Deer herbivory and habitat type influence long-term population dynamics of a rare wetland plant

Pinpointing the factors that alter the population viability of long-lived organisms, such as perennial plants, is especially useful for informing conservation management policies for threatened and endangered species. In this study, I used 4 years of demographic data on rare plant Polemonium vanbruntiae (Eastern Jacob’s ladder, Polemoniaceae) to determine how white-tailed deer herbivory and habitat type (wet meadow and forest seep) affect long-term population viability. I incorporated these factors into matrix population models to estimate the deterministic and stochastic growth rates (λ and λs, respectively), stable stage distribution (SSD), the reproductive value for each stage class, the cumulative probability of extinction, and the elasticity values for all vital rates under each browsing and habitat scenario. Population growth rates of P. vanbruntiae in wet meadow sites are expected to increase at a slightly faster rate than at forest seep sites. Herbivory significantly decreased the predicted population growth rate under stochastic conditions. However, P. vanbruntiae ramets are expected to increase in the future as the population growth rate (λ) > 1 under both “browse” and “no browse” scenarios, but deer herbivory increased the extinction risk to a detectable level. Deer preferentially browsed vegetative and reproductive adult ramets over yearlings and seedlings, and browsing significantly reduced fertility of reproductive ramets and increased the probability of stasis for small and large vegetative ramets. Browsing shifted the elasticity values of vital rates and changed the potential for younger life histories stages, such as seedlings, to change future population growth. Under herbivore pressure, survival and stasis of large vegetative ramets have the largest potential impact on future population growth. This study provides empirical evidence that white-tailed deer are an important ecological factor affecting long-term population dynamics of rare plant populations and offers management suggestions for remaining populations of P. vanbruntiae.