|HOME SCHEDULE AUTHOR INDEX SUBJECT INDEX|
Predation risk influences relative strength of top-down and bottom-up impacts in phytophagous insects.
Denno, Robert*,1, Gratton, Claudio2, Dobel, Hartmut1, Finke, Deborah1, 1 Department of Entomology, College Park, Maryland, USA2 Department of Entomology, Madison, Wisconsin, USA
ABSTRACT- Elucidating the relative strength of top-down and bottom-up forces in communities of phytophagus insects has been a major historical focus. Current consensus is that both forces play a role, but it is poorly known if these forces act differently on herbivores in the same community. Using manipulative experiments with an assemblage of sap-feeding phytophagous insects (six species of planthoppers, leafhoppers, and heteropteran bugs) inhabiting intertidal Spartina marshes, we examined the association between herbivore behavior, risk of predation, and ultimately the relative impact of top-down (wolf spider predation) and bottom-up factors (host-plant nutrition) on the population density of each sap-feeding herbivore. Bottom-up effects prevailed in this community, whereby the density of all six sap-feeders increased when the nitrogen content of their Spartina host plant was elevated. By contrast, wolf-spider addition significantly suppressed populations of only Prokelisia planthoppers, and had little impact on the other four sap-feeder species in the community. Functional response experiments and behavioral studies revealed that certain species (Prokelisia planthoppers) were at much higher risk of attack by wolf spiders than other sap-feeders in the assemblage, and that risk of predation was associated with the particular escape-defensive behavior of a species. Moreover, risk of spider predation was linked to the strength of top-down impacts in the field, because species with ineffective escape behaviors and a high risk of spider attack (Prokelisia planthoppers) were the only sap-feeders whose populations were suppressed by spider predation in the field. Thus, specific behavioral characteristics of the sap-feeders on Spartina influenced risk of predation and the relative strength of top-down and bottom-up impacts on their population dynamics. These results call into question the overall pervasiveness of top-down forces and underscore the primacy of basal resources in structuring this community of phytophagous insects.
Key words: bottom-up control, top-down control, risk of predation, food web dynamics