Weak climatic associations among British plant distributions


Daniel S. Chapman, Centre for Ecology and Hydrology, Bush Estate, Edinburgh EH26 0QB, UK.
E-mail: dcha@ceh.ac.uk


Aim  Species distribution models (SDMs) are used to infer niche responses and predict climate change-induced range shifts. However, their power to distinguish real and chance associations between spatially autocorrelated distribution and environmental data at continental scales has been questioned. Here this is investigated at a regional (10 km) scale by modelling the distributions of 100 plant species native to the UK.

Location  UK.

Methods  SDMs fitted using real climate data were compared with those utilizing simulated climate gradients. The simulated gradients preserve the exact values and spatial structure of the real ones, but have no causal relationships with any species and so represent an appropriate null model. SDMs were fitted as generalized linear models (GLMs) or by the Random Forest machine-learning algorithm and were either non-spatial or included spatially explicit trend surfaces or autocovariates as predictors.

Results  Species distributions were significantly but erroneously related to the simulated gradients in 86% of cases (P < 0.05 in likelihood-ratio tests of GLMs), with the highest error for strongly autocorrelated species and gradients and when species occupied 50% of sites. Even more false effects were found when curvilinear responses were modelled, and this was not adequately mitigated in the spatially explicit models. Non-spatial SDMs based on simulated climate data suggested that 70–80% of the apparent explanatory power of the real data could be attributable to its spatial structure. Furthermore, the niche component of spatially explicit SDMs did not significantly contribute to model fit in most species.

Main conclusions  Spatial structure in the climate, rather than functional relationships with species distributions, may account for much of the apparent fit and predictive power of SDMs. Failure to account for this means that the evidence for climatic limitation of species distributions may have been overstated. As such, predicted regional- and national-scale impacts of climate change based on the analysis of static distribution snapshots will require re-evaluation.