Interactive Effects Of Predation Research Articles

Community variability in pond metacommunities: interactive effects of predators and isolation on stochastic community assembly

In the absence of environmental heterogeneity, spatial variation among local communities can be mostly attributed to random dispersal, demographic stochasticity and species interactions causing historical contingency in colonization (e.g. priority effects). The consequences of demographic stochasticity and random dispersal are highly dependent on community size, regional richness and local richness, which, along with historical contingency can be strongly affected by spatial isolation and the presence of predators. Here, we tested whether and how the presence of a generalist predatory fish and spatial isolation can change spatial community variability in artificial ponds by manipulating the presence/absence of fish (redbreast tilapia) at three different distances from a source wetland. We then used null models to compute how much the observed community variability within treatments deviates from the expected from randomly reassembled communities (i.e. beta‐deviation). We found that spatial isolation can have both negative and positive effects on community variability, and their importance may depend on the presence of predatory fish. Negative effects likely occurred because predatory insects cannot always successfully colonize highly isolated ponds, causing the abundance of herbivores and detritivores to increase, consequently increasing community size, which is known to decrease the importance of demographic stochasticity. However, when fish are absent, the more isolated communities exhibit more community variability than is expected from random community assembly. We believe that such an increase in variability is most likely due to an increase in the importance of historical contingency in colonization, generating more distinct communities in isolated ponds. Such effects were absent in the presence of generalist predatory fish, likely because fish prevented any early colonizing taxa from becoming too abundant. Our findings show that the presence of generalist predators can change how community variability responds to spatial isolation.

Interacting forces of predation and fishing affect species' maturation size.

Fishing is a strong selective force and is supposed to select for earlier maturation at smaller body size. However, the extent to which fishing‐induced evolution is shaping ecosystems remains debated. This is in part because it is challenging to disentangle fishing from other selective forces (e.g., size‐structured predation and cannibalism) in complex ecosystems undergoing rapid change.Changes in maturation size from fishing and predation have previously been explored with multi‐species physiologically structured models but assumed separation of ecological and evolutionary timescales. To assess the eco‐evolutionary impact of fishing and predation at the same timescale, we developed a stochastic physiologically size‐structured food‐web model, where new phenotypes are introduced randomly through time enabling dynamic simulation of species' relative maturation sizes under different types of selection pressures.Using the model, we carried out a fully factorial in silico experiment to assess how maturation size would change in the absence and presence of both fishing and predation (including cannibalism). We carried out ten replicate stochastic simulations exposed to all combinations of fishing and predation in a model community of nine interacting fish species ranging in their maximum sizes from 10 g to 100 kg. We visualized and statistically analyzed the results using linear models.The effects of fishing on maturation size depended on whether or not predation was enabled and differed substantially across species. Fishing consistently reduced the maturation sizes of two largest species whether or not predation was enabled and this decrease was seen even at low fishing intensities (F = 0.2 per year). In contrast, the maturation sizes of the three smallest species evolved to become smaller through time but this happened regardless of the levels of predation or fishing. For the four medium‐size species, the effect of fishing was highly variable with more species showing significant and larger fishing effects in the presence of predation.Ultimately our results suggest that the interactive effects of predation and fishing can have marked effects on species' maturation sizes, but that, at least for the largest species, predation does not counterbalance the evolutionary effect of fishing. Our model also produced relative maturation sizes that are broadly consistent with empirical estimates for many fish species.

Interactive effects of predation and facilitation during the settlement process of non-indigenous species

Interactive effects of predation and facilitation during the settlement process of non-indigenous species

Ecology of Caribbean Sponges: Are Top-Down or Bottom-Up Processes More Important?

Benthic-pelagic coupling and the role of bottom-up versus top-down processes are recognized as having a major impact on the structure of marine communities. While the roles of bottom-up processes are better appreciated they are still viewed as principally affecting the outcome of top-down processes. Sponges on coral reefs are important members of the benthic community and provide a critically important functional linkage between water-column productivity and the benthos. As active suspension feeders sponges utilize the abundant autotrophic and heterotrophic picoplankton in the water column. As a result sponges across the Caribbean basin exhibit a consistent and significant pattern of greater biomass, tube extension rate, and species numbers with increasing depth. Likewise, the abundance of their food supply also increases along a depth gradient. Using experimental manipulations it has recently been reported that predation is the primary determinant of sponge community structure. Here we provide data showing that the size and growth of the sponge Callyspongia vaginalis are significantly affected by food availability. Sponges increased in size and tube extension rate with increasing depth down to 46 m, while simultaneously exposed to the full range of potential spongivores at all depths. Additionally, we point out important flaws in the experimental design used to demonstrate the role of predation and suggest that a resolution of this important question will require well-controlled, multi-factorial experiments to examine the independent and interactive effects of predation and food abundance on the ecology of sponges.

