PS 7-65
Variation in the viability of populations of the rare endemic Calochortus tiburonensis
The Threatened plant Calochortus tiburonensis (Liliaceae) is known from only one hilltop in the world (Marin County, CA) and is confined to disjunct patches of stressful serpentine soils that vary in their nutrient and heavy metal concentrations. Each patch of serpentine supports a C. tiburonensis population and there appears to be very little if any gene flow between populations. Populations vary in size from a few dozen plants on the most stressful patches of serpentine to several thousand plants in areas where soil nutrient concentration is higher. We are using matrix models to quantify the rate at which each population is growing or declining and elasticity analyses to determine the cause of growth/decline. We are also using projections to make predictions about how this species is likely to fare as drought increases in frequency and severity.
Results/Conclusions
There are considerable differences in how each population is performing. Larger populations are increasing (λ=1.011 – 1.023), attributable to very survival of plants once they reach flowering (≈4 yrs old) while the smallest, most isolated population is declining (λ=0.91) due to low reproductive output (Allee effects) and a failure of seedlings to establish. Interestingly, data from a related set of experiments and observational studies suggest plants in this declining population are locally adapted to the more stressful conditions. The traits under selection are water and nitrogen use efficiency – traits that are likely to be highly advantageous in a drier climate but may disappear if this population goes extinct. Dormant bulbs appear to play a key role in buffering these populations from rapid decline as the climate becomes warmer and drier. C. tiburonensis has limited dispersal abilities and is restricted to patchily distributed outcrops of serpentine soil, meaning that C. tiburonensis is very unlikely to migrate with its preferred climate and so must adapt in situ or go extinct. Preliminary results from the projections show that the larger populations are likely to persist over the next century but will be smaller; the smallest populations are predicted to go extinct within 60 years.