Detecting the influence of climatic variables on species distributions: a test using GIS niche-based models along a steep longitudinal environmental gradient

Aim  To investigate the influence of climate variables in shaping species distributions across a steep longitudinal environmental gradient.
Location  The state of Oklahoma, south-central United States.
Methods  We used Geographical Information Systems (GIS) niche-based models to predict the geographic distributions of six pairs of closely related amphibian and reptile species across a steep longitudinal environmental gradient. We compared results from modelling with actual distributions to determine whether species distributions were primarily limited by environmental factors, and to assess the potential roles of competition and historical factors in influencing distributions.
Results  For all species pairs, GIS models predicted an overlap zone in which both species should occur, although in reality in some cases this area was occupied by only one of the species. We found that environmental factors clearly influence the distributions of most species pairs. We also found evidence suggesting that competition and evolutionary history play a role in determining the distributions of some species pairs.
Main conclusions  Niche-based GIS modelling is a useful tool for investigating species distribution patterns and the factors affecting them. Our results showed that environmental factors strongly influenced species distributions, and that competition and historical factors may also be involved in some cases. Furthermore, results suggested additional lines of research, such as ecological comparisons among populations occurring inside and outside predicted overlap zones, which may provide more direct insight into the roles of competitive interactions and historical factors in shaping species distributions.