Predator-pathogen interactions: synergy between mortality causes and failure of the healthy herds hypothesis

Ecology can be studied at multiple scales (e.g. population ecology vs. behavioural ecology), yet studies that focus on the relationships between dynamics at different scales are critical for understanding the functions and processes of ecosystems (Levin 1992). Fine-scale variables, such as behavioural and physiological traits of individual organisms, can have major effects on broader-scale dynamics, such as population and evolutionary processes. For example, the foraging activities of, and prey selection by, predators serve as selective filters that can alter the distribution of phenotypic traits within their prey populations. Such predator-driven natural selection is widely acknowledged to increase the fitness of prey herds by weeding out the sicker and weaker animals, resulting in a ‘healthy herd’ (Packer et al. 2003). At broader scales, predation can also alter the composition of and interactions within prey assemblages if some species are more impacted by predation than others (Chase et al. 2002). Mechanisms such as these provide insights into the important connections between behavioural ecology and evolutionary patterns, and understanding such interactions is necessary to inform management and conservation efforts in the face of broad-scale changes in ecosystems worldwide. However, the complexity of ecosystems raises the question of how variation in simple, fine-scale dynamics, such as the filter of selective predation, translates upward to dynamics at higher ecological scales. As the number of species in an ecological community change, perhaps driven by anthropogenic change, the web of interactions among these species will likewise change, and the direct and indirect effects of these species on one another will take new forms. Indirect effects, such as predator-induced shifts in pathogen–host interactions, are difficult to observe or predict but are ubiquitous in natural communities (Wootton 1994; Werner & Peacor 2003). In this volume, Duffy et al. (2011) present a study that highlights the complexities of, and vagaries associated with, interaction webs in ecological communities. They demonstrate a counterexample to the aforementioned ‘healthy herds’ hypothesis of predator-mediated population dynamics, a fact that has important consequences for our understanding of many ecological processes. They show that the predator-avoidance mechanism of an herbivore renders it more susceptible to a pathogen, demonstrating that, contrary to the classical hypothesis, predators can have negative effects on the health of prey populations. Daphnia dentifera, an herbivorous cladoceran, allocated more resources to growth when in the presence of a gape-limited predator, the larva of Chaoborus phantom midges. This increase in growth allows Daphia to more quickly outgrow the susceptible size classes. However, increased growth corresponded with increased food intake, which also increased the uptake and spore yield of a pathogenic yeast, Metschnikowia, thereby increasing the spread of the pathogen in the Daphnia population. Thus, the net effect of the predator in this system is actually a decrease in the average health of its prey population. This presents a serious challenge to the ‘healthy herd’ hypothesis and calls for a more dynamic hypothesis to explain the effects of predators on prey populations. The ‘healthy herd’ hypothesis can be regarded as prey-centric. That is, it views the health or fitness of herbivore populations as the consequences of a range of phenotypes within the population (e.g. susceptibility to disease), among which predation is the agent of natural selection. However, if herbivore health is determined by the action of a pathogen, rather than by the phenotype of the host alone, a positive predator–pathogen interaction hypothesis may be a more accurate way to model the resulting dynamics. In this light, the ChaoborusDaphnia-Metschnikowia system of Duffy et al. (2011) is analogous to examples of interactions and emergent effects among predators, of which there is a sizable literature available (Soluk & Collins 1988; Losey & Denno 1998; Nystrom et al. 2001; Cardinale et al. 2003). This literature contains examples of predator-avoidance mechanisms that effectively reduce the likelihood of being consumed by one predator, only to increase vulnerability to a second predator (Soluk & Collins 1988; Losey & Denno 1998). Such interactive effects of predators on prey populations are termed ‘risk-enhancing’ because of the increased risk of mortality for the prey (Sih, Englund & Wooster 1998; Schmitz 2007), and ‘trait-mediated’ because it is a phenotypic trait that modulates the interaction among organisms. The study of Duffy et al. (2011) fits in this same category and is particularly noteworthy because, *Correspondence author. E-mail: kdwe222@uky.edu

