Complex responses within a desert bee guild (Hymenoptera: Apiformes) to urban habitat fragmentation

Urbanization within the Tucson basin of Arizona during the past 50+ years has fragmented the original desert scrub into patches of different sizes and ages. These remnant patches, like the surrounding desert, are dominated by Larrea tridentata (creosote bush), a long-lived, evergreen shrub whose flowers are visited by more than 120 native bee species across its range. Twenty-one of these bee species restrict their pollen foraging to L. tridentata. To evaluate the response of this native bee fauna to fragmentation, we compared species incidence and abundance patterns for the bee guild visiting Larrea at 59 habitat fragments of known size (0.002-5 ha) and age (up to 70 years), and in adjacent desert. The 62 bee species caught during this study responded to fragmentation heterogeneously and not in direct relation to their abundance or incidence in undisturbed desert. Only a few species found in continuous desert outside the city were entirely absent from urban fragments. Species of ground-nesting Larrea specialists were under-represented in smaller fragments and less abundant in the smaller and older fragments. In contrast, cavity-nesting bees (including one Larrea specialist) were over-represented in the habitat fragments, probably due to enhanced nesting opportunities available in the urban matrix. Small-bodied bee species were no more likely than larger-bodied species to be absent from the smaller fragments. The introduced European honeybee, Apis mellifera, was a minor faunal element at >90% of the fragments and exerted little if any influence on the response patterns of native bee species to fragmentation. Overall, native bee response to urban habitat fragmentation was best predicted by ecological traits associated with nesting and dietary breadth. The consequences for the floral host and its pollination interactions are not intuitive but await investigation.