Climate is one of the main drivers of species distribution. However, as different environmental factors tend to co-vary, the
effect of climate cannot be taken at face value, as it may be either inflated or obscured by other correlated factors. We used
the favourability models of four species (Alytes dickhilleni, Vipera latasti, Aquila fasciata and Capra pyrenaica) inhabiting
Spanish mountains as case studies to evaluate the relative contribution of climate in their forecasted favourability by using
variation partitioning and weighting the effect of climate in relation to non-climatic factors. By calculating the pure effect of
the climatic factor, the pure effects of non-climatic factors, the shared climatic effect and the proportion of the pure effect of
the climatic factor in relation to its apparent effect (r), we assessed the apparent effect and the pure independent effect of
climate. We then projected both types of effects when modelling the future favourability for each species and combination
of AOGCM-SRES (two Atmosphere-Ocean General Circulation Models: CGCM2 and ECHAM4, and two Special Reports on
Emission Scenarios (SRES): A2 and B2). The results show that the apparent effect of climate can be either inflated (overrated)
or obscured (underrated) by other correlated factors. These differences were species-specific; the sum of favourable areas
forecasted according to the pure climatic effect differed from that forecasted according to the apparent climatic effect by
about 61% on average for one of the species analyzed, and by about 20% on average for each of the other species. The pure
effect of future climate on species distributions can only be estimated by combining climate with other factors. Transferring
the pure climatic effect and the apparent climatic effect to the future delimits the maximum and minimum favourable areas
forecasted for each species in each climate change scenario.
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