Abstract
AbstractAimsTo investigate spatial congruence between ecological niches and genotype in two allopatric species of desert tortoise that are species of conservation concern.LocationMojave and Sonoran Desert ecoregions; California, Nevada, Arizona, Utah, USA.MethodsWe compare ecological niches of Gopherus agassizii and Gopherus morafkai using species distribution modelling (SDM) and then calibrate a pooled‐taxa distribution model to explore local differences in species–environment relationships based on the spatial residuals of the pooled‐taxa model. We use multiscale geographically weighted regression (MGWR) applied to those residuals to estimate local species–environment relationships that can vary across the landscape. We identify multivariate clusters in these local species–environment relationships and compare them against models of (a) a geographically based taxonomic designation for two sister species and (b) an environmental ecoregion designation, with respect to their ability to predict a genotype association index for these two species.ResultsWe find non‐identical niches for these species, with differences that span physiographic and vegetation niche dimensions. We find evidence for two distinct clusters of local species–environment relationships that when mapped, predict an index of genotype association for the two sister taxa better than did either the geographically based taxonomic designation or an environmental ecoregion designation.Main conclusionsExploring local species–environment relationships by coupling SDM and MGWR can benefit studies of biogeography and conservation. We find that niche separation in habitat selection conforms to genotypic differences between sister taxa of tortoise in a recent secondary contact zone. This result may inform decision making by agencies with regulatory or land management authority for the two sister taxa addressed here.
Highlights
The relationships between the distributions of species and their ecological properties have long been central to biogeographic in‐ quiry (Grinnell, 1917; MacArthur, 1972)
The single set of variables selected to describe the ecological niche of each species included the following: precipitation of the driest month (Ppt_dry), precipitation seasonality (Ppt_CV), surface texture (Surf_Text), soil moisture (S_moist), temperature evenness (Isotherm), photosynthetic activity (Veg_Amp), topographic position index (Topo_Index) and surface material (Surf_Mat; see Table 1 for descriptions), and resulted in models for G. agassizii and G. morafkai with test AUC scores of 0.733 and 0.875, respectively (Figure 3)
We explore ecological niche differences between two allopat‐ ric species of conservation concern, G. agassizii and G. morafkai, and find that while these two species occupy broadly similar
Summary
The relationships between the distributions of species and their ecological properties have long been central to biogeographic in‐ quiry (Grinnell, 1917; MacArthur, 1972). We develop a case study of two sister taxa to explore geo‐ graphic patterns of niche differences between them, with focus on their differing conservation status These two species, Gopherus agassizii (Agassiz's desert tortoise) and Gopherus morafkai (Morafka's desert tortoise) diverged approximately 6 Ma due to geographic iso‐ lation by the Bouse embayment, a putative marine transgression of the ancestral Gulf of California along the lower Colorado River, which has resulted in allopatric speciation (Murphy et al, 2011). The results of this study will inform conservation planning across the transition zone of these two species
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