Interactive effects of predators and a pesticide on aquatic communities

Chemical contaminants are ubiquitous in nature and a major goal of ecologists has been to understand and predict their impacts on natural communities. While direct toxic effects can be garnered from single‐species laboratory studies, the full suite of possible effects can only be observed when organisms are embedded within a community. In this study, we manipulated the concentration of malathion (a broad‐spectrum insecticide) to determine the impacts on aquatic communities containing phytoplankton, periphyton, and 27 species of animals (16 zooplankton, 5 snails, 3 tadpoles and 3 predatory insects). Using relatively low concentrations (0.13 to 0.46 mg l−1), we found important direct (and interactive) effects of predators and malathion on the food web as well as a number of apparent density‐ and trait‐mediated indirect effects. Malathion initiated an indirect effect by decreasing zooplankton diversity and abundance, which propagated an increase in phytoplankton, a decrease in periphyton, and a subsequent decrease in the growth of leopard frog tadpoles. There also was an apparent trait‐mediated indirect effect whereby increased amounts of the pesticide reduced predation rates on amphibians without affecting the survival of the primary amphibian predator (larval Anax dragonflies). In contrast, snail survival and growth was unaffected by the pesticide but there were strong, species‐specific effects from their primary predator (adult Belostoma water bugs). This is one of few studies to examine the impacts of malathion on aquatic communities across a range of concentrations, despite the fact that it is currently the most commonly applied insecticide in the United States, it is applied around the world, and it can be legally directly sprayed over aquatic habitats to control the mosquitoes that carry malaria and West Nile virus. Our results suggest several mechanisms by which a wide variety of pesticides with similar modes of action might impact aquatic communities.

Interactive effects of predators and a pesticide on aquatic communities

Interactive effects of predators and a pesticide on aquatic communities

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.

Episodic disturbance events modify predator–prey interactions in soft sediments

Physical disturbance events are common in shallow soft-sediment habitats and can have significant effects on predator–prey interactions. While several studies have reported on predator aggregations following disturbance events, few studies have investigated the mechanisms and interactive effects of predation and physical disturbance on prey survival in shallow soft-sediment habitats. In this study the interactive effects of sediment resuspension and predation by two contrasting epibenthic predator species were tested on the survival of the amphipod Corophium volutator in a laboratory experiment. The shrimp Crangon crangon and juvenile plaice Pleuronectes platessa were used as predators, both numerical dominants in shallow soft sediments on the Swedish west coast. In addition we quantified epibenthic predator aggregation in the field following small-scale disturbances. In the laboratory, synergistic negative effects of predation and non-lethal disturbance on Corophium survival were found with both predator species, and rapid aggregation of several mobile epibenthic predator species following disturbance was demonstrated in the field. Abundances of C. crangon, the numerically dominant predator in the field, were doubled in disturbed patches within 2 min following disturbance. Our study emphasises the importance of considering episodic small-scale disturbances when interpreting predation effects and trophic interactions in shallow soft-sediment systems.

Interactive Effects of Predation and Dispersal on Zooplankton Communities

Communities of coexisting organisms are formed by both dispersal among habitat patches in a region and local interactions within patches. The supply of colonists from the regional pool has the potential to alter the outcome of local interactions by providing species that fill different ecological roles. I examined the consequences of invasion by fish (juvenileLepomis macrochirus) and insect (Notonecta undulata) predators on experimental plankton communities that were either connected to or isolated from the regional pool of zooplankton species. The effects of predators on zooplankton composition and diversity depended on dispersal by members of the regional zooplankton species pool. Fish and notonectids reduced the abundance of large zooplankton and thereby facilitated invasion by new species from the surrounding region. Both predators reduced zooplankton species richness in the absence of dispersal, while fish had positive effects and notonectids had no effects with rapid dispersal. Fish facilitated invasion by more species than they excluded, while similar numbers of species invaded with notonectids as were driven extinct. The effects of predators on planktonic trophic structure (zooplankton size structure and biomass, phytoplankton biomass) were independent of the level of dispersal. Both predators reduced zooplankton mean body size, but only fish enhanced phytoplankton density. Invasion by members of the regional species pool slightly increased zooplankton mean body size and reduced phytoplankton biomass; however, the effects were small and independent of the predator treatment. Predator-facilitated invasion of new species altered the effects of predators on zooplankton species composition and diversity, but not on plankton trophic structure. The results suggest that spatial heterogeneity in predator abundance promotes regional coexistence among zooplankton. Connection to a diverse regional species pool via dispersal may therefore be important for determining the impact of local interactions such as predation on communities.

