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Predators indirectly induce stronger prey through a trophic cascade.

ABSTRACT: Many prey species induce defences in direct response to predation cues. However, prey defences could also be enhanced by predators indirectly via mechanisms that increase resource availability to prey, e.g. trophic cascades. We evaluated the relative impacts of these direct and indirect effects on the mechanical strength of the New Zealand sea urchin Evechinus chloroticus We measured crush-resistance of sea urchin tests (skeletons) in (i) two marine reserves, where predators of sea urchins are relatively common and have initiated a trophic cascade resulting in abundant food for surviving urchins in the form of kelp, and (ii) two adjacent fished areas where predators and kelps are rare. Sea urchins inhabiting protected rocky reefs with abundant predators and food had more crush-resistant tests than individuals on nearby fished reefs where predators and food were relatively rare. A six-month long mesocosm experiment showed that while both food supply and predator cues increased crush-resistance, the positive effect of food supply on crush-resistance was greater. Our results demonstrate a novel mechanism whereby a putative morphological defence in a prey species is indirectly strengthened by predators via cascading predator effects on resource availability. This potentially represents an important mechanism that promotes prey persistence in the presence of predators.

SUBMITTER: Spyksma AJP

PROVIDER: S-EPMC5698640 | biostudies-literature | 2017 Nov

REPOSITORIES: biostudies-literature

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Predators indirectly induce stronger prey through a trophic cascade.

Spyksma Arie J P AJP Shears Nick T NT Taylor Richard B RB

Proceedings. Biological sciences 20171101 1866

Many prey species induce defences in direct response to predation cues. However, prey defences could also be enhanced by predators indirectly via mechanisms that increase resource availability to prey, e.g. trophic cascades. We evaluated the relative impacts of these direct and indirect effects on the mechanical strength of the New Zealand sea urchin <i>Evechinus chloroticus</i> We measured crush-resistance of sea urchin tests (skeletons) in (i) two marine reserves, where predators of sea urchin ...[more]

PMID: 29093219

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Project description:When prey are differentially affected by intra and interspecific competition, the cooccurrence of multiple prey species alters the per capita availability of food for a particular prey species which could alter how prey respond to the threat of predation, and hence the overall-effect of predators. We conducted an experiment to examine the extent to which the nonconsumptive and overall effect of predatory water bugs on snail and tadpole traits (performance and morphology) depended on whether tadpoles and snails cooccurred. Tadpoles and snails differed in their relative susceptibility to intraspecific and interspecific competition, and predators affected both prey species via consumptive and nonconsumptive mechanisms. Furthermore, the overall effect of predators often depended on whether another prey species was present. The reasoning for why the overall effect of predators depended on whether prey species cooccurred, however, differed for each of the response variables. Predators affected snail body growth via nonconsumptive mechanisms, but the change in the overall effect of predators on snail body growth was attributable to how snails responded to competition in the absence of predators, rather than a change in how snails responded to the threat of predation. Predators did not affect tadpole body growth via nonconsumptive mechanisms, but the greater vulnerability of competitively superior prey (snails) to predators increased the strength of consumptive mechanisms (and hence the overall effect) through which predators affected tadpole growth. Predators affected tadpole morphology via nonconsumptive mechanisms, but the greater propensity for predators to kill competitively superior prey (snails) enhanced the ability of tadpoles to alter their morphology in response to the threat of predation by creating an environment where tadpoles had a higher per capita supply of food available to invest in the development of morphological defenses. Our work indicates that the mechanisms through which predators affect prey depends on the other members of the community.

Project description:Documenting trophic niche partitioning and resource use within a community is critical to evaluate underlying mechanisms of coexistence, competition, or predation. Detailed knowledge about foraging is essential as it may influence the vital rates, which, in turn, can affect trophic relationships between species, and population dynamics. The aims of this study were to evaluate resource and trophic niche partitioning in summer/autumn between the endangered Atlantic-Gaspésie caribou (Rangifer tarandus caribou) population, moose (Alces americanus) and their incidental predators, the black bear (Ursus americanus) and coyote (Canis latrans), and to quantify the extent to which these predators consumed caribou. Bayesian isotopic analysis showed a small overlap in trophic niche for the two sympatric ungulates suggesting a low potential for resource competition. Our results also revealed that caribou occupied a larger isotopic niche area than moose, suggesting a greater diversity of resources used by caribou. Not surprisingly, coyotes consumed mainly deer (Odocoileus virginianus), moose, snowshoe hare (Lepus americanus), and occasionally caribou, while bears consumed mainly vegetation and, to a lesser extent, moose and caribou. As coyotes and bears also feed on plant species, we documented trophic niche overlap between caribou and their predators, as searching for similar resources can force them to use the same habitats and thus increase the encounter rate and, ultimately, mortality risk for caribou. Although the decline in the Gaspésie caribou population is mostly driven by habitat-mediated predation, we found evidence that the low level of resource competition with moose, added to the shared resources with incidental predators, mainly bears, may contribute to jeopardize the recovery of this endangered caribou population. Highlighting the trophic interaction between species is needed to establish efficient conservation and management strategies to insure the persistence of endangered populations. The comparison of trophic niches of species sharing the same habitat or resources is fundamental to evaluate the mechanisms of coexistence or competition and eventually predict the consequences of ecosystem changes in the community.

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Predators indirectly induce stronger prey through a trophic cascade.

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Predators indirectly induce stronger prey through a trophic cascade.

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