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).