2.5. Niche differentiation
To assess the degree of climate niche overlap among species and genetic groups, two randomization tests were executed in ENMTools ver. 1.0.4 (Warren & Dinnage 2021): the niche identity test and the niche similarity test or background test (Warren et al. 2008). The identity test was performed to examine whether the habitat suitability scores generated from the ecological niche models of species and genetic populations are significantly different than expected if they were generated from the same distribution. The similarity test was also performed to examine the degree to which environmental niches are conserved over time, between M. geocarpum and genetic groups.
To quantify niche overlap for pair-wise comparisons, we used Schoener’s D (Rödder & Engler 2011; Schoener 1968; Warren et al. 2008) metric, calculated using generalized linear model (GLM) with 100 randomized pseudoreplicates following the model below:
\begin{equation} D\left(p_{x}+p_{y}\right)=1-\frac{1}{2}\sum_{i}\left(|p_{x,i}-b_{y,i}|\right)\nonumber \\ \end{equation}
Where px,i and py,i denote the probability assigned in a species distribution model computed for species X and Y to grid cell i . D value ranged from 0 to 1.
The observed values of D in both the niche identity test and the background similarity test were compared to the mean values of five randomized runs using t-test (Warrenet al. 2008). The environmental niches were considered significantly different if the observed values of niche overlap were less than 95% or 99% (alpha = 0.05 and 0.01, respectively) of the overlap values derived from the pseudo replicates.