Niche comparisons reveal significant divergence despite narrow endemism in Leavenworthia, a genus of rare plants

Background and aims: Quantifying niche similarity among closely related species offers myriad insights into evolutionary history and ecology. In this study, our aim was to explore the interplay of geographic and niche space for rare, endemic plant species and determine if endemic habitats were environmentally similar or unique.

Methods: We characterized the niche of all Leavenworthia species, a genus of rare plants endemic to rocky glades in the eastern United States, using WorldClim data, surface geology, elevation, and slope. We calculated the area of range overlap and estimated niche similarity between species in their total occupied niche space and the subset of niche space shared by both species. We used Linear Discriminant Analyses to determine which niche dimensions differed the most between species. We used niche dimensions with consistently high discriminatory power to perform a Random Forest classification analysis and Principal Component Analysis. Using a linear model, we related geographic distance to distance in niche space.

Key results: Most species comparisons concluded that species' niches had diverged, with niche similarity increasing linearly with range overlap. Temperature variation, precipitation amount and seasonality, and surface geology were the most divergent niche dimensions among all species comparisons. Geographic distance explained 42% of the variation in niche space distance. Sites that were closer in niche space than expected were oriented east-west due to the strong correlation between latitude and PC1 scores.

Conclusions: Despite being endemic to seemingly very similar habitat, niche similarity is low among Leavenworthia species. Low niche similarity, combined with low geographic overlap suggests that this lineage of rare plants potentially diversified in isolation but across a very small geographic area. The correlation between geographic space and niche space has received considerable attention, but our results suggest that geographic distance is a weak predictor of distance in niche space.