Plants underlie the structure of biodiversity. Regardless of whether biodiversity is an ecosystem service intrinsically or a functional support of ecosystem services, depletion of biodiversity is a concern. Within plant communities, one to several plant species are often dominant. The loss of dominant plants within a community may indicate structural and functional changes in community function, with resulting changes in provisioning of ecosystem services. We examined the predicted loss of dominant plant species within six U.S. Southwest ecoregions, as indicated by richness metrics under current (baseline) and two future climate scenarios for the period of 2041-2060. The future scenarios, based on the Community Climate System Model v4.0 (CCSM4) scenarios 4.5 and 8.5, represent lower and higher end projections of projected temperature increase. For each scenario, we modeled the likelihood of occurrence of 70 plants that characterized 30 major plant communities representing 79% of the ecoregions (735,335 km2). We categorized the models to presence/absence values using a likelihood threshold unique to each species. The change between each future scenario and baseline richness was calculated and represented as the portion lost from baseline conditions. Additionally, we calculated predicted richness for the baseline model by summing all contributing species models.
The highest richness predicted for the baseline model was 27 species; however, 80% of the modeled area had richness of nine or fewer species (mean 6.7, SD 3.9). In comparison, among the 30 contributing plant communities, the average dominant species identified for model input was four. The threshold used for each species model was the species’ mean likelihood score within the plant community for which it was dominant. We interpreted the baseline map as indicating where species were dominant or associate. The future scenarios predicted a mean loss of species across the ecoregions of 2.8 (SD 1.9) and 3.2 (SD 2.2) for the CCSM4 4.5 and 8.5 conditions, respectively. Dominant species richness could decrease by 50% or more over 43.4% of the study area under the low temperature scenario and 53% under the high temperature scenario. These losses were predicted in parts of all six ecoregions. Generally, the CCSM4 8.5 scenario included areas of richness loss predicted for the CCSM4 4.5 scenario, with additional areas contiguous to the CCSM4 4.5 scenario predictions. Continuing analysis includes identifying the public lands associated with loss of dominant plant richness and those species contributing to either stability or decline of biodiversity for those areas.