Habitat fragmentation reduces bee taxon richness and temporal beta-diversity in a hotspot of bee biodiversity

Background/Question/Methods

Evidence is mounting that pollinator assemblages are affected by human activities such as urbanization. Recent meta-analyses reveal that human-mediated habitat degradation tends to decrease pollinator diversity and preferentially extirpate specialist pollinators. However, few studies have examined how disturbance affects the seasonal turnover of pollinators, despite the fact that pollinator communities generally exhibit marked compositional changes across the blooming season of their co-occurring plant communities. Additionally, despite the knowledge that pollinator assemblages in different ecosystem types often respond idiosyncratically to disturbance, data are lacking on how habitat fragmentation affects highly diverse pollinator assemblages that include numerous rare or patchily-distributed species. We surveyed the bee fauna in coastal sage scrub ecosystems of southern coastal San Diego to address (1) how habitat fragmentation affects bee species richness and community composition in this bee diversity hotspot, and (2) how habitat fragmentation affects the temporal beta-diversity of bee assemblages in our system. We collected bees via netting and bowl traps in scrub fragments embedded in urban areas and in relatively intact natural reserves through the spring and summer in 2011 and 2012, and assigned species identity and life history traits to each bee specimen we collected.

Results/Conclusions

We collected 11,760 native bees (12,260 bees total) in 2011 and 2012, representing more than 250 bee species. Bee species richness was ca. 35% lower in habitat fragments compared to natural reserves. While bee abundances appeared similar in reserves and fragments, fragments had a higher proportion of species with broadly generalized diets. Bee temporal beta diversity was significantly lower in fragments compared to reserves and, at least for 2011, declined as the season progressed. This decline in temporal beta diversity with habitat fragmentation may be caused by the increased proportion of broad generalist bee species in fragments—fragments and reserves did not differ in seasonal turnover when broad generalist species were excluded from the analyses. Our findings are consistent with the emerging pattern in which habitat fragmentation causes a decline in bee species diversity, especially for species that are not candidate “human commensals” (i.e., those that do not have broadly generalized diets). Our results also suggest that studies aiming to elucidate the responses of strongly seasonal organisms to anthropogenic disturbances may need to account not only for snapshot-level effects of disturbance on groups of organisms, but also for potential effects of disturbance on the phenology-dependent interactions of organisms in the community.