Individual differences in predators but not producers mediate the magnitude of a trophic cascade

Top-down community dynamics are commonly mediated by the foraging mode of the dominant predator. However, functional intraspecific variation in foraging mode may also mediate these interactions, particularly if the magnitude of intraspecific variation mimics or exceeds that of interspecific variation. Here, we simultaneously assess the role of intraspecific variation in predator activity level and plant constitutive defenses in mediating the presence and magnitude of a trophic cascade. In experimental mesocosms, we constructed tri-trophic ecological modules containing eight herbivores (two cabbage loopers, two beet armyworms, two darkling beetles, and two banded crickets) and lettuce plants of one of three cultivars which vary in their constitutive latex defense. In a portion of the mesocosms, we added individual cursorial spiders which vary consistently in their activity level (i.e., foraging mode). We then measured herbivore mortality and average plant leaf damage via herbivory after 1 week. The presence of a spider alone did not reduce plant damage, yet communities containing more active spiders had up to a 50 % reduction in plant damage compared with communities with more sedentary spiders. Notably, spider activity level did not influence herbivore mortality; thus, the decrease in plant damage associated with predatory activity appears to be the outcome of indirect non-consumptive effects. Increases in constitutive plant latex content did not decrease herbivory, possibly because the cabbage looper uses a countermeasure called "leaf trenching." Together, these data reveal that the effects of functional intraspecific variation on a trophic cascade can be dramatic and varied, depending on the trophic level under consideration.