Can environmental variation generate positive indirect effects in a model of shared predation?
Brassil, Chad*,1, 1 Michigan State University, Hickory Corners, MI
ABSTRACT- Classic models of apparent competition predict negative indirect effects between prey with a shared enemy. If predator per capita growth rates are nonlinear, then endogenously generated periodic cycles are predicted to generate less negative, or even positive indirect effects between prey. Here I determine how exogenous mechanisms such as environmental variation could modify indirect effects. I find that exogenous variation can have a broader range of effects on indirect interactions than endogenously generated cycles. For models in which the per capita growth rates of the predator species is a linear function of population densities, indirect effects are altered by variation in parameters that affect the response of the predator per capita growth rate to prey consumption. Even in this simple model, temporal variation that affects the interaction of predators and prey or the response of predators to prey consumption can lead to large increases or decreases in the indirect effect between prey, dependent on how prey populations co-vary with the parameter changing due to environmental variation. Positive indirect effects can occur when the period of environmental variation is close to the natural period of the biological system and shifts in subharmonic resonance occur with the addition of the second prey. Models that include nonlinear numerical responses generally lead to indirect effects that are sensitive to environmental variation in more parameters and across a wider range of frequencies.
Key words: extrinsic forcing, subharmonic resonance, apparent competition, noise amplification
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