Variation in the demographics of a rare central Oregon endemic, Astragalus peckii Piper (Fabaceae), with fluctuating levels of herbivory

Understanding variation in plant vital rates (survival, growth, and reproduction) and population demographic parameters for rare plant taxa facilitates effective management for long-term persistence. We evaluated demographics of the rare plant Astragalus peckii (Fabaceae), a state-listed Threatened plant in Oregon, USA, with particular emphasis on how a microlepidopteran herbivore, Sparganothis tunicana , impacted vital rates and population growth. Stage-based transition matrix models were used to compute population growth rate ( λ ) and elasticity from 2006 to 2009 at two populations: Bull Flat, which was located in the main population center; and Chiloquin, a naturally isolated population. Population growth at Bull Flat was stable to slightly declining ( λ = 0.96, 95 % CI 0.91–1.00) whereas at Chiloquin, the isolated population, population growth was increasing ( λ = 1.20, 95 % CI 1.15–1.24). Microlepidopteran herbivory was associated with different plant responses in each population. At Bull Flat, plant survival was lower with greater herbivore presence. At Chiloquin, reproduction was reduced in plants when herbivores were active earlier in the growing season. Despite these effects on plant vital rates, we found lower population growth only during one transition period at Bull Flat when we compared matrices with and without herbivory. In addition to herbivory, we also address the potential role precipitation plays as a contributor to site differences and temporal variation within sites. Overall, we illustrate how two populations can have different responses to the same disturbance factor and highlight implications for management of different populations across the landscape.