Insular Biogeography of Vernal Pools in the Central Valley of California

Abstract

Over 100 vernal pools distributed in seven California counties were studied for the patterns of generic and species diversity. Data on the presence/absence of different plant genera and species, and on the physical environmental parameters were analyzed using multiple regression. Size of vernal pool had a small but significant effect on species richness; percentage bare ground and pool depth were important in certain equations. Regional variation in species composition was highly correlated with a latitudinal north-south gradient. Using information measure of species diversity (H'), analysis of variance components showed that the within-pools, between-pools within a region, and among regions variability was 55%, 17%, and 28%, respectively. The same partitioning at the generic level of diversity gave 63%, 17%, and 20% for the three components. Lack of sympatry along with a lowered between-pools diversity at the generic level suggests that niche partitioning occurs largely at the level of genera; within each genus species replace each other largely along the regional pattern. Monte Carlo simulation was used for estimating the species/genus ratio per pool in samples drawn from three different distributions of species number per genus. Comparisons between these theoretical and observed values suggested a rather large random sampling component in the taxonomic diversity of vernal pools as most of them have as few as 10 to 12 species; competition between related species in the area of their overlapping geographic distributions might account for the observed patterns of generic diversity.