How does an extreme cold event affect the bee community in a Creosote shrubland?

Background/Question/Methods

Larrea tridentata (creosote bush) dominates the warm deserts of North America providing floral resources to  over 120 native bee species, both oligolectic and generalist. In turn, outcrossing results in significantly higher seed set than self-pollination, hence increases in pollinator availability and abundance improve seed production. In recent years, due to increasing winter T_minin the American Southwest, creosote populations have been experiencing a northward expansion. However, periodic extreme cold events, predicted to occur more frequently in coming years, might negatively affect these communities if they reach the temperature tolerance limits for creosote. Reduction in creosote will affect dependent bee communities. During the winter of 2011, temperatures at the Sevilleta National Wildlife Refuge (SNWR) fell well below average, causing widespread creosote dieback. Here we ask whether the initial lack of floral resources and subsequent significant reduction of floral resources affected the bee communities in this area. Previous data on the bee guild at the creosote shrubland site at the SNWR was compared to data collected after the extreme cold event. Twice daily 15 minute surveys were conducted on 5 bushes, two days per week, during the creosote flowering period. All collected bees were pinned and identified to species.

Results/Conclusions

Massive creosote die-backs were observed, with populations at shrubland sites across the SNWR experiencing almost 100% loss of canopy. The following spring creosote did not flower during the regular early spring flowering time and no bees were observed. However, this extreme cold event did not lead to creosote mortality: early summer regrowth was observed on most shrubs, and flowers were seen as early as the fall of 2011 as well as in the following spring flowering seasons. Compared to the earlier data, bee community abundance, as well as diversity, was significantly reduced following the cold snap. The total number of bees collected in 2005 was 1506, this number was drastically reduced to 202 in 2012. Creosote is a valuable resource for numerous bee species and, as a dependable and abundant resource in the desert shrubland at the SNWR, it has been shown to be heavily utilized by generalist bees when other resources are scarce, as well as specialist species. This extreme cold event and subsequent creosote defoliation negatively affected not only specialist species but the entire bee community.