Climate change is expected to create opportunities for exotic species, either by creating open niche space or altering existing niche relationships. Long term data sets have been leveraged to show community-scale responses to climate change, but thus far have rarely been utilized to demonstrate changes in community composition and differential responses of native and exotic species. Using a long-term data set collected in a California grassland from 2006 to 2016, we quantified the magnitude and directionality of changes and in community composition for native and exotic species assemblages using non-metric dimensional scaling and permutation based statistical comparisons. These communities exist on two distinct soil types; stressful serpentine soils which are generally resilient to most invading species, and non-serpentine soils which harbor a large compliment of noxious weeds. Reductions in species richness have been documented on both soil types, although the effect has been much larger on non-serpentine soils. This loss of richness can primarily be attributed to reductions in plot-scale occupancy for rare native forbs. We therefore expected to find changes in the composition of native plant communities on both soils, with little change in the assemblage of exotic species.
Results/Conclusions
Both serpentine and non-serpentine plant communities have undergone directional change in composition from 2006 to 2016. The magnitude of change is much larger in serpentine communities, consistent with the greater loss of species richness observed on these soils. When assemblages of native and exotic species were considered separately, native species on non-serpentine soils exhibit no change in composition, whereas exotic species exhibit strong, directional change in composition across years. On serpentine soils, both native and exotic species assemblages showed directional change over time, but the magnitude of this change was much larger in exotic species. These analyses, in conjunction with previous work, paint a picture of a relatively stable community of native plants for which rare species are becoming more so. In contrast, we see an assemblage of exotic species exhibiting strong responses to changes in climate over the past decade. In this community it therefore appears unlikely that changes in climate are benefiting all exotic species at the expense of native species. High stress tolerance and local adaptation of plants found on serpentine soils may explain the relative stability of this community in response to a changing climate.