Importance of local vs. geographic variation in salt marsh plant quality for arthropod herbivore communities

Abstract

Summary An important recent advance in food web ecology has been the application of theory regarding spatial gradients to studies of the factors that affect animal population dynamics. Building on extensive studies of the Spartina alterniflora food web at the local scale, we hypothesized that geographic variation in S. alterniflora quality is an important bottom-up control on food web structure and that geographic variation in S. alterniflora quality would interact with the presence of predators and top omnivores to mediate herbivore densities. We employed a four-factor fully crossed experiment in which we (i) collected plants from high- and low-latitude locations and grew them in a common garden and varied (ii) plant fertilization status (mimicking the plant quality differences due to marsh elevation), (iii) mesopredator density and (iv) omnivore density. Our results suggest that the single most important factor mediating insect herbivore densities is local variation in plant quality – induced in our experiment by fertilization and demonstrated repeatedly as a consequence of marsh elevation. Top-down effects were generally weak and in those cases where predators did exert a significant suppressing effect on herbivores, that impact was itself mediated by host-plant characteristics. Finally, despite observed variation in plant quality with latitude, and the separately measurable effects of this variation on herbivores, geographic-scale variation in plant quality was overwhelmed by local conditions in our experiments. Synthesis. We suggest that a first-order understanding of variation across large latitudinal ranges in the Spartina alterniflora arthropod food web must begin with local variation in plant quality, which provides strong bottom-up forcing to herbivore populations. A second-order understanding of the arthropod food web should consider the role of predation in controlling herbivores feeding on low-quality plants. Finally, while latitudinal variation in plant quality probably explains some variation in herbivore densities, it is probably more of a response to herbivore pressure than a driver of the herbivore dynamics. Although extrapolating from local to geographic scales presents multiple challenges, it is an essential task in order for us to develop an understanding that is general rather than site-specific.