The Tiburon mariposa lily, Calochortus tiburonensis (Liliaceae), is a Threatened plant that is known from only one hilltop in Marin Co., CA. Although it is managed as a single population, it is actually several populations confined to disjunct patches of serpentine soils, each of which hosts populations that range in size from a few hundred plants (four populations) to several thousand (two populations). Resource managers rarely have multi-site demographic data to make conservation decisions. We have been collecting detailed demographic data for 5 years and are using transition matrix models to both to estimate asymptotic population growth rates (lambda) over three full transitions for all of the populations, and to compare population structures. Such data can be used to design highly effective conservation plans.
Results/Conclusions
Results from the six populations were strikingly different. One of the largest populations suffered declines in all three years (lambda = 0.86, 0.91 and 0.92), the elasticity of lambda to the persistence of large vegetative plants and their advancement to flowering had a composite value of 0.52, and this population deviated from its stable stage distribution, with an excess of plants entering dormancy following flowering. The other large population grew in all three years (lambda = 1.04, 1.02, 1.03), no one transition had a large elasticity value (range 0.01 – 0.19), and the population was near its stable stage distribution. Population growth rates for the small populations varied in synchrony with each other. They all declined in year one (lambda < 1.0), grew in year two (lambda > 1), and declined again in year three. All of these small populations had an excess of young plants and flowering plants. These data suggest that the same conservation strategy is not likely to be appropriate for all of the populations. The large population that has consistently grown despite pronounced drought is not in need of aggressive intervention, while the large population that has consistently declined is. For this declining population, targeting mature plants for protection would be the most effective approach. Although the small populations contain very few plants, our models also indicate that they contribute significantly to the persistence of the species as a whole.