UC San Diego

Pollination is an important ecosystem function threatened by habitat fragmentation, a globally widespread form of anthropogenic habitat modification. Despite general recognition that habitat fragmentation tends to reduce pollinator species richness, few studies have examined how fragmentation impacts pollinator temporal and functional diversity, or how changes to pollinator diversity may influence the structure and function of plant-pollinator interactions. I compared study plots in coastal sage scrub habitat fragments and large natural reserves with respect to the temporal, functional, and landscape-level diversity of bees, the most important guild of pollinators in this region. I also documented plant-pollinator interactions in the same study system to examine how loss of pollinator diversity influences the structural properties of plant-pollinator interaction networks. Lastly, I used a meta-analysis to examine the role of the western honey bee, a globally introduced pollinator, in natural habitats worldwide. I found that compared to natural reserves, habitat fragments harbored bee assemblages that are taxonomically and functionally distinct from those in reserves, with consistently reduced taxonomic diversity throughout the study season, lower turnover of bee taxa as the season progresses, and lower functional diversity. However, fragments and reserves harbored similar abundances of bees, and exhibited similar spatial turnover of bee assemblages with respect to both taxonomic and functional diversity. Plant-pollinator interaction networks in fragments exhibited lower interaction selectivity and higher nestedness compared to those in reserves, but networks in fragments and reserves were otherwise structurally similar. Honey bees were numerically dominant across all study sites in my study system, a globally uncommon phenomenon despite the presence of honey bees in most surveyed natural habitats worldwide. The patterns of pollinator diversity loss I documented suggest that large, intact natural reserves are essential for conserving the regional distinctiveness of bee faunas in San Diego as well as the functions they perform. On the other hand, conserving plant-pollinator interactions in habitat fragments appears both possible, thanks to the continued persistence of structurally robust plant-pollinator interaction networks therein, and potentially rewarding, thanks to the high functional and taxonomic beta diversity among fragments.