TOP-DOWN CASCADE FROM A BITROPHIC PREDATOR IN AN OLD-FIELD COMMUNITY

We tested the hypothesis that a bitrophic (third and fourth level) art hropod predator can exert a cascading, top-down influence on other art hropods and plants in an early successional old field. First-stadium m antids, Tenodera sinensis, were added to replicated open-field plots i n numbers corresponding to naturally occurring egg hatch density and a llowed to remain for approximate to 2 mo. Sticky-trap dispersal barrie rs around both control and mantid-addition plots allowed us to monitor emigration of arthropods continuously during the experiment. Biomass of herbivores, carnivores, and plants, and abundances of arthropod tax a within plots were determined at the beginning, middle, and end of th e experiment. The impact of mantids on the community was a top-down tr ophic cascade, beginning at the fourth trophic level and evident at ea ch of the lower three levels. Mantids induced marked behavioral respon ses in other predators, but interference among predators did not preve nt the trophic cascade. The most common predators, cursorial spiders, emigrated from mantid addition plots in significantly greater numbers than from controls. This behavioral response may have resulted from av oidance of predation or competition. Mantids decreased biomass of herb ivorous arthropods through predation, and this decrease in turn increa sed biomass of plants. Therefore, these generalist predators were able to decrease herbivory enough to affect plant growth. This and other r ecent studies indicate that top-down effects can be important in struc turing terrestrial communities. Ours is the first example of a top-dow n cascade by a generalist arthropod predator in a nonagricultural ecos ystem and illustrates the importance of detecting behavioral responses in studies of trophic interactions.