Smith, Reynolds and LeBuhn 2005

The relative importance of local and landscape factors in determining patterns of biodiversity has been a focus of recent conservation research (e.g. Steffan-Dewenter et al 2002).  The majority of these investigations have been single-year, cross-sectional studies which relate species richness patterns to a set of abiotic  and biotic factors that vary in space.  Our work on bumble bees in the northern Sierra suggests that temporal variability among years magnifies spatial variability and that single-year; cross-sectional studies may illuminate only part of the story.   

Since 2002, my lab has studied pollinator communities comprised of 13 species of bumble bees in the northern Sierra Nevada of California (Hatfield and LeBuhn, submitted).  We have found that there is dramatic variation in the species found in any given meadow from one year to the next even though there is stability in the plant community (LeBuhn et al. in prep).  For example, many meadows had an over 50% change in bumble bee species composition from 2002 to 2003, two years with very similar patterns of rainfall and snow melt.  Not only does the species composition change, the total number of species found in a meadow can vary dramatically.  These data suggest that conclusions drawn from snapshots of community data may be incomplete. 

Bumble bees of the Sierra Nevada are univoltine and queens over-winter. Annual variation is thus, not attributable to a 'seed bank' and rather must be explained by differences in nest success and productivity, dispersal and settlement patterns. I propose to determine the relative contribution of local and regional processes in structuring local community structure and turnover using experimental and observational methods.  To understand the processes shaping variability in species composition, I will 1) characterize current patterns of dispersal and exchange of individuals among meadows using micro-satellite markers and mark and re-capture techniques; and 2) evaluate the importance of habitat variables at different scales for species turnover;  3) characterize settlement and post-settlement survivorship by following survivorship and reproduction in experimental nests as well as natural nests, and, 4) evaluate the importance of dispersal, settlement and post-settlement process on community turnover. I will also examine the effects of bumble bee density and resource supplementation on both settlement and post-settlement survivorship of bumble bee communities. These multi-year data on dispersal and settlement patterns are essential to understanding the relative contribution of local and landscape-level factors to species diversity and, ultimately, to designing effective conservation strategies for these important pollinators.