Ecological niche divergence
ENMs sufficiently predicted habitat suitability within each lineage’s
buffered minimum convex polygon, as indicated by mean AUC scores:S. atlantis (mean = 0.809, s.e. = 0.084), northern S.
hesperis (mean = 0.810, s.e. = 0.063), and southern S. hesperis(mean = 0.803, s.e. = 0.085) (Fig. 4). Visual inspection of predicted
habitat suitability surfaces across the entire study landscape suggests
that the three genomic lineages are divergent in their respective
habitat associations and ecological niches, and that the highest density
of suitable habitat for each lineage is generally found within and
adjacent to their buffered minimum convex polygon. Relative
contributions of geographic and environmental predictor variables to
ENMs, measured as the drop in AUC scores after each variable was
randomly permuted, are reported in Table 2. For each of the three
lineages, land cover and growing degree days had the greatest
contribution to ENMs. Contributions of other variables varied
considerably among lineages.
Linear mixed effects models indicated there was a significant reduction
in the power of each lineage’s ENMs when predicting the localities of
each of the other two lineages. The coefficient of the “within-vs. between-lineage” binary predictor variable was significantly
negative for each pairwise comparison: S. atlantis and northernS. hesperis (β = -0.292; p = 0.028), S. atlantisand southern S. hesperis (β = -0.6811; p <
0.00), and northern and southern S. hesperis (β = -0.333;p = 0.023). Together, these results indicate that all three
lineages are significantly divergent in their habitat associations and
ecological niches.
These analyses were repeated using null ENMs built with randomly
generated localities confined to each lineage’s buffered minimum convex
polygon. Linear mixed effects models addressing resulting AUC scores
indicated no significant reduction in the power of each lineage’s null
ENMs when predicting the random localities generated for the other
lineages. Specifically, the coefficient of the “within- vs.between-lineage” binary predictor variable was non-significant forS. atlantis and northern S. hesperis (β = -0.012; p= 0.822), S. atlantis and southern S. hesperis (β = 0.020;p = 0.751), and northern and southern S. hesperis (β =
0.015; p = 0.810). These results indicate that observed
differences in habitat associations and ecological niches cannot be
attributed to biases arising from differences in available habitat among
the three lineages.