Downscaling European species atlas distributions to a finer resolution: implications for conservation planning

Aim: One of the limitations to using species' distribution atlases in conservation planning is their coarse resolution relative to the needs of local planners. In this study, a simple approach to downscale original species atlas distributions to a finer resolution is outlined. If such a procedure yielded accurate downscaled predictions, then it could be an aid to using available distribution atlases in real-world local conservation decisions. Location: Europe. Methods: An iterative procedure based on generalized additive modelling is used to downscale original European 50 x 50 km distributions of 2189 plant and terrestrial vertebrate species to c. 10 x 10 km grid resolution. Models are trained on 70% of the original data and evaluated on the remaining 30%, using the receiver operating characteristic (ROC) procedure. Fitted models are then interpolated to a finer resolution. A British dataset comprising distributions of 81 passerine-bird species in a 10 x 10 km grid is used as a test bed to assess the accuracy of the downscaled predictions. European-wide, downscaled predictions are further evaluated in terms of their ability to reproduce: (1) spatial patterns of coincidence in species richness scores among different groups; and (2) spatial patterns of coincidence in richness, rarity and complementarity hotspots. Results: There was a generally good agreement between downscaled and observed fine-resolution distributions for passerine species in Britain (median Jaccard similarity = 70%; lower quartile = 36%; upper quartile = 88%). In contrast, the correlation between downscaled and observed passerine species richness was relatively low (rho = 0.31) indicating a pattern of error propagation through the process of overlaying downscaled distributions for many species. It was also found that measures of model accuracy in fitting original data (ROC) were a poor predictor of models' ability to interpolate distributions at fine resolutions (rho = -0.10). Although European hotspots were not fully coincident between observed and modelled coarse-resolution data, or between modelled coarse resolution and modelled downscaled data, there was evidence that downscaled distributions were able to maintain original cross-taxon coincidence of species-richness scores, at least for terrestrial vertebrate groups. Downscaled distributions were also able to uncover important environmental gradients otherwise blurred by coarse-resolution data. Main conclusions: Despite uncertainties,