Aim To identify the ecological factors associated with the range borders of two amphibian species, to investigate geographical variation in environmental parameters that determine species range limits, and to develop a testable hypothesis on the species’ biogeographical history.
Location Portugal and Spain.
Methods We documented the distribution of two species of marbled newt (Triturus marmoratus and Triturus pygmaeus) from atlas data, survey data and six latitudinal transects. Countrywide ecogeographical models were constructed from presence-only data and a suite of 19 environmental parameters. Local ecogeographical models were constructed along the contact zone.
Results Triturus marmoratus and T. pygmaeus share a parapatric range border from coastal Portugal to central Spain. Ecogeographical models performed better in descriptive mode (for the country where they were developed) than in predictive mode (when extrapolated for the neighbouring country). This suggests the existence of spatial variation for parameters that determine the species range borders. Local models showed good performance in Spain and western Portugal. Parameters dominating the models were climatic (precipitation, temperature and solar radiation), geographical (altitude) and geophysical (soil permeability). Enclaves in the distribution of T. marmoratus/T. pygmaeus also corresponded with environmental parameters. Poor-fitting local models were found in central Portugal, where the mutual range border of the two species coincides with the River Tejo.
Main conclusions The fit and predictive performance of the ecogeographical models suggest that equilibrium conditions have been reached over most of the contact zone. Parameter selection, field observations and natural history knowledge led us to identify, in terms of reproductive strategy, a stable species with regular, but low annual recruitment (T. marmoratus) vs. an opportunistic species with fluctuating ‘all-or-nothing’ annual recruitment (T. pygmaeus). Ecological parapatry derives from the use of permanent ponds for breeding in T. marmoratus vs. ephemeral ponds used by T. pygmaeus. Enclaves in the distribution show that, historically, T. pygmaeus has been displacing T. marmoratus. In central Portugal, where the model fit is poor, the advance of T. pygmaeus is currently impeded by the River Tejo. We predict that once this barrier has been crossed, the contact zone will move c. 40 km to the north. As T. marmoratus and T. pygmaeus are hybridizing species, we predict the occasional presence of T. marmoratus genes in areas that have changed occupancy.