facilitation among the predator species. Furthermore, the strength of predator interactions and resulting prey mortality may change with habitat type. We experimentally examined predation on amphipods in rock and algal habitats by two species of intertidal crabs, Hemigrapsus sanguineus (top predators) and Carcinus maenas (intermediate predators). Algae provided a safer habitat for amphipods when they were exposed to only a single predator species. When both predator species were present, mortality of amphipods was less than additive in both habitats. However, amphipod mortality was reduced more in rock than algal habitat because intermediate predators were less protected in rock habitat and were increasingly targeted by omnivorous top predators. We found that prey mortality in general was reduced by (1) altered foraging behavior of intermediate predators in the presence of top predators, (2) top predators switching to foraging on intermediate predators rather than shared prey, and (3) density reduction of intermediate predators. The relative importance of these three mechanisms was the same in both habitats; however, the magnitude of each was greater in rock habitat. Our study demonstrates that the strength of specific mechanisms of interference between top and intermediate predators can be quantified but cautions that these results may be habitat specific. [ABSTRACT FROM AUTHOR]

Understanding whether multiple predator species have independent effects on shared prey is critical for understanding community dynamics. We describe the prevalence and strength of non-independence between predators by quantifying the prey’s risk of predation and the degree to which it deviates from the risk predicted from a null model of independent predator effects. Specifically, we document how frequently non-independent effects occur among ten different multiple predator combinations with mayfly larvae as prey. These predator combinations vary both predator density and predator species richness. Overall, the predator effects were non-independent and translated to an average of 27% fewer prey being consumed compared to independent predator effects. Non-independence of this magnitude is likely to have population level consequences for the prey and influence the distribution or prey preference of predators. Closer inspection shows that much of the risk reduction in this system is weak, to the point of being indistinguishable from independent predator effects, while few effects are strong. This pattern of many weak interactions and few strong ones parallels the pattern of interaction strengths documented previously in intertidal communities. Consequently, understanding strong interactors in multiple predator systems may help us understand the importance of a species. [ABSTRACT FROM AUTHOR]

Interactions between different size classes of predator species have the potential to influence survival of prey species in intraguild predation (IGP) systems, but few studies test for these effects. Using a substitutive design in a field setting, I measured the effects of two size classes of IG predators (large and small larvae of the dragonfly Anax junius) on the mortality of IG prey (larvae of the dragonfly Pachydiplax longipennis). I also examined whether combinations of large A. junius and P. longipennis and small A. junius and P. longipennis had substitutable effects on shared prey (larvae of the damselfly Ischnura verticalis). The presence of both size classes of A. junius, when alone and in combination with P. longipennis, significantly increased mortality of I. verticalis. In the presence of P. longipennis, large and small A. junius had similar effects on the mortality of I. verticalis, and effects of size-structured assemblages of A. junius were similar to the effects of each size class alone at the same density. The effects of the two size classes of A. junius on P. longipennis differed, and P. longipennis mortality was lower when exposed to size structured assemblages of A. junius than when exposed to only large A. junius at the same density. Results were similar to those in a laboratory study, although the effect of P. longipennis on I. verticalis was much lower in the field setting. These results demonstrate that interactions between different size classes of IG predators promote the survival of IG prey and highlight the importance of within-species size structure as a characteristic that may promote the coexistence of predators in IGP systems. [ABSTRACT FROM AUTHOR]

Investigating how prey density influences a prey’s combined predation risk from multiple predator species is critical for understanding the widespread importance of multiple predator effects. We conducted experiments that crossed six treatments consisting of zero, one, or two predator species (hellgrammites, greenside darters, and creek chubs) with three treatments in which we varied the density of mayfly prey. None of the multiple predator effects in our system were independent, and instead, the presence of multiple predator species resulted in risk reduction for the prey across both multiple predator combinations and all three levels of prey density. Risk reduction is likely to have population-level consequences for the prey, resulting in larger prey populations than would be predicted if the effects of multiple predator species were independent. For one of the two multiple predator combinations, the magnitude of risk reduction marginally increased with prey density. As a result, models predicting the combined risk from multiple predator species in this system will sometimes need to account for prey density as a factor influencing per-capita prey death rates. [ABSTRACT FROM AUTHOR]