The roles of top and intermediate predators in herbivore suppression: contrasting results from the field and laboratory

Abstract

Intraguild predators can have unpredictable effects in food webs: they can either disrupt or enhance herbivore suppression, depending on their direct effects on herbivores. Furthermore, intraguild predation is not always unidirectional, with top predators eating more effective intermediate predators. In a vineyard field experiment and a series of laboratory experiments, the effect of intraguild interactions between a likely top predator (the spider Cheiracanthium mildei) and an intermediate predator (the spider Anyphaena pacifica) on suppression of their shared leafhopper prey (Erythroneura spp.) was examined. In the field experiment, C. mildei drove the suppression of leafhoppers while reducing numbers of the other spider A. pacifica. The results of laboratory experiments confirmed the predatory impacts of C. mildei on both leafhoppers and A. pacifica. As a top intraguild predator, C. mildei appears to dominate predator–prey interactions in the vineyard ecosystem, although results indicate that its impact may be dampened by cannibalism. By contrast, A. pacifica preyed on leafhoppers and smaller C. mildei in laboratory experiments, but had no measurable impacts in the field experiment. The larger average size and wide‐ranging hunting mode of C. mildei may provide a predatory advantage in the field that did not emerge over short time periods in small laboratory cages. These results highlight the pitfalls of extrapolating from the controlled setting of the laboratory to more heterogeneous conditions in the field.

Cascading effects of cannibalism in a top predator

“…Holes on either side of the cages allowed access to vines, and could be opened and resealed with metal clips. Previous studies found no cage effects on spiders or leafhoppers when using this cage design (Hogg & Daane, , ).…”

“…The voracity of C. mildei may explain this discrepancy. Cheiracanthium mildei appears to be a particularly voracious species (Hogg & Daane, , ). Hunger can increase the frequency of cannibalism in spiders (Samu et al , ; Petersen et al , ), and C. mildei may have a higher satiation threshold than many other spider species.…”

“…assessing cannibalism within A. pacifica or the effects of A. pacifica on C. mildei were not objectives). Interactions between C. mildei and A. pacifica appear to be largely unidirectional, with C. mildei acting as the intra‐guild predator (Hogg & Daane, ). Cheiracanthium mildei is larger than A. pacifica at adulthood, and in previous surveys was 34% larger (by body length) than A. pacifica on average in vineyards where both species occurred (B. Hogg and K. Daane, unpublished).…”

“…In previous experiments, C. mildei was unaffected by the presence of other spiders in cages but experienced high overall mortality (Hogg & Daane, ), and preyed more heavily on conspecifics than on its apparent intra‐guild prey, the abundant native wandering spider Anyphaena pacifica Banks (Anyphaenidae) (Hogg & Daane, ). Cheiracanthium mildei dominates the predator community in Napa vineyards (Hogg et al , ; Hogg & Daane, ), where the most abundant herbivore prey for spiders are commonly leafhoppers, Erythroneura variabilis Beamer, and E. elegantula Osborn (Hogg & Daane, , ). Cheiracanthium mildei has been shown to suppress these leafhoppers and other spider species found in the vineyard ecosystem (Hogg & Daane, , ).…”

“…Cheiracanthium mildei dominates the predator community in Napa vineyards (Hogg et al , ; Hogg & Daane, ), where the most abundant herbivore prey for spiders are commonly leafhoppers, Erythroneura variabilis Beamer, and E. elegantula Osborn (Hogg & Daane, , ). Cheiracanthium mildei has been shown to suppress these leafhoppers and other spider species found in the vineyard ecosystem (Hogg & Daane, , ). Indeed, C. mildei appears to be a particularly efficient consumer of leafhoppers, and reductions in its numbers may be linked directly to increases in leafhopper populations.…”