Keystone Predation and Interaction Strength: Interactive Effects of Predators on Their Main Prey

The application of basic ecological concepts to fields of conservation biology and applied environmental sciences is a healthy sign, but before these concepts are widely used, ecology must provide operational definitions and quantifiable methods. Keystone species and interaction strength are concepts with deep practical and theoretical implications. We studied the strength of predation on mussels (Mytilus trossulus) by the keystone seastar Pisaster ochraceus and the whelks Nucella emarginata and N. canaliculata under different environmental conditions in the Oregon intertidal zone. We attempted to determine: (1) the sensitivity of keystone predation to the presence of other predators in the system; (2) the role of other predators in the presence and absence of a keystone species; and (3) the per capita and population—level variability in interaction strengths of strong (keystone) vs. weak interactors. Predation intensity on mussels was measured by recording the survival of mussels transplanted to areas from which seastars, whelks, or both, had been either manually removed or left undisturbed at natural densities. Whelk experimental units were nested within those for the seastar treatment to account for the much larger body size and greater mobility of seastars. Each combination of seastar and whelk treatment was replicated four times in both wave—exposed and wave—protected habitats of two sites that differed in predator densities, primary productivity, and recruitment and growth rates of prey species. Predation intensity by the keystone predator was strong under all site $\times$ wave exposure combinations, and was unaffected by the presence of whelks. Whelks, in contrast, had ecologically important effects on mussel survival in the absence, but not in the presence, of the keystone predator. Population (total) interaction strength between seastars and mussels was 2—10 times stronger than that between whelks and mussels across sites and wave exposures. Per capita interaction strength of seastars was two to three orders of magnitude larger than that of whelks. However, per capita effects of seastars were more variable between sites and wave exposures, probably because simple density values grossly underestimate the ability of mobile predators to localize prey. Such interactive effects and variability in interaction strengths between keystone and weak predators may characterize all keystone predator—dominated systems, but data currently are insufficient to test this proposition. Negative effects of seastars on whelk density were observed <4 mo following initiation of Pisaster removals. Seastars also had a negative effect on whelk sizes, which took longer to appear, after 6 mo of continuous Pisaster removal. Negative effects of seastars on whelks appeared to be stronger in places with higher densities of predators, partially explaining the reduced predation intensity of whelks observed in the presence of seastars. Our results support the idea that in keystone—dominated systems, species other than the keystone species have only minor, if any, effects on the rest of the community, and thus might be cited by some as redundant species. However, our results also indicate that, after the loss of a keystone species, previously redundant species can partially compensate for the reduced predation and adopt a major role in the altered system. Such responses are potentially an important force in stabilizing communities. Further, such possible compensatory capabilities of alternative consumers suggests that, at least for predators, the term redundant species conveys an inaccurate image of the potential importance of weak interactors and should be abandoned.

Demographic and life history response of the cladoceranBosmina longirostris to variation in predator abundance.

Population dynamics, demography and body size of the cladoceranBosmina longirostris were examined in an experimental study in which the abundance of its predator (the cyprind fishPhoxinus eos) was varied in an unproductive lake. Four densities of fish were used, encompassing the biomass of fish in the lake.Bosmina was most abundant at low and medium fish densities (1.06 and 2.12 g fish biomass · m-3) and less abundant when fish were either absent or present at high density (3.71 g fish biomass · m-3). The unimodal response to predator abundance resulted from effects on both birth and death rates.Bosmina birth rates increased as fish biomass increased, in response to increasing food (phytoplankton) biomass. Death rates were highest at high fish biomass (because of fish predation) and in the absence of fish (because of predation by the dipteranChaoborus, which was most abundant in the absence of fish). Size-frequency distributions revealed that fish eliminated the larger size classes ofBosmina, and mean carapace length ofBosmina populations was inversely proportional to fish biomass.Bosmina initiated reproduction at smaller size in the presence of fish than in their absence, and size at maturity was inversely proportional to fish biomass. Size at birth also tended to decrease with increasing fish biomass, but this trend was not as strong as that of size at maturity. Decreased size at maturity apparently allowedBosmina individuals to reproduce before becoming vulnerable to fish predation. Flexibility in size at maturity, together with low abundance of invertebrate predators and large herbivores (which were preyed upon by fish), allowedBosmina to become abundant in low and medium fish treatments. In the high fish treatment, mortality due to fish predation was too severe to be offset by decreased size at maturity, andBosmina population density was low. The net response ofBosmina populations to fish predation results from interactive effects of predation on mortality, natality, and life history traits.