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. Habitat loss has often been shown to have a negative effect on the number and composition of species in plant-pollinator communities. Although we have a general understanding of the negative consequences of habitat loss for species diversity, much less is known about the effects of habitat loss on the pattern of interactions in mutualistic networks. Networks of mutualistic interactions often form patterns of relatively high nestedness and low modularity; these patterns are thought to confer stability on communities. With the growing threat of environmental change, it is important to expand our understanding of the factors that affect biodiversity and the stability of the communities that provide critical ecosystem functions and services. 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. A metacommunity is a set of local communities whose dynamics are linked by the dispersal of multiple interacting species, and it can be affected by the structure of the landscape on which species interact. Theory predicts that biodiversity should be enhanced by increasing the amount and connectivity of a focal habitat in the landscape, however results from empirical studies have been highly variable. Moreover, the quality of the habitat between patches of focal habitat (i.e. the "matrix") has been shown to affect biodiversity within a focal habitat. Increasing the quality of the matrix may benefit biodiversity by increasing the ease of dispersal among patches or providing supplementary resources. Less well understood is the ability of the matrix to serve as a source of colonizing species that can interact with focal communities. The inconsistent results from prior empirical work may be because few metacommunity studies have addressed the importance matrix habitats for local community structure and function; I know of no other study that has experimentally examined within a single system the effects of habitat amount, arrangement, and matrix quality on community structure and ecosystem function. I developed a microcosm system of oak leaf litter communities in miniature 1 m2 landscapes. I used a fully factorial 2x2x2 design, manipulating oak litter patch size (large or small), arrangement (connected or isolated), and matrix quality (either bare ground or an alternative matrix of pine litter). Rather than identifying species in the litter visually, I used the molecular technique terminal-restriction fragment length polymorphism to characterize communities and quantify species richness and composition. 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. Species interact directly through predation, mutualism, and competition. However, species can also interact indirectly if a direct effect on one species alters an effect on another. A set of indirect interactions links the dynamics of all species in a network, regardless of whether they interact directly or not, and these indirect interactions can affect species coexistence. The great complexity of most real ecological interaction networks provides multiple indirect pathways between species. The goal of analyzing this model was to examine how the strength of a negative indirect effect (i.e., apparent competition) is partitioned among two simultaneously acting pathways. 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.