To analyse a source of uncertainty when forecasting the possible effects of climate change on species distributions, which may appear when the species studied have doubtful taxonomy or are subdivided into subspecies with different environmental requirements. Iberian newts (Triturus pygmaeus and Triturus marmoratus, a recently identified species) were used as examples. Environmental favourability models were performed on the occurrence of the newts according to two taxonomic options: (1) the two species separately; and (2) the two species together, as they were considered a single species by taxonomists until recently. The models were projected to three time periods between 2011 and 2100 within a context of climate change, using two different general circulation models and two emission scenarios. We calculated the discrepancy between forecasts produced with the different taxonomic options and their consistency under the same climate change scenario. The model based on the two species together did not distinguish between particular environmental requirements of either of the two species. Discrepancy values between taxonomic options increased over time. A reduction in areas favourable to T. pygmaeus and its north-eastward displacement were only predicted when this species was analysed separately. Nevertheless, the uncertainty derived from taxonomic ambiguity barely affected the predictions for T. marmoratus. Qualitatively and quantitatively different distribution forecasts for two newt species in mainland Spain were obtained depending on the taxonomic option considered. Taxonomic uncertainty also affected other sources of uncertainty. Some guidelines are suggested to aid in similar cases.
