Four consecutive years of historic drought in California have led to substantial declines in the size of the largest remaining population of the rare serpentine endemic, Streptanthus glandulosus niger (Brassicaceae). Only one additional population persists in the wild and there is currently no gene flow between the two. At present, little is known about the remaining genetic diversity of the wild populations or whether a seed bank persists and whether it harbors any genetic diversity absent in the standing plant population. This information is critical to understanding the threats to this species and designing an effective conservation plan. We used periodic matrix models to describe the multiple, within-year transitions typical of an annual plant and to calculate annual population growth rate. We collected soil samples and germinated seed from it to assess the size of the seed bank. We used 10 polymorphic microsatellite loci to assess the genetic diversity in the largest population and the plants that emerged from the seed bank.
Results/Conclusions
Our study population declined in size by 14% in 2013-14 and 17% in 2014-15. Only eight seedlings emerged from the 150 soil samples collected at this site; all of these emerged from the same sample. This suggests that four consecutive years of drought may have nearly depleted the seed bank. We used simulations to estimate the degree to which the seed bank could buffer the population from steep declines over a period of decades. Our simulations indicate that the current seed bank is far too small to reduce the risk of extinction. Preliminary results indicate that this population deviates significantly from Hardy-Weinberg equilibrium, has low genetic diversity (I = 0.45) and is highly inbred (FIS = 0.81). Preliminary results also do not indicate that the plants that emerged from the seed bank contain any novel genetic diversity. This is particular worrisome because S. glandulosus niger is serpentine-restricted and thus cannot migrate with its preferred climate but rather must adapt in situ as the climate continues to change, or go extinct.