Niche overlap and null hypothesis test
One key assumption for applying ENMs is that species niche changes very slowly across space and time (Warren et al. 2008). These tests are based on two similarity metrics (Warren’s I and Schoener’s D), we calculated these metrics in ENMTools v1.4.3, using 100 replicates to generate a pseudoreplicated null distribution (Warren et al. 2010). The null hypothesis of niche equivalency is rejected when empirical values are significantly less than the critical values for both the niche equivalency and similarity tests (Warren et al. 2010).
E-space climate niche overlap between lineages of S. boulengeriwas estimated using the PCA-env approach proposed by Broennimann et al. (2012). An unbiased estimate of the Schoener’s D metric can be calculated for our data and is ensured to be independent of the resolution of the grid, statistical confidence in niche overlaps was then tested through a bi-directions niche similarity test (Broennimann et al. 2012).