Most animals undergo substantial changes in body size and morphology during ontogeny. Body size determines many aspects of ecological performance, including foraging rate, fecundity, range size and predation risk. Size structure in prey populations can strongly influence predator-prey interactions, and prey size selectivity by predators can strongly influence long term population dynamics and community structure. Indeed, predator consumption rates are commonly hump-shaped functions of prey size such that intermediate sized prey are most vulnerable. Prey, therefore, pass through windows of vulnerability to size-specific predators as they grow through ontogeny, and growth rates can be strong determinants of survival. Prey growth rates are, in part, driven by resource availability. Consequently, resources may impose bottom up control on the outcome of size-structured predator-prey interactions. Here we ask whether resource availability affects the interactions between red-eyed treefrog (Agalychnis callidryas) tadpoles and two different size limited predators—dragonfly nymphs and giant water bugs.
To accomplish this, we develop a simulation from short-term empirical studies that predicts size- and density-specific growth and mortality of A. callidryas at different resource levels. We first used a response surface approach to quantify size-specific functional responses for larval dragonfly and water bug predators. Both predators have hump shaped size-specific predation rates, and the most vulnerable sized prey for water bugs is larger than for larval dragonflies. We then used another response surface experimental approach to integrate studies of density- and size-specific growth rates of red-eyed treefrog tadpoles across a resource gradient. These results were incorporated into our simulation model to examine whether resource availability can improve prey survival by decreasing the time it takes the prey to reach a size-refuge and by differentially affecting the shapes of the predators’ functional responses. We show that resource availability can affect tadpole survival and size structure via its effects on tadpole growth rates. Our findings suggest that resources can play an important role in driving higher level trophic interactions. We also highlight the utility of response surface approaches for attaining a more comprehensive understanding of consumer – resource interactions and for estimating parameters that enable us to scale up our empirical findings.