Abstract
Understanding the factors governing predation remains a top priority in ecology. Using a dragonfly nymph-tadpole system, we experimentally varied predator density, prey density, and prey species ratio to investigate: (i) whether predator interference varies between prey types that differ in palatability, (ii) whether adding alternate prey influences the magnitude of predator interference, and (iii) whether patterns of prey selection vary according to the predictions of optimal diet theory. In single-prey foraging trials, predation of palatable leopard frog tadpoles was limited by prey availability and predator interference, whereas predation of unpalatable toad tadpoles was limited by handling time. Adding unpalatable prey did not affect the predator’s kill rate of palatable prey, but the presence of palatable prey increased the influence of predator density on the kill rate of unpalatable prey and reduced unpalatable prey handling time. Prey selection did not change with shifts in the relative abundance of prey types. Instead, predators selected easy-to-capture unpalatable prey at low total densities and harder-to-capture palatable prey at high densities. These results improve our understanding of generalist predation in communities with mobile prey, and illustrate that characteristics of the prey types involved govern the extent to which alternate prey influence the predator’s kill rate.
Highlights
Understanding the factors governing predation remains a top priority in ecology
Natural systems vary considerably in the abundance of prey and predators, as well as in the diversity of available prey types. Understanding how these factors interact to influence predation within a community has been a longstanding priority of ecologists[1,2,3]. Key developments in this area include recognition of non-linearities in the relationship between prey density and a predator’s per capita kill rate, refinement of functional response models to account for predator-predator interactions that depress per capita kill rate (i.e., interference5–7), and optimal foraging theory which helped explain prey selection based on prey availability and the relative profitability of alternate p rey[3,8]
Prey selection was influenced by total prey density, not the relative availability of either prey type, and shifted from a preference for unpalatable mobile prey at low densities to a preference for relatively inactive palatable prey when prey density was high
Summary
Understanding the factors governing predation remains a top priority in ecology. Using a dragonfly nymph-tadpole system, we experimentally varied predator density, prey density, and prey species ratio to investigate: (i) whether predator interference varies between prey types that differ in palatability, (ii) whether adding alternate prey influences the magnitude of predator interference, and (iii) whether patterns of prey selection vary according to the predictions of optimal diet theory. Predators selected easy-to-capture unpalatable prey at low total densities and harder-to-capture palatable prey at high densities These results improve our understanding of generalist predation in communities with mobile prey, and illustrate that characteristics of the prey types involved govern the extent to which alternate prey influence the predator’s kill rate. Frequency-dependent switching generates distinct patterns in indices of selective predation as well as in the predator’s functional response (i.e., a sigmoidal shaped response reflecting a region of density-dependent predation) and has been linked to increased stability in multiprey communities (e.g.16; but s ee[17,18,19]) Another way predators adjust their foraging behaviour is through shifts in the amount of active prey search with changes in total prey density (e.g.20,21), which can influence the functional response s hape[22]. While it has been demonstrated that alternate prey can critically influence the extent of predator interference[12], this seemingly important result has subsequently received limited attention
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