Effects of nutrients and predators on an old-field food chain: interactions of top-down and bottom-up processes

How top-down and bottom-up processes interact in terrestrial systems is an important ecological question, vet well designed studies addressing this question are rare. We tested the interaction of these two processes in an old-field community by simultaneously manipulating nutrient level and large cursorial spider density in a replicated field experiment. Spider density and nutrient level interacted to determine overall growth of plants, with the greatest plant biomass occurring in plots with normal predator density and elevated nutrients. Plant growth was increased for a short period of time in all plots with elevated nutrient levels. However, with predator abundance reduced, Homoptera were able to increase in nutrient addition plots and cause a subsequent reduction in plant growth. Other herbivorous arthropods appeared unaffected by our experimental treatments. Spider reduction also significantly reduced plant diversity while nutrient effects on plant diversity were it on significant. Manipulation of predators and nutrients affected the biomass and behavior of smaller cursorial spiders. Biomass of these smaller predators increased with top spider reduction, which in turn enhanced emigration of these smaller spiders. This study demonstrates that top-down and bottom-up processes can interact to affect the biomass of several different trophic levels and the behavior of organisms within those trophic levels. However, only some components of this diverse community were significantly affected by our manipulations. indicating that strong trophic interactions can exist within a framework of weaker interactions in a diverse system. Researchers of trophic interactions need to recognize that top-down and bottom-up processes are not acting in isolation. Therefore. results of studies investigating top-down or bottom-up processes alone must be interpreted cautiously, and ideally these processes should not be studied independently.