Increased drought frequency alters the optimal management strategy of an endangered plant

Many rare plant and animal species require frequent habitat disturbance to ensure population persistence, but humans often suppress or modify natural disturbance regimes, such as floods and fires, which harms disturbance-adapted species. To manage these populations, demographic models are often used to quantify the optimal disturbance frequency of the habitat. Increasing drought frequency due to climate change could alter optimal management strategies by decreasing the benefit of a recently disturbed habitat. We ask whether disturbance and drought interactively affect population dynamics and optimal management of Astragalus bibullatus, a Federally endangered legume endemic to limestone cedar glades in Tennessee. We used matrix population models to determine the independent and interactive effects of drought and management of woody canopy cover on A. bibullatus' probability of extinction, time to extinction and future population size. Under current drought frequencies, A. bibullatus' probability of extinction was minimized with the most frequent management, since open habitats promote recruitment. However, as drought frequency increases, the probability of extinction was minimized with less frequent management, since canopy cover decreased mortality of older plants during droughts. We recommend managing disturbance-dependent habitats by maintaining two types of environments; those that promote high survivorship during climatically unfavorable years, and those that promote increased reproduction and recruitment during climatically favorable years. Varying management goals across the landscape will decrease species-wide risk by preventing a single drought from harming all populations.