The effects of landscape structure on biodiversity, network architecture, and ecosystem function

The amount, composition, and configuration of habitat in the surrounding landscape can affect the structure of local communities, how those communities interact, and the ecosystem functions they perform. In the following chapters I report the results of three studies that use the analysis of experimental and observational data and mathematical modeling to explore these ecological themes in a number of systems.;In chapter two I describe the results of a large-scale field study on the effects of habitat loss on the architecture of plant-pollinator interaction networks. In the first empirical study on the effects of habitat loss on plant-pollinator network architecture, I found that regional habitat loss contributes directly to species loss and indirectly to the re-organization of interspecific interactions in a local community. Networks became less nested and more modular with habitat loss. Species loss was the primary driver of variation in network architecture, though species composition also affected modularity. Previous theory suggests that a reduction in nestedness and an increase in modularity with habitat loss may threaten community stability and that such a loss of stability may contribute to an extinction debt in communities already affected by habitat loss.;With my third chapter, I report the results of an experimental microcosm study in which I used leaf litter metacommunities to examine the effects of landscape structure on local communities, and the resulting consequences for ecosystem function. I developed a microcosm system of oak leaf litter communities in miniature 1 m2 landscapes. The rate of oak leaf litter decomposition was measured using litter bags. After one year, oak patch isolation and presence of a pine litter matrix both affected species composition and increased species richness. Patch size had marginally significant effects on richness and composition. Landscape effects on richness and composition translated to an effect on the rate of decomposition. The presence of a pine litter matrix, where species richness was greater, slowed the rate of oak leaf litter decomposition. Isolated patches, where species richness was greater, also had a slower rate of decomposition. Results also show that matrix quality can mediate the effects of patch size and arrangement on local communities via interactions with communities originating in the matrix, suggesting that matrix communities can be integral parts of the metacommunity, which can have consequences for ecosystem function. Therefore, integrating variation in matrix quality will be a key part of moving metacommunity concept forward, especially for applying the metacommunity concept to studies of habitat loss and fragmentation.;In chapter four, I use mathematical models to examine how an indirect effect is simultaneously propagated through multiple pathways of an interaction network and the consequences for species coexistence. I used Lotka-Volterra competition and predation equations to model the population dynamics in a 5-species community: a generalist predator (e.g., a spider) preys on two species (e.g., bees), which provide mutualistic services to their specialist partners (e.g., plants), which are competitors for a shared resource. I focus on the two species in the intermediate trophic level (e.g., the bees) to examine how the net indirect effect between them is partitioned between the predator and their competing mutualistic partners, and the consequences for species coexistence. I use the inverse of the community (Jacobian) matrix to quantify net effects of each species on the other and then use the conjugate variable approach to partition the indirect effect between the two pathways. Model results show that when both pathways are acting at once, the partial indirect strength of interaction depends on the strength of mutualism between the focal species and their respective partners. The presence of both pathways increases the area of parameter space in which both species can coexist, suggesting that the effects of multiple pathways of indirect effects are not additive. The presence of a shared predator, which generally results in the exclusion of the species least able to withstand predation, can instead mediate coexistence even in the absence of a trade-off. This suggests that understanding the mechanisms underlying an indirect effect is important for targeted species management. (Abstract shortened by UMI.)...