Type: report

Article abstract: Coastal southern California has been identified as an area supporting high biological diversity (Myers et al. 2000). Among the habitat types contributing to the high diversity of species in southern California are vernal pools. Vernal pools result from an unusual combination of soil, topography, and climate, and are inhabited by a wide variety of plant and animal taxa wholly or partly restricted to such habitats. Vernal pools form in areas with 1) relatively flat topography; 2) impermeable soils such as clay near the surface; and 3)pronounced seasonal variation in rainfall. During the rainy season (November-April in southern California) water pools atop relatively impermeable soils, and remains pooled on the surface for a period of time ranging from days to weeks. During and shortly after this period of inundation, annual plants and invertebrates characteristic of vernal pools grow and reproduce, producing propagules that can withstand a long period of desiccation. The pools and underlying soil soon dry up, and the seeds, spores, cysts, and eggs of vernal pool flora and fauna remain dormant in the soil until the next rainy season. The annual period of inundation inhibits growth of upland plants in vernal pool areas, conversely the annual period of desiccation precludes the development of a typical community of marsh plants and animals. Therefore a particular group of species tolerant of the pattern of seasonal inundation and desiccation has evolved to occupy vernal pools. Due to the rarity of vernal pools and the low dispersal capability of their characteristic biota, vernal pool animals and plants show a high degree of endemism between groups of pools. Because of this high endemism, and the specialization of vernal pool species for their unusual habitat, vernal pools contribute a disproportionately high number of species to southern California's overall biological diversity, relative to their limited area. The rapid growth in southern California's human population and concomitant development have greatly reduced the area of vernal pools and the abundance and distribution of their unique taxa. Remaining vernal pools are threatened by development­ related impacts such as trash dumping, off-road vehicle travel, and invasion by exotic plant species. Several vernal pool species in southern California are listed as threatened or endangered under the Endangered Species Act. In response to the decline in extent of vernal pools