| This dissertation focuses on coexistence in patchy environments. Coexistence is the basis of biological diversity. Today's increasingly fragmented landscapes make it imperative that we view coexistence in a spatial context. My objectives are two-fold. On the one hand I seek to understand mechanisms underlying puzzling patterns of coexistence. On the other hand I seek to develop general insights into the spatial dimension of coexistence. I use a novel mathematical approach, the perturbation theorem for dynamical systems, to identify key points of contact between ecological and evolutionary theories on coexistence and diversity. These connections provide for a framework that brings together several important concepts: population structure, spatial heterogeneity in fitness, and temporal scales over which local and spatial processes influence diversity. The framework yields comparative predictions that can serve as guidelines in biodiversity management. One key result to emerge from this general theoretical approach is that when consumers engage in asymmetric competition, coexistence results from an interaction between local dynamics and spatial processes. In my empirical research I investigate how spatial processes interact with local dynamics to shape coexistence in a spatially structured host-parasitoid community. I test both local and metapopulation hypotheses. The local hypothesis, intraguild predation, predicts coexistence if the inferior larval competitor is superior at finding unparasitized hosts. Hence, the superior larval competitor should be absent from patches of low host productivity. The metapopulation hypothesis, dispersal-competition trade-off, predicts coexistence if the inferior competitor is a superior disperser. Hence, the superior larval competitor should be absent from patches isolated by distance. Dispersal experiments demonstrate that parasitoid coexistence occurs even in the absence of a dispersal advantage to the inferior competitor. Field surveys show that patches from which the superior larval competitor is absent are not the most isolated, but the least productive. In a natural experiment, loss of the superior larval competitor was not associated with habitat loss or fragmentation that increases distance among occupied patches, but with a large reduction in host productivity. Taken together, these findings strongly suggest that parasitoid coexistence occurs via local interactions rather than spatial processes. The results have implications for multiparasitoid food webs in patchy environments, and offers practical insights regarding the release of multiple parasitoid species in pest control. |
