Insular biogeography of vernal pools in the Central Valley of California [Vascular flora]

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.