Latitudinal patterns of alpha, beta, and gamma diversity in California vernal pools

Background/Question/Methods

Decreasing species richness with increasing latitude has been a well-established pattern found in a wide range of taxonomic groups and ecosystems. Temporary aquatic ecosystems, however, have been understudied in this regard. In California, precipitation increases and temperature decreases with increasing latitude.  Consequently, temporary aquatic ecosystems may be larger and persist longer with increasing latitude, which may result in a reverse gradient. I used data from California vernal pools to examine alpha, beta, and gamma diversity patterns of aquatic invertebrate taxa along a latitudinal gradient. Taxonomic groups were also categorized as having passive or active dispersal strategies and pool characteristics (water depth and surface area) were attained. The data consisted of approximately 400 vernal pools from 11 sites across 5 degrees of latitude in California. To test for patterns along the latitudinal gradient, I used regression analysis to examine whether diversity patterns changed linearly and used analysis of similarity to examine how species composition changed.

Results/Conclusions

Water depth and surface area both increased with increasing latitude.  Alpha diversity increased, but gamma diversity did not change, with increasing latitude.  Alpha diversity was positively related to gamma diversity and to water depth.  Passive-disperser alpha diversity increased with increasing gamma diversity, and both active- and passive-disperser alpha diversity increased with increasing water depth.  Species composition was significantly different among sites, but these differences were not associated with distances among sites.  Beta diversity decreased with increasing latitude and increased with variation of vernal pool depths within sites.  These results suggest that variation in local-scale characteristics (pool depth and surface area) is likely causing diversity patterns across the latitudinal gradient.  This study has implications for the scale at which management and conservation occurs in California vernal pools, a habitat similar to other temporary aquatic habitat, which supports high levels of endemism and species diversity.