Bee community response to an extreme cold event: A four year study of 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 likely 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 absence and subsequent significant reduction of floral resources affected the bee community 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 for four years. Twice daily, 15 minute surveys were conducted on 5 bushes, two days per week, during the spring creosote flowering period. All collected bees were identified to species.

Results/Conclusions

Massive Creosote die-backs were observed, with populations experiencing almost 100% loss of canopy. Creosote did not flower during the regular 2011 spring flowering time and no bees were observed. However, this extreme cold event did not lead to creosote mortality: summer regrowth was observed, 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. In the four years following, abundance has gradually increased to higher levels and richness has remained lower but shows an overall increasing trend. However, a shift in community composition has occurred. Generalist bee species abundance and richness have remained low whereas specialist bee species abundance increased dramatically and richness has recovered to initial numbers. Creosote is a dependable and abundant resource in the desert shrubland at the SNWR; it has been shown to be heavily utilized by generalist bees, as well as specialist species. This extreme cold event and subsequent creosote defoliation initially affected the entire bee community, but lasting effects were observed on generalist bee species.