Environmental Explanatory Variables

Publication Date

2019-06-05

Creation

2018-11-21

Inman, R.D., and Esque, T.C., 2019, Local ecological niche models, genotype associations and environmental data for desert tortoises: U.S. Geological Survey data release, https://doi.org/10.5066/P91V2S8C.

We developed a two-step modeling approach drawing on the strengths of both species distribution modelling (SDM) and multiscale geographically weighted regression (MGWR) to explore spatial patterns in species-environment relationships in Gopherus agassizii and Gopherus morafkai across this secondary contact zone. In the first step, we use SDM to develop range-wide ecological niche models for each species separately and test existing hypotheses that the niches of these two species are more difference than would be expected by chance. We then pool both species and develop a single model of their combined ecological niche and treat the mapped residuals from this pooled model as a measure of local deviation. We assume that, if the two species exhibit different ecological niches, residuals from a pooled model will represent how poorly their combined niche predicts habitat at a given location. In the second step, we use MGWR to explore spatial patterns in the relationships between these residuals and hypothesized explanatory variables that may enumerate differences between the two species and their hybrids within the focal study area. While the interpretation of model coefficients for residuals as the response variable is perhaps less intuitive than interpreting those for habitat suitability, residuals offer a local measure of deviation from pooled niches that may illuminate landscape gradients in local niche differences. We use SDM and MGWR in a coupled modeling approach to identify differences in the ecological niches of G. agassizii and G. morafkai, and explore spatially varying species-environment relationships in the recent secondary contact zone. We 1) formally test for differences in their ecological niches, 2) identify boundaries represented by differences in their niches, and 3) determine which of three delineations better describes landscape patterns of genotypic variation. These delineations include A) the Colorado River (the current geographic boundary defining each species), B) the Mojave and Sonoran Basin and Range ecotone, and C) geographic patterns in local niche differences identified in this study. The results of this study will inform conservation planning across the transition zone of these two species.

• USGS Western Ecological Research Center

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