Climate-induced shifts in the niche similarity of two related spadefoot toads (genus Pelobates)

Abstract

Of the four species encompassing the genus Pelobates, only two overlap along a narrow contact zone, i.e., Pelobates fuscus and Pelobates syriacus. Our study investigated the shifts in niche similarity of these two closely related species from the Last Interglacial towards the end of the twenty-first century. We computed climatic suitability models using Maxent and projected them onto future and past climates. We used fossil occurrences to test the predictive accuracy of past projections. Niche similarity was assessed between the studied species using Schoener’s D index and a background similarity test. Finally, we evaluated niche differentiation by contrasting the species occurrences using a logistic regression analysis. The ecological niches are slightly extended outside the present geographical ranges in the Caucasus and the Balkans, south for P. fuscus and north and west for P. syriacus, suggesting that their present distribution is not at equilibrium with the climate. The Last Interglacial distribution of P. fuscus included British Isles and broad areas in western, central, and northern Europe, while P. syriacus extended northwards in the Balkans. The validation with fossil records revealed good predictive performance (omission error = 4.1 % for P. fuscus and 16.6 % for P. syriacus). During the Last Glacial Maximum, climatic suitability persisted in refugia in southern Europe, Pannonian Basin, and Caucasus for P. fuscus, and Israel, southern Balkans, and Caucasus for P. syriacus. Present potential distributions revealed a low similarity of species’ ecological niches, comparable with Last Interglacial, but projections towards 2080 revealed a sharp increase.