Masters Thesis

The role of fire in shaping pollinator communities : a post burn analysis of bees in serpentine habitat

Bees (Hymenoptera) represent one of the primary pollinators in most natural and managed ecosystems world-wide. Current concerns for a global decline in bee populations emphasize the need to better understand the factors that shape bee communities. Although habitat disturbance is believed to play an important role in bee community structure, little is known about how bees respond to fire, one of the most common disturbances in terrestrial ecosystems. I assessed the effects of a natural wildfire, the Biscuit Fire of 2002, on shaping bee communities in serpentine habitats of northwestern California and southwestern Oregon. In the summer of 2005 I surveyed bee communities in eight unburned and eight burned areas that each included a xeric, upland component and a wetland component (i.e., Darlingtonia fen). I also surveyed the vegetation at each site to determine if differences in floral diversity, abundance or composition could explain any differences in bee community structure between unburned and burned sites. Average bee species richness was 1.4× greater in unburned sites than in burned sites, although this difference was only marginally significant. Bee community composition differed between unburned and burned sites, however these differences were also weakly significant. In contrast, flowering plant communities did not differ in diversity, abundance or composition between unburned and burned sites and only two plant species differed in their abundance between sites. Since floral resources were comparable in unburned and burned sites, they do not explain the observed, albeit modest, differences in bee richness. Variation in the bee community is likely explained by changes in available nesting resources following the Biscuit Fire, however, I did not assess nesting resources directly in my study. My work supports a growing body of literature suggesting that serpentine plant communities are less affected by fire than plant communities found in other habitats, perhaps because open, low productivity habitats experience a relatively small release from aboveground competition that typically follows fire. Similarly, the effects on bee community structure I found are modest in comparison to some previous work assessing the effects of fire on bees in other habitats and are likely a reflection of the lack of significant post-fire changes in associated serpentine plant communities.