A letter from the frontier: forecasting species expansions

Abstract

As the global climate changes, species will have to move to where the weather suits their needs. But can we forecast where that is – can we predict the future distribution of a species based on the anticipated distribution of its preferred habitat and the current ability of that species to move from one site to the next? Although several researchers have tried to do this, a new paper by Chris Thomas and co-workers 1xEcological and evolutionary processes at expanding range margins. Thomas, C.D. et al. Nature. 2001; 411: 577–581Crossref | PubMed | Scopus (501)See all References1 suggests that this approach might be too simplistic.Working on four insect species that have recently expanded their ranges in Britain, Thomas et al. show that populations at the range margin have enlarged the spectrum of habitat types that they can colonize, and have improved their potential for dispersal. Of two species of butterfly, the silver-spotted skipper Hesperia comma previously lived on warm, south-facing chalk hillsides in southeast England. With climate warming, the species now thrives on slopes of all aspects, resulting in more patches to colonize and shorter distances between them. This has led to a much faster rate of expansion than was expected based on the distribution of the original habitat of the butterfly. The second butterfly, the brown argusbrown argus Aricia agestis, has also benefited from warmer weather by increasingly turning to widespread host plants in habitats that were previously too cool for it. This has allowed the argus to cross previously unbridgeable gaps between patches of its original host plant. In two species of bush crickets Conocephalus discolor and Metrioptera roeselii, Thomas et al. found a much higher proportion of longer- than shorter-winged forms in recently established populations than in old ones. This, the researchers argue, reflects how populations at expanding range fronts can be dominated by particularly dispersive individuals, hence significantly increasing the rate of expansion. Behind the advancing front, less dispersive but more fecund forms might again be favoured.Fig. 1Brown argus Aricia agestis, Robert WilsonView Large Image | Download PowerPoint SlideClearly, the paper by Thomas et al. should not lull us into believing that we live in the best of all possible worlds, where all species are able to adjust their ranges even quicker than we ever thought possible. Rather, as emphasized by Stuart Pimm 2xEntrepreneurial insects. Pimm, S. Nature. 2001; 411: 531–532Crossref | PubMed | Scopus (21)See all References2, the results should make us distrust our prophetic abilities, because global climate change might affect different species in different ways and through processes that we did not know to expect. The particular processes described by Thomas et al. also require some initial expansion before they come into play, and many species might fail to take these first crucial steps. But once a population gets going, transient evolutionary changes in habitat range and migration rate might well turn out to be common phenomena. But just how common? That remains to be discovered.