Does Spatial Scale Affect the Incidence of Density Dependence? A Field Test with Insect Parasitoids

Abstract

We tested the hypothesis that spatial scale of investigation has an effect on the probability of detecting density dependence in the field. Specifically, we examined parasitism rates by two different parasites of one host (the salt—marsh planthopper Prokelisia) with different dispersal capabilities. One parasite, the wasp Anagrus, attacked host eggs and flew widely over the marsh, traveling on the order of kilometres. The other, the strepsipteran Elenchus, parasitized host nymphs and adults. Females of this parasite cannot fly, and dispersal is by triungulinid nymphs, which walk on host plants and can only disperse on the order of metres from their spent maternal corpse. We found no dispersing plant—hoppers to be carrying Elenchus. We expected no density dependence by Elenchus on a large scale, that is, over an area of 1—2 km2 but possibly some evidence of density dependence on a small scale, that is, within a few metres. By similar logic, we expected Anagrus perhaps to show density dependence at both scales because it disperses so well. Parasitism rate by both insect species varied widely both in space, even between adjacent sites, and through times. We found little evidence of density dependence for either type of parasite at any spatial or temporal scale. Failure to detect density dependence may be due less to the effect of looking at the wrong spatial scale than to the idiosyncratic behavior of individual parasite species.