Climatic niche shifts between species' native and naturalized ranges raise concern for ecological forecasts during invasions and climate change

Abstract

Aim: Correlative models that forecast extinction risk from climate change and invasion risks following species introductions, depend on the assumption that species’ current distributions reflect their climate tolerances (‘climatic equilibrium’). This assumption has rarely been tested with independent distribution data, and studies that have done so have focused on species that are widespread or weedy in their native range. We use independent data to test climatic equilibrium for a broadly representative group of species, and ask whether there are any general indicators that can be used to identify when equilibrium occurs. Location: Europe and contiguous USA. Methods: We contrasted the climate conditions occupied by 51 plant species in their native (European) and naturalized (USA) distributions by applying kernel smoothers to species’ occurrence densities. We asked whether species had naturalized in climate conditions that differ from their native ranges, suggesting climatic disequilibrium in the native range, and whether characteristics of species’ native distributions correspond with climatic equilibrium. Results: A large proportion of species’ naturalized distributions occurred outside the climatic conditions occupied in their native ranges: for 22 species, the majority of their naturalized ranges fell outside their native climate conditions. Our analyses revealed large areas in Europe that species do not occupy, but which match climatic conditions occupied in the USA, suggesting a high degree of climatic disequilibrium in the native range. Disequilibrium was most severe for species with native ranges that are small and occupy a narrow range of climatic conditions. Main conclusions: Our results demonstrate that the direct effects of climate on species distributions have been widely overestimated, and that previous large-scale validations of the equilibrium assumption using species’ native and naturalized distributions are not generally applicable. Non-climatic range limitations are likely to be the norm, rather than the exception, and pose added risks for species under climate change.