UC San Diego

Habitat fragmentation due to human activity has led to pollinator declines worldwide, yet little is known about how this diversity loss affects pollination services in natural ecosystems. In this study, we investigate the mechanistic links between habitat fragmentation, a proxy for pollinator diversity loss, and the quantity (conspecific pollen deposition) and quality (heterospecific pollen proportion) of pollination services in coastal sage scrub habitats in the San Diego region. We documented pollinator visitation and pollen deposition across ten focal plant species in six natural reserve and six scrub fragment plots. At the level of the community as a whole, habitat fragmentation per se was not a significant driver of conspecific pollen deposition nor was it a significant driver of heterospecific pollen proportion. However, habitat type (reserves vs. fragments) formed statistically significant interactions with other variables in both conspecific and heterospecific proportion models, suggesting that fragmentation can indirectly affect pollination services. The western honeybee, Apis mellifera, was the most numerically dominant floral visitor across all study plots. Due to its high abundance, A. mellifera could influence both conspecific pollen deposition and heterospecific pollen proportion among reserve and fragment plots, potentially altering pollination services within coastal sage scrub habitats. Overall, habitat fragmentation can impart complex effects within plant-pollinator networks, as habitat type was shown to affect plant species differently with regards to changing pollinator variables. These differences in responses from plant species and pollinators could potentially result in a restructuring of plant-pollinator networks.