COS 12-6 - Characterizing the climatic and edaphic niches of the Temperate North American clade of Asclepias

Abstract: Niche divergence has been considered an important driver of speciation and diversification that has consequences for species coexistence. By characterizing the niches of species across a whole group it becomes possible to assess niche differences among closely related species and gain insight into the role that niches play in lineage diversification, as well as how that leads to coexistence across a landscape. To evaluate the role of niche differentiation in lineage diversification, I studied the Temperate North American Clade of Asclepias (milkweeds), which contains approximately 57 species. These species exhibit a broad range of ecological variation, from open deserts to forest understories. I hypothesized that 1) species diverged along both climatic and edaphic niche axes, primarily precipitation and soil texture, and 2) niche differentiation among species in the southwestern United States and Mexico is greater than in the Eastern Temperate Forest. Here, I characterized niches by 19 bioclimatic variables, elevation, % clay content, % silt content, % sand content, coarse fragment content, nitrogen content, pH, and electrical conductivity. I used multivariate analyses, including principal component analysis (PCA), linear discriminant function analysis (LDA), and ecological niche modeling, to characterize niches and evaluate species differences. The degree of niche overlap between species was measured using Warren’s I.

Preliminary results from PCA and LDA show that species of the Temperate North American Clade are differentiated primarily along a climatic niche axis with mean annual temperature and precipitation being the most important. Soil characters, while still important for differentiation among some species pairs, did not contribute to differentiation as much as climate. In addition, species found in the southwestern United States and Mexico have high niche differentiation from each other, likely on account of the high ecological heterogeneity of the region. Counter to my original hypothesis, however, the niche differentiation of the species found in these regions is not significantly higher than among species in the Eastern Temperate Forest. These results indicate that the majority of the ecological variation of this group is not contained in one part of this group’s distribution, but rather is found across its range. This could suggest that the rapid diversification of this group, potentially associated with niche, may be consistent across their evolutionary history as well.