There is a growing body of evidence indicating that predators can have important non-lethal effects on prey populations, particularly with regard to morphological and behavioral traits of the prey that affect their dispersal. Spatial or landscape ecologists, however, have ignored the role of non-lethal predator effects even though dispersal is thought to be a key process in promoting predator-prey persistence. In this study, we report on the effects of spider predators on the dispersal and mortality of Prokelisia crocea planthoppers (Hemiptera: Delphacidae) in a prairie landscape. All arthropods were removed from 48 small, naturally occurring cordgrass patches (Spartina pectinata), the sole host plant of P. crocea. Patches were then re-stocked with a constant density of planthoppers (2 adult females/stem). The spider assemblage associated with cordgrass, divided into three foraging guilds (cursorial, sit-and-wait, and web building), was added to the patches at one of three densities (none, ambient levels, or triple ambient levels). One half of the patches were caged to prevent planthopper emigration, and the other half were left open. In the caged patches, there was a gradual but non-significant decline in planthopper recaptures with increasing spider density (all guilds combined), suggesting that mortality effects of spiders were weak. In contrast, planthopper recaptures in open patches declined by 85% between the none and triple ambient spider addition treatments. This steep, significant decline was mostly attributed to predator-induced emigration: as spider density increased, planthopper emigration increased. These data suggest that patch connectivity for planthoppers may depend strongly on spider densities.
Based on a three-generation survey of 147 cordgrass patches, the size of the patch, but not its isolation from other cordgrass, or matrix composition (mudflat, mixture of native grasses, or the exotic smooth brome) affected spider densities. For cursorial and web-building spiders, densities 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 a planthopper patch extinction increased significantly with the density of web-building spiders. Small cordgrass patches with high densities of web-building spiders were especially at risk to extinction. Because small cordgrass patches support high spider densities and favor high planthopper emigration rates, the non-lethal 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.
Keywords: dispersal, emigration, extinction, mainland-island, mark-recapture, metapopulation, patch dynamics, planthoppers, Prokelisia, source-sink, Spartina pectinata, spatial distribution, tall-grass prairie