Micro-endemic species of snails and amphipods show population genetic structure across very small geographic ranges.

Understanding variation in population genetic structure, even across small distances and for species with extremely limited ranges, is critical for conservation planning and the development of effective management strategies for imperiled species. Organisms that occupy the same geographic extent can maintain different population structures, ranging from highly diverged to panmictic. Such differences can result from differences in biological characteristics such as dispersal ability or demographic history. We used microsatellite loci to evaluate population genetic structure and variation of four desert spring invertebrates having high to low dispersal ability: the lung snail Physa acuta, two species of gilled snails (Juturnia kosteri and Pyrgulopsis roswellensis; family Hydrobiidae) and the amphipod Gammarus desperatus. The study location represents entire species ranges for the micro-endemic hydrobiids and G. desperatus, while P. acuta is ubiquitous throughout much of North America. We found little evidence of significant population genetic structure for P. acuta and J. kosteri, but much more for P. roswellensis and G. desperatus. Our results demonstrate differences in habitat preference and/or dispersal ability between the species. This information provides insight into how gene flow shapes varying population genetic structure between species across small spatial scales (<100 km 2 ). Most importantly, our results suggest that conservation agencies should not consider these micro-endemic species to be composed of single populations, but rather, that management plans for such species should account for population genetic variation across the species' ranges.