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Food availability and prey age alter trophic cascade strength

Predator-prey interactions impact not only the organisms directly involved, but can also have cascading effects on organisms throughout an ecosystem. These trophic cascades can be caused by actual predation in which predators reduce prey numbers via consumption, or by fear of predators, leading to behavioral changes in prey such as reduced foraging and growth. Adding complexity, these interactions are also predicted to depend on resource abundance and prey state (age, size, etc.), but little empirical data is available to support these predictions.

To further understanding of these complex predator-prey interactions, MSC Postdoctoral Researcher Catherine Matassa, PhD Candidate Sarah Donelan, and Professor Geoffrey Trussell, along with Dr. Barney Luttbeg of Oklahoma State University, investigated how resource abundance and prey age impact the strength of trophic cascades in a rocky shore food web.

In the study, recently published in the journal Oikos, Matassa and colleagues conducted an experiment in the running seawater lab at the MSC to test the effects of green crab predation risk on the behavior and feeding habits of its prey, the intertidal snail the Atlantic dogwhelk.

Results indicated that snails avoided risky habitats and reduced their foraging rates on barnacles when crabs were present or when risky habitats were not profitable (low food availability). By manipulating barnacle abundance and snail age in conjunction with predation risk, the researchers found that this trophic cascade was strengthened in situations where there was more food and snails were more in a more mature life stage.

The study also showed that juvenile and adult snails react differently to these trade offs, perhaps due to differences in their energy reserves, ability to gather food, or investment in reproductive development.

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Two snails (Nucella lapillus) forage on barnacles (Semibalanus balanoides) in the rocky intertidal zone. In their study, “Resource levels and prey state influence antipredator behavior and the strength of nonconsumptive predator effects,” Matassa and colleagues examine how barnacle abundance affects the responses of juvenile and larger, sub-adult snails to predatory risk cues from the green crab (Carcinus maenas). Photo credit: Catherine Matassa

The authors suggest that the ecological consequences of predators scaring prey may be more pronounced in systems with abundant food and that the demographic composition of prey populations can shape trophic cascades driven by predation risk. This research illustrates the importance of taking functional traits into account when studying food webs and trophic cascades, and provides an example of using a more holistic approach to studying the effects of predators within ecological communities.

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