Habitat Suitability as a Limiting Factor for Establishment in a Narrow Endemic: Abronia alpina (Nyctaginaceae)

Understanding the causes of narrow endemism is crucial to conservation, particularly in biodiversity hotspots like the California Floristic Province. The loss of rare species as a consequence of climate change could result in substantial reductions in biodiversity, especially in high-elevation systems. We investigated causes of range restriction by using the narrow alpine endemic Abronia alpina (Ramshaw Meadows sand verbena) as a case study. This study examined habitat suitability as a limiting factor for the establishment of Abronia alpina, an endemic native to only 2 meadow systems in the Sierra Nevada Mountains, Inyo National Forest, California, USA. We tested habitat suitability by (a) using vegetation community composition and diversity as a proxy; (b) measuring key physical environmental characteristics (i.e., soil moisture, pH, soil texture, and surface soil temperature); and (c) establishing experimental field and greenhouse germination tests that exposed seeds to soils from several potentially suitable meadows. These 10 apparently suitable meadow systems were compared to the currently occupied meadow. Our results suggest that though there is some difference in suitable habitat among the 10 meadows studied, most meadows are relatively similar, for the rest of the measured parameters, to the native meadow. Vegetation community composition in the occupied meadow differed significantly from 8 of the surveyed meadows but was similar to 2 meadows. However, species diversity and richness in the native meadow did not generally differ from other surveyed meadows. Seven of 10 meadows were significantly colder than the occupied meadow, but 3 meadows with similar elevation ranges were equivalent. Range restriction does not appear to be a result of physical soil conditions, but cooler minimum temperatures may play a role. Although seeds only germinated in 2 of the tested meadows in field trials, dispersal limitation may be important in the species' restricted range, as seed germination was not significantly different among any meadow soils. Thus, nonhabitat factors, such as seed dispersal or recent speciation, may be the cause for the narrow range of A. alpina. Given stable climatic conditions, we recommend 3 meadow systems for establishment if recent population declines continue and relocation is deemed necessary. We also identify 7 other meadows that may be suitable, given a changing climate.