Demographic and genetic variation in Dodecahema leptoceras (Gray) Rev. and Hardham (Endangered species)

Dodecahema leptoceras is an annual forb endemic to alluvial deposited substrates in just three counties in southern California. Mining, agriculture, and expansion of urban infrastructure have altered the alluvial floodplains in which this species occurs such that approximately 80% of the previously known occurrences of D. leptoceras have been extirpated. The severe reduction in population numbers has led to the state and federal listing of this species as endangered. An understanding of the demography and the genetic variability of this species is necessary to formulating and implementing a management plan to ensure its persistence. Three years of demographic monitoring of 4 populations in 3 geographic locations have shown that annual survivorship and fecundity are environmentally-controlled. Habitat heterogeneity both within and between sites appears to be an important factor in population fitness in any given year as plant performance differs between sites under differing environmental conditions. There are no indications that pollinator availability limits the fecundity of this species, which is often a concern where habitat fragmentation has occurred. However, seed production appears to be limited by unpredictable environmental conditions that truncate the flowering period. Gene diversity is high in this species relative to other endemic and annual plant species. Genetic variability and polymorphism were examined in two generations of seed rain and two generations of germinating plants in order to indirectly assess the genetic diversity of the seed bank. Levels of variability were similar in both generations of seed rain, however, seed rain was significantly less genetically variable than both generations of germinating plants. Increased gene diversity and reduced levels of inbreeding (as determined by F-statistics) were evident in germinating plants in 1999, a year of exceptionally harsh environmental conditions. This finding suggests that selective germination occurs under temporally variable and unpredictable environmental conditions. It appears that the soil seed bank functions both to restore populations demographically and to replenish gene diversity in standing populations.