The effects of isolation on mutualistic interaction networks.

Abstract

Global environmental changes, such as habitat fragmentation, are negatively impacting biodiversity and species interactions. The conservation of species and their interactions is essential to preserve the ecosystem system services they provide, such as pollination. This thesis examines how pollinators and their interactions with flowering plants respond to habitat isolation. In order to predict what will happen to mutualistic interactions in the face of global change, recent community- and metacommunity level studies have examined plant-animal mutualistic networks. I took a similar approach, by first examining changes in the pollinator community composition and the resulting consequences for pollination services, measured through seed set. This was followed by an investigation of plant-pollinator interactions using a network approach, specifically examining how the structure of plant visitation and pollen transport networks and interactions at the species level changed with increasing isolation. To do this, I established a field experiment with plant patches at varying degrees of isolation on a sheep farm pasture in Oxford, New Zealand. Observations were conducted over a three-month period, during which all flower-visiting insects were collected to be identified and to have their body size measured and pollen load quantified. Each plant’s seed set was also measured to calculate pollination success at patches. My findings showed that isolation had a generally negative effect on pollinators, plants and their interactions. The body size of pollinators was smaller at isolated patches, as was the richness of the pollen they carried. The patterns were driven by a compositional shift of pollinator relative abundances, with a significant decline in the large generalist Bombus species being observed. These changes in species composition decreased one measure of the functioning of the system; plant seed set significantly declined with isolation for several of the patch plant species. Through analysis of plant interactions at the network level, I found that a decline in plant species seed set correlated with plant generality, and specialist plant species were more negatively affected by isolation than were generalist plant species. I also found that pollinator body size was positively correlated with the number of plant species or pollen grains with which a pollinator interacted. In addition, network connectance and nestedness declined with isolation. The trends observed could have important implications for the long-term stability and functioning of isolated plant communities, and their interactions with mutualists such as pollinators.