Abstract
The enemy‐free space hypothesis (EFSH) contends that generalist predators select for dietary specialization in insect herbivores. At a community level, the EFSH predicts that dietary specialization reduces predation risk, and this pattern has been found in several studies addressing the impact of individual predator taxa or guilds. However, predation at a community level is also subject to combinatorial effects of multiple‐predator types, raising the question of how so‐called multiple‐predator effects relate to dietary specialization in insect herbivores. Here, we test the EFSH with a field experiment quantifying ant predation risk to insect herbivores (caterpillars) with and without the combined predation effects of birds. Assessing a community of 20 caterpillar species, we use model selection in a phylogenetic comparative framework to identify the caterpillar traits that best predict the risk of ant predation. A caterpillar species' abundance, dietary specialization, and behavioral defenses were important predictors of its ant predation risk. Abundant caterpillar species had increased risk of ant predation irrespective of bird predation. Caterpillar species with broad diet breadth and behavioral responsiveness to attack had reduced ant predation risk, but these ant effects only occurred when birds also had access to the caterpillar community. These findings suggest that ant predation of caterpillar species is density‐ or frequency‐dependent, that ants and birds may impose countervailing selection on dietary specialization within the same herbivore community, and that contingent effects of multiple predators may generate behaviorally mediated life‐history trade‐offs associated with herbivore diet breadth.
Highlights
The “enemy‐free space hypothesis” (EFSH) holds that the evolution of dietary specialization of small herbivorous arthropods is selected by fitness benefits arising from the use of specific host plants for de‐ fense or refuge from generalist predators (Bernays & Graham, 1988)
We ask whether ant preda‐ tion biased toward dietary specialist herbivores and contingency on bird predation are community‐wide patterns that oppose the com‐ munity‐level prediction of the EFSH. Are these patterns driven by dominant, outlier herbivore species, with most species conforming to the EFSH prediction of reduced ant predation on dietary special‐ ists? How do these patterns relate to other putatively important her‐ bivore traits, such as behavioral defenses, mobility, body size, and abundance, which all may correlate with diet breadth? In particular, we address the hypothesis that dietary specialization trades off with the strength of behavioral defenses in the herbivore community
As expected based on their similar effects in models accounting for bird‐exclusion effects, picFBR and picHPD were positively cor‐ related across all 20 caterpillar species, showing that dietary generalists were more likely than specialists to respond behaviorally when attacked
Summary
The “enemy‐free space hypothesis” (EFSH) holds that the evolution of dietary specialization of small herbivorous arthropods is selected by fitness benefits arising from the use of specific host plants for de‐ fense or refuge from generalist predators (Bernays & Graham, 1988). The palatability experiments used in previous EFSH studies eliminated or reduced possible effects of be‐ havioral defenses by using freshly killed caterpillars (e.g., Bernays & Cornelius, 1989) or live caterpillars removed from their host plants and experimentally placed near ant nests or foraging trails (e.g., Dyer, 1995, 1997; Dyer & Floyd, 1993) These palat‐ ability experiments successfully tested one specific mechanism of the EFSH, that is, dietary specialization imparts EFS via superior co‐opting of plant allelochemicals, while offering limited opportuni‐ ties to test alternative mechanisms. Based on evidence that dietary generalist cater‐ pillars and tortoise beetle larvae exhibit more behavioral defenses than specialists do (Bernays, 1988; Coley, Bateman, & Kursar, 2006; Vencl, Nogueira‐de‐Sa, Allen, Windsor, & Futuyma, 2005), we hy‐ pothesize that behavioral defenses associated with dietary general‐ ization might offset or trade off with unpalatability associated with dietary specialization To address this hypothesis, we measure ant predation risk in situ coupled with laboratory behavioral assays for each caterpillar species.
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