Abstract
According to apparent competition theory, sharing a predator should cause indirect interactions among prey that can affect the structure and the dynamics of natural communities.Though shifts in prey dominance and predator resource use along environmental gradients are rather common, empirical evidence on the role of indirect prey–prey interactions through shared predation particularly with increasing productivity, is still scarce.In an 8‐week lake mesocosm experiment, we manipulated both the addition of inorganic nutrients and the presence of generalist fish predators (crucian carp, Carassius carassius L.), to test for the effects of indirect interactions through shared predation along a productivity gradient.We found that apparent mutualism (indirect positive interaction) between benthic and pelagic prey strongly affected short‐term responses of aquatic food webs to increasing productivity in the presence of a generalist fish. Increasing productivity favoured the relative abundance of benthic prey, following trends in natural productive lake systems. This led to a shift in fish selectivity from pelagic to benthic prey driven by changes in fish behaviour, which resulted in apparent mutualism due to the lower and delayed top‐down control of pelagic prey at increasing productivity.Our results show empirical evidence that the coupling of multiple production pathways can lead to strong indirect interactions through shared predation, whereby prey dynamics on short time‐scales are highly dependent on the foraging behaviour of generalist predators. This mechanism may play an important role in short‐term responses of food webs across environmental gradients.
Highlights
Food webs are one of the main functional units that regulate ecosystem responses to environmental change
This led to a shift in fish selectivity from pelagic to benthic prey driven by changes in fish behaviour, which resulted in apparent mutualism due to the lower and delayed top-down control of pelagic prey at increasing productivity
Our results demonstrate that apparent mutualism (+,+) can play a role in structuring benthic and pelagic communities over short time periods, where increasing proportions of benthic prey at increasing productivity led to lower top-down control of alternative pelagic prey in experimental mesocosms
Summary
Food webs are one of the main functional units that regulate ecosystem responses to environmental change. Natural food webs are often dominated by generalist predators that feed on multiple prey items thereby coupling different energy pathways (Polis & Strong, 1996; Rooney et al, 2008; Wolkovich et al, 2014) Under such circumstances, the theory of apparent competition predicts that additional prey will lead to indirect prey–prey interactions that alter the top-down regulation of the alternative food web pathways (Holt, 1977; Holt & Bonsall, 2017). We hypothesize that short-term apparent mutualism drives responses of benthic and pelagic communities to increasing productivity, whereby increasing relative abundance of Chironomidae along the nutrient gradient will lead to a weakening of top-down control on alternative Cladocera prey. We hypothesize that short-term apparent mutualism will drastically affect the structure and dynamics of benthic and pelagic prey, with potentially important consequences for the seasonal dynamics of aquatic communities
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