COS 141-4
Trophic processes influence the spatial distribution pattern of a chewing herbivore
Most source-sink models assume implicitly that landscape heterogeneity influences patch quality, which in turn drives a species’ distribution across habitat patches. These models describe a species distribution in a phenomenological manner without addressing the actual mechanism underlying heterogeneity of quality. In this study, we assessed bottom-up and top-down effects influencing habitat quality for a chewing herbivore (Lepidopera: Arctiidae) across previously described source and sink habitat patches. We caged early and late instars during the 2012 and 2013 field seasons on the preferred larval host plant at our study site using cages constructed from two different mesh sizes. The larger mesh allowed small invertebrate predators access to half of the experimental caterpillars, while the smaller mesh excluded them. Upon termination of the experiment, we recorded survival, pupal weight, and emerged parasitoids. We also collected leaf samples and measured trichome density and foliar nutrient concentrations.
Results/Conclusions
We found that top-down but not bottom-up effects varied across source and sink habitats. Abundances of parasitoids and parasitoid-attributable caterpillar mortality were higher in sources than sink habitats. This finding suggests that parasitoids track but do not drive P. virginalis population dynamics in this system. Though bottom-up effects did not vary significantly between sources and sinks, we found that foliar nutrients and mechanical defenses were good predictors of caterpillar growth rates. For early instar caterpillars, we found higher survival rates in source than sink habitat. These results show that the importance of trophic forces varies across habitats and ontogenetic stages. This work indicates that trophic processes underlying habitat quality at the local scale can drive species distributions at the landscape scale, consistent with predictions under a source-sink dynamics model.