While poleward species migration in response to recent climatic warming is widely documented, few studies have examined entire range responses of broadly distributed sessile organisms, including changes on both the trailing (equatorward) and the leading (poleward) range edges. From a detailed population census throughout the entire geographical range of Aloe dichotoma Masson, a long-lived Namib Desert tree, together with data from repeat photographs, we present strong evidence that a developing range shift in this species is a ‘fingerprint’ of anthropogenic climate change. This is explained at a high level of statistical significance by population level impacts of observed regional warming and resulting water balance constraints. Generalized linear models suggest that greater mortalities and population declines in equatorward populations are virtually certainly the result, due to anthropogenic climate change, of the progressive exceedance of critical climate thresholds that are relatively closer to the species’ tolerance limits in equatorward sites. Equatorward population declines are also broadly consistent with bioclimatically modelled projections under anticipated anthropogenic climate change but, as yet, there is no evidence of poleward range expansion into the area predicted to become suitable in future, despite good evidence for positive population growth trends in poleward populations. This study is among the first to show a marked lag between trailing edge population extinction and leading edge range expansion in a species experiencing anthropogenic climate change impacts, a pattern likely to apply to most sessile and poorly dispersed organisms. This provides support for conservative assumptions of species’ migration rates when modelling climate change impacts for such species. Aloe dichotoma's response to climate change suggests that desert ecosystems may be more sensitive to climate change than previously suspected.
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