Intraspecific differentiation: Implications for niche and distribution modelling

Aim: Mounting evidence suggests that failure of species distribution models to integrate local adaptation hinders our ability to predict distribution ranges, raising the
<br />question of whether modelling should be performed at the level of species (clade
<br />models) or intraspecific lineages (subclade models), characterized by the restricted
<br />availability of occurrence points. While Ensembles of Small Models (ESMs) offer an
<br />attractive framework for small datasets, their evaluation remains critical. We address
<br />these issues in the case of very small datasets inherent to subclade models and discuss
<br />which modelling strategy should be applied based on niche overlap among lineages.
<br />Location: Sweden.
<br />Taxon: Mosses.
<br />Methods: Ensembles of Small Models were evaluated by null models built from randomly sampled presence points. We compared the extent of suitable area predicted
<br />by the projections of clade and subclade models. Niche overlap was quantified using
<br />Schoener's D and Hellinger'sImetrics, and the significance of these metrics in terms
<br />of niche conservatism or divergence was assessed by similarity tests.
<br />Results: We introduced a simple procedure for evaluating ESMs based on the pooling of the statistics used to assess model accuracy from the replicates. Despite fairly
<br />high AUC and TSS values, 2 of the 23 subclade models did not perform better than
<br />null models and should be discarded. Combined predictions from subclade models
<br />contributed, on average, five times more than clade models to the total suitable area
<br />predicted by the combination of subclade and clade models. The D and I metrics averaged 0.45 and 0.71, with evidence for niche conservatism in half of the species and
<br />no signal for niche divergence.
<br />Main conclusions: In addition to the assessment of ESM accuracy based on the simple procedure described here, we recommend that ESMs should be systematically
<br />evaluated against null models. Lumping or splitting occurrence data at the intraspecific level substantially impacted model projections. Given the poor performance of
<br />models based on small datasets, even when employing ESMs, we pragmatically suggest that, in the absence of evidence for niche divergence during diversification of
<br />closely related intraspecific lineages, SDMs should be based on all available occurrence data at the species level.