Northwestern University, Chicago Botanic Garden Glencoe, IL, United States
Background/Question/Methods
Species that are numerically rare occur in all biomes across the planet, yet our understanding of their population dynamics is nascent. Theory suggests that population characteristics such as low abundance should result in high extinction risk. Empirically however, rare species persist and account for nearly half of all species across global communities. Here we employ a multidecadal, rare species monitoring dataset from the Chicago Botanic Garden to investigate population dynamic characteristics of 68 populations of 40 rare species. We related qualitative population characteristics to extinction risk of the same populations, which we quantitatively assessed using density dependent and density independent population viability analyses. Specifically, we asked how population size and population trends over time relate to extinction risk.
Results/Conclusions
We found that only 24 of our populations exhibited concerning extinction risks ( >20%). The key to these relatively low extinction risks for so many rare, listed species was frequency dependence, a form of density dependence. We found that qualitative population characteristics including population size and population trend were poor predictors of extinction risk overall, though their relationships to extinction risk did agree with theoretical expectations. Our results highlight the importance of density dependence in rare species persistence and the need for quantitative species extinction risk assessment.
