Implications of Population Growth Rate Projections and Pollen Limitation for the Conservation of a Threatened Dioecious Plant

The effective conservation of threatened and endangered plants requires an understanding of population dynamics and the evaluation of factors that could reduce population growth. I constructed and analyzed a stage structured demographic model for Euphorbia telephioides, a threatened dioecious perennial herb, to determine the current status of three populations, compare projections of population growth using different methods, and determine the effects of pollen limitation in the population dynamics of this species. Dioecious plants are prone to pollen limitation due to their inability to self-pollinate. Studies indicate that pollen limitation reduces seed set in plants due to insufficient quantity or quality of pollen, which can reduce population growth rate due to the decrease in fecundity. I combined experimental tests for pollen limitation with construction and analysis of structured demographic models, to examine how increased levels of pollen limitation would affect population growth rates. Determining the current status of populations, and simulating the consequences of possible threats, such as pollen limitation, provides a quantitative basis for conservation actions. I compared deterministic and stochastic projections of a stage structured demographic model to examine how environmental variation affects population growth rates, and I examined the effects of parameterizing the model excluding demographic measures of randomly marked individuals in the population growth rates (Lambda). The majority of estimated lambdas and their 95% confidence intervals indicate that these three populations are projected to decline. Lambda estimated excluding randomly marked individuals overestimated population growth because adult plants had 100% survival. I did not find evidence of significant pollen limitation of fruit or seed production, and simulations of increased levels of pollen limitation reduce Lambda at a modest rate between 0.17% to 1.91%. The main advantage of constructing a structured demographic model is that these models allow us to integrate data on different stages of a complex life cycle. In the case of E. telephioides elasticity analysis indicates that increasing stasis of non-flowering plants could lead to increasing population growth rates.