This is raster file (base_cats_new3.asc) that was used to generate the environmental resistance surface with the ResistanceGA R package (Peterman, 2018) to evaluate models of environmental resistance to between-population movement of saltwater crocodiles Crocodylus porosus in the Northern Territory of Australia, represented by individual pairwise genetic distances among individuals. ResistanceGA models pairwise genetic distances in response to pairwise ‘ecological distances’ using linear mixed effects models with a maximum-likelihood population effects (MLPE) random effects structure (Clarke, Rothery, & Raybould, 2002), represented by individual ID in our models. We used Smouse and Peakall (1999) pairwise genetic distance as the response variable for this purpose.We estimated resistance surfaces that optimised random-walk commute distances (Etten, 2018) among the locations of sampled individuals as an explanatory variable in models of pairwise genetic distances among individuals. We ran a single surface optimisation in ResistanceGA (Peterman, 2018) to generate resistance values for the six environmental cover categories and stopped each model after 25 consecutive generations of no improvement in log-likelihood.

We created a categorical resistance surface layer using a 3 km x 3 km cell size raster (with 325 x 202 cells) with cells classified as sea, dry land, and the different types of habitats for C. porosus. We classified habitats into ‘core breeding habitat’, ‘marginal breeding habitat’, ‘core non-breeding habitat’ or ‘marginal non-breeding habitat’, following the definitions in the literature (Fukuda & Cuff, 2013; Fukuda et al., 2007; Webb, 1991). Breeding of C. porosus is highly seasonal during the wet season (November-April) and constrained to temporarily flooded, freshwater waterbodies which are not necessarily the most suitable habitat for saltwater crocodiles outside the breeding period (Campbell et al., 2013; Fukuda & Cuff, 2013; Fukuda et al., 2007; Webb, 1991). The core breeding habitats are the most favourable nesting areas represented by particular vegetation types as defined by Fukuda et al. (2007), while the marginal breeding habitats were identified by broader vegetation communities occasionally used for nesting (Fukuda & Cuff, 2013). The core non-breeding habitats are the most favourable waterbodies that tend to persist outside the breeding season (Fukuda & Cuff, 2013) and does not include known nesting areas. We defined the marginal non-breeding habitats by buffering the core non-breeding habitats by 3 km so that these habitats would include temporary waterbodies that may dry up during the dry season (May-October) or coastal areas with salinity levels similar to seawater (typically 35 parts per thousand). Although C. porosus is highly adapted to the saline environment (Cramp, Meyer, Sparks, & Franklin, 2008; Grigg, Taplin, Harlow, & Wright, 1980; Taplin, 1985), the species occurs in much higher density in brackish or fresh water (Fukuda et al., 2011; Webb & Manolis, 1989) and nesting females and embryos require access to freshwater (Webb, Manolis, Buckworth, & Sack, 1983; Webb, Messel, & Magnusson, 1977). Although some individuals access sea, especially when moving between the rivers (Campbell et al., 2010; Fukuda, Webb, Manolis, Lindner, & Banks, 2019), it is considered less favoured than brackish or freshwater habitats, and dry land is almost inaccessible to crocodiles as suggested by the previous tracking by satellites (Fukuda et al., 2019).

References

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