The importance of negative density dependence for rare species persistence

Determining the predictors of extinction risk is a major goal of conservation biology. Theory suggests that population characteristics such as low abundance and declining trends should equate to high extinction risk. However, rare species persist and account for nearly half of all species across global communities. Here we employ the Chicago Botanic Garden's long-term, rare species monitoring dataset to investigate population dynamics and hypothesized predictors of extinction risk of 73 populations of 43 rare species. Specifically, we ask how important negative density dependence is for rare species persistence and how well population size, population trends, life cycle duration, and clonality predict estimates of extinction risk in rare species. Extinction risk was estimated using density-dependent and density-independent population viability analyses. Based on our simulations, we found that only 33 of our populations had concerning extinction risks (>20 %). The key to these relatively low extinction risks for so many rare, listed species was negative density dependence. Population size, population trends, life cycle duration, and clonality were not good predictors of extinction risk based on our modeling efforts, though their relationships to extinction risk did agree with theoretical expectations. Our results highlight the potential importance of negative density dependence for rare species persistence and the importance of incorporating density dependence in population projection models for extinction risk assessment.