Spider effects on planthopper mortality, dispersal, and spatial population dynamics
Nonlethal (trait-mediated) effects of predators on prey populations, particularly with regard to prey dispersal, scarcely have been considered in spatial ecological studies. In this study, we report on the effects of spider predators on the mortality, dispersal, and spatial population dynamics of Prokelisia crocea planthoppers (Hemiptera: Delphacidae) in a prairie landscape. Based on a three-generation survey of host-plant patches (Spartina pectinata; Poaceae), the density of cursorial and web-building spiders declined significantly with increasing patch size (a pattern the opposite of that for the planthopper). Independent of patch size effects, an increase in the density of web-building and cursorial spiders had a negative effect on planthopper density in one of three generations each. Finally, the likelihood of extinction of local (patch) populations of planthoppers increased significantly with an increase in the density of web-building spiders. Planthoppers in small host-plant patches with high densities of web-building spiders were especially at risk of extinction. To evaluate whether spider effects on planthopper spatial dynamics were mediated by predation and/or spider-induced dispersal, we performed a field experiment in which host-plant patches were either caged or left open and received one of three spider density treatments (removal, ambient levels, or high = triple ambient levels). For the caged patches, there was a nonsignificant decline in planthopper recaptures with increasing spider density, suggesting that mortality effects of spiders on planthoppers were weak. In contrast, planthopper recaptures in open patches declined by 85% between the removal and high spider treatments. This significant decline was mostly attributed to spider-induced emigration. We conclude that, at high spider densities, spiders are likely to have a greater impact on planthopper densities through induced emigration than consumption. Because small cordgrass patches support high spider densities and favor high planthopper emigration rates, the nonlethal effects of spiders may play a very important role in determining critical patch size, source-sink properties of cordgrass patches, and the spatial distribution and spread of planthoppers.