Aggregation of invertebrate predators in complex-structured habitats: Role of altered cannibalism, intraguild predation, prey availability, and microclimate

In general, habitat complexity (number of different structural elements per unit volume,) is thought to promote increases in natural enemy abundance. Meta-analysis verified the positive association between abundance and habitat complexity for a diversity of invertebrate predators and parasitoids (Chapter 1). Several possible mechanisms are thought to underlie the accumulation of natural enemies in structurally-complex habitats, although few have been tested rigorously. These mechanisms include: (1) refuge from predation, (2) enhanced capture success of prey, (3) access to a more favorable microclimate, and (4) access to alternative food sources. Using the wolf spicier Pardosa littoralis, I examined how the aforementioned mechanisms influence the abundance of this salt marsh-inhabiting predator.; Like many other natural invertebrate predators, Pardosa littoralis accumulates in complex-structured habitats. Specifically, this spider abounds in Spartina (cordgrass) habitats replete with leaf litter (thatch). My research documented that thatch promotes the immigration of this spider (Chapter 4) and retards its emigration from habitats independent of prey availability (Chapter 5). The combination of thatch and prey presence has a synergistic effect on the residence time of P. littoralis in a habitat and further promotes its aggregation there (Chapter 3). The most likely mechanism underlying the accumulation of P. littoralis in complex-structured habitats is refuge from predation, whereby cannibalism, and to some extent intraguild predation from other spider species, are diminished in complex-structured habitats with thatch (Chapter 2). More favorable abiotic conditions that are associated with thatch, including elevated humidity or reduced temperatures, do not promote spider accumulation in thatch-rich habitats (Chapter 4).; Because structurally-complex habitats dampen antagonistic interactions among predators (cannibalism) and promote their aggregation, then top-clown impacts on herbivores and trophic cascades may be more frequent in terrestrial food webs than was originally expected. With a better knowledge of the mechanisms promoting natural enemy aggregation in complex habitats, including an understanding of how specific structural elements can be modified to moderate antagonistic interactions among members of the predator complex, we may be better able to conserve natural enemies and at the same time increase their collective effectiveness in suppressing agricultural pests.