A tribute to Hubert Saint Girons: niche separation between Vipera aspis and V. berus on the basis of distribution models

Interspecific competition in contact areas is a major topic in distribution studies, but few data about this issue are available for European vipers. The aim of this study is the analysis of relative distributions of two viperids in an alpine area using habitat suitability models to verify which ecological factors affect their occurrence and to assess a possible niche separation. The study area covered about 585 km2 and it is located in lower Valtellina, the largest alpine valley in Lombardy (Northern Italy), that is West-East oriented. It included the bottom of the valley and both sides of the mountain slopes (200 - 3600 m a.s.l.). Fiftynine presence records (Vipera aspis N = 19; V. berus N = 40) came from direct surveys carried out from 2000 to 2008. Five non-correlated potential predictors were included in the model: annual precipitation, elevation, solar radiation, slope, and habitat type. All the variables were resampled to 100 m spatial resolution. Potential distribution was modelled using the maximum entropy method, and performed by the software MAXENT 3.3.1 (http://www.cs.princeton.edu/~schapire/maxent/). We used the bootstrap procedure to generate ten different models each with 20% of localities as test sample. We then averaged single model results. The models fit quite well (Vipera aspis: AUC = 0.938 ± 0.024; V. berus: AUC = 0.953 ± 0.015). The suitable area at the 10th percentile threshold is respectively 23.9% and 8.5% of the study area. Overlapping surface accounts for 5.5% of the cumulative suitable area. The most informative variables are: solar radiation (26.6%), elevation (25.7%), habitat (20.9%) and slope (19.3%) for the asp viper; habitat (62.4%) and elevation (26.3%) for the adder. Woods, meadows and urban areas have a positive effect on V. aspis distribution as wetlands and meadows do on V. berus. The two species are separated according to an altitudinal gradient at 1500 m a.s.l.. Asp viper prefers sunny and plain sites while the adder does not seem to be influenced by these factors. The two vipers are mutually exclusive and are separated by ecological constraints, as already observed by Saint Girons in 1975. Vipera aspis is more thermophilic and lives at low altitude, while Vipera berus prefers cool and humid areas typical of alpine pastures. A conservation problem could arise from anthropic activities at low elevations that dramatically reduce habitat suitability for the asp viper. On the other hand, the adder does not seem to be threatened by human presence thanks to its altitudinal preferences.