PARENT SESSION
Oral Session # 20: Biogeography II: Microorganism and Insect Communities.
Presiding: R Winfree
Tuesday, August 5. 8:00 AM to 11:30 AM, SITCC Meeting Room 106.

Trophic cascades dampened in complex food webs with intraguild predation.

Finke, Deborah^*,1, Denno, Robert¹, ¹ University of Maryland, College Park, Maryland, USA

ABSTRACT- The prevalence of trophic cascades in terrestrial systems has been debated. Food web structure is one factor that may mediate the ability of predators to suppress herbivores and thus indirectly enhance plant primary productivity. In complex food webs with reticulate interactions such as intraguild predation, the strength of cascading predator effects may attenuate from one trophic level to the next, precluding a trophic cascade. Thus, trophic cascades may only occur largely in communities with low species diversity and with only a few trophic links among species. The objective of our study was to determine the impact of increasing food web complexity on the strength of trophic cascades on a mid-Atlantic salt marsh. We manipulated food web complexity by increasing predator diversity, both the number of predator species and the number of intraguild predators present. Four food webs of varying complexity were established: plants only (Spartina cordgrass), plants + herbivores (Prokelisia planthoppers), plants + herbivores + single predator (the mirid bug Tytthus), and plants + herbivores + a predator complex including both strict predators (Tytthus bugs and the web-building spider Grammonota) and intraguild predators (the wolf spiders Pardosa and Hogna). After two months of interaction, the effects of the food web complexity treatments on the size of the herbivore population and plant biomass were assessed. In the simple food web with only one predator, a trophic cascade occurred. Mirid bugs drastically reduced the planthopper population and indirectly enhanced plant biomass. In contrast, in the complex food web with a diverse predator assemblage, suppression of the planthopper population was diminished precluding a trophic cascade. This was due in part to the intraguild predation of mirid bugs by spiders. Thus, reticulate interactions such as intraguild predation can dampen the strong cascading effects of predation and potentially stabilize food-web dynamics.

Key words: intraguild predation, trophic cascade, food web structure, top-down forces