The threats of landscape homogenization to ephemeral pool communities and species integrity

Background/Question/Methods

Ephemeral wetlands are insular habitats in both space and in time, and face similar environmental challenges as other island systems.  Vernal pools in southern California represent one class of ephemeral wetlands that are arranged in archipelagos: pools that are tens of meters apart or less are clustered into complexes typically separated by tens of kilometers.  The pools appear with winter/spring rains, may last for days or weeks, and may dry and fill several times a season.  Differences in species richness and composition are seen between complexes, between pools within a complex, and between filling events within a pool.  Species sort by pool longevity much as oceanic island species sort by island size.  Unfortunately, these islands are also vulnerable to many of the same adverse affects brought on by anthropogenic forces as their oceanic counterparts.  For over a decade, we have sampled the crustacean communities in vernal pool complexes across southern California focusing particularly on the endangered San Diego fairy shrimp Branchinecta sandiegonensis.

Results/Conclusions

We have found evidence that disturbance, particularly by vehicles, has produced a homogenization cascade in the system.  Disturbed pools have reduced richness and lack many species that are present but numerically rare in functional ecosystems. This disturbance further threatens the already endangered fairy shrimp by introducing a formerly allopatric congener (Branchinecta lindahli) and facilitating hybridization.  The potential results are biotic, taxonomic and genetic homogenization, loss of species integrity and eventually loss of ecosystem resilience.