Plant production and alternate prey channels impact the abundance of top predators

Ali Arab, Gina M. Wimp

While numerous studies have examined the effects of increased primary production on higher trophic levels, most studies have focused primarily on the grazing food web and have not considered the importance of alternate prey channels. This has happened despite the fact that fertilization not only increases grazing herbivore abundance, but other types of consumers such as detritivores that serve as alternate prey for generalist predators. Alternate prey channels can sustain generalist predators at times when prey abundance in the grazing food web is low, thus increasing predator densities and the potential for trophic cascades. Using arthropod data from a fertilization experiment, we constructed a hierarchical Bayesian model to examine the direct and indirect effects of plant production and alternate prey channels on predators in a salt marsh. We found that increased plant production positively affected the density of top predators via effects on lower trophic level herbivores and mesopredators. Additionally, while the abundance of algivores and detritivores positively affected mesopredators and top predators, respectively, the effects of alternate prey were relatively weak. Because previous studies in the same system have found that mesopredators and top predators rely on alternate prey such as algivores and detritivores, future studies should examine whether fertilization shifts patterns of prey use by predators from alternate channels to the grazing channel. Finally, the hierarchical Bayesian model used in this study provided a useful method for exploring trophic relationships in the salt marsh food web, especially where causal relationships among trophic groups were unknown.