Effects of Habitat Fragmentation on the Nesting Dynamics of Desert Bees

Abstract

Anthropogenic changes in land use and habitat fragmentation are thought to be greater drivers of declines in the abundance and species richness of pollinating insects such as bees (Hymenoptera: Apiformes) than agricultural intensification and pesticide use. Yet, few studies have examined how habitat fragmentation by urbanization affects pollinator nesting dynamics. We investigated the nesting dynamics of native solitary bee species that inhabit fragments of Sonoran Desert in Tucson, AZ. Specifically, we compared floral densities, nest completion, and nest attack rates of cavity-nesting bees in large (>2 ha) and small (<0.5 ha) fragments. In small fragments, where resource density was high, more nests were occupied and females responded by provisioning offspring more quickly than in larger fragments where resource density was low. However, nests in small fragments were more likely to be attacked by natural enemies than those in large fragments. In this desert system, habitat size, the most-often used metric for examination of how pollinators respond to habitat fragmentation, affects nesting dynamics and demographics through its interactions with nest-site availability, abundance of floral resources, and natural enemy attack. As landscape fragmentation increases, pollinator conservation should not only consider food sources but also nesting requirements and interactions with natural enemies.