Drivers of rarity and the conservation of Astragalus L. (Fabaceae) in the southeastern United States of America

The prevalence of species in space and time, especially species that are rare, is consistently pertinent in the study of evolution and ecology. However, in the ongoing biodiversity crisis, increasing rates of rarity and extinction are creating an increasing challenge to understand why species are being lost and how to better conserve them. Astragalus is the largest genus of flowering plants worldwide and is known for high rates of rarity and narrow endemism. In the southeastern United States of America (SEUS), Astragalus are represented by a relatively few species, resulting in a unique opportunity to study the evolutionary and ecological drivers of rarity and to improve conservation decisions. We first use a phylogenetic approach using next generation sequencing to resolve the regional relationships and biogeographic history of 11 of 13 Astragalus taxa in the SEUS. For one rare SEUS species, A. michauxii, we experimentally test for drivers of low reproduction and use population models to explore the effects of fire season and frequency on population dynamics. We recovered three distinct clades of SEUS taxa with Pleistocene origin, each with a distinct geographic phylogenetic signal. Each clade is comprised of rare and SEUS endemic species, particularly associated with the Atlantic Coastal Plain biodiversity hotspot. For A. michauxii, we found low reproductive success is not the result of pollen or resource limitation, the two most likely causes of reduced fitness. Finally, we found that season of fire and fire return interval are key drivers of population dynamics in A. michauxii, with only dormant season fires producing positive population growth. Throughout this volume, we discuss the implications of these evolutionary and ecological drivers on conservation measures needed in the SEUS, and specifically to reduce the probability of extinction for A. michauxii.