Complex species interactions in a human world : the impacts of agricultural intensification on species interaction network structure
Keywords: Biology.

Species interactions are the backbone of ecological communities and the driving force behind ecosystem processes. Even though anthropogenic disturbances, such as agricultural intensification, are causing changes in biodiversity and species interactions worldwide, our understanding of how disturbances impact the structure and stability of species interaction networks is still nascent. In this work, I explore the impacts of agricultural intensification on species interaction network structure and stability using empirical datasets of plant-hummingbird interactions (chapter 1) and plant-pollinator and plant-herbivore interactions (chapters 2-5). All three datasets were collected along agricultural intensification gradients; the plant-hummingbird network was collected in a coffee-producing region of southern Costa Rica, and the plant-pollinator and plant-herbivore networks were collected in the central coast of California in an area of predominantly industrial agricultural production. I found that agricultural intensification significantly altered interaction network structure, typically leading to an increase in generalist species and generalist interactions (chapter 1-3) both because reciprocally specialized interactions were lost from the community and because species altered their interaction behavior to become more generalist in agricultural habitats. In a comparison of the ecological and topological change within the plant-pollinator and plant-herbivore networks (chapter 2) along the intensification gradient, I found that the ecological drivers underlying network change were distinct between the two network types. Unexpectedly, however, their topological responses to agricultural intensification were similar. By connecting the plant-pollinator and plant-herbivore networks into a single dataset of "hybrid" networks, I was able to provide the first empirical example of how hybrid network structure and robustness (a critical component of community stability) is affected by anthropogenic disturbance (chapter 3). I also explore the roles of plant species connecting the plant-pollinator and the plant-herbivore hybrid networks (chapter 4) and found that as agricultural intensification increases, so did similarity in plant species roles (e.g. number of interaction partners or importance in the network) within the two sub-networks. This indicates that in agriculturally intensive areas, each plant species is more critical to maintaining overall network structure, and that the indirect interactions pollinators and herbivores may be stronger in agricultural areas. Taken together, the findings from chapter 3 and 4 highlight that there are many insights into the structure-stability relationship to be gained from studying hybrid interaction networks and provide new metrics for quantifying hybrid network structure. In the final chapter (chapter 5) I focus exclusively on the plant-pollinator networks and find that the linkage rules determining the realization of interactions between plants and their pollinators are predominantly trait-based in natural areas but are predominantly abundance-based in agricultural areas. The transition toward abundance-based linkage rules could potentially be attributed to a decline in floral trait diversity and an increase in the representation of generalists. Ultimately, in this thesis I use three novel and extensive empirical datasets of to further our understanding of the impacts of anthropogenic disturbance, specifically agricultural intensification, on species interactions, biodiversity, and the structure-stability relationship.