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.