Modelling the distribution of the Boid snakes, Epicrates cenchria alvarezi and Boa constrictor occidentalis in the Gran Chaco (South America)

The viviparous snake species Epicrates cenchria alvarezi and Boa constrictor occidentalis inhabit the Gran Chaco region of South America. Ecological factors determining their distribution are poorly known. GIS-based modelling of a species' environmental requirements using occurrence records provides essential information of the species' distribution. We modelled the geographical distribution of E. c. alvarezi and B. c. occidentalis in the Gran Chaco and analyzed the degree to which their distribution is associated with different environmental variables ( precipitation, normalized difference vegetation index (NDVI), land surface temperature and altitude) using Logistic Regression Analysis. We found that E. c. alvarezi and B. c. occidentalis would be more frequent in areas with high levels of NDVI ( positive relationship with NDVI1), with low levels of precipitation ( negative relationship with PRE1) and with lower altitude ( negative relationship with ALT). However, the analysis also revealed specific differences in the environmental requirements of the snakes, showing that B. c. occidentalis is more frequent than E. c. alvarezi at sites with higher levels of NDVI. Additionally, E. c. alvarezi is more frequent at sites with lower temperatures in the dry season. The maps obtained show that both are highly likely to be present in the Dry subregion of the Gran Chaco. The results of the present study can be an important contribution to a better understanding of the ecological requirements of the species and of the impact of global environmental change on its distribution.