Background
The authors write that "climate warming threatens the survival of species at their warm, trailing-edge range boundaries but also provides opportunities for the ecological release of populations at the cool, leading edges of their distributions," so that "as the climate warms," as they continue, "leading-edge populations are expected to utilize an increased range of habitat types, leading to larger population sizes and range expansion."

What was done
Seeking evidence for or against this scenario, Oliver et al. tested "the hypothesis that the habitat associations of British butterflies have expanded over three decades of climate warming." This they did by characterizing "the habitat breadth of 27 southerly distributed species from 77 monitoring transects between 1977 and 2007 by considering changes in densities of butterflies across 11 habitat types."

What was learned
In response to the overall climate warming that occurred between 1977 and 2007, but "contrary to expectation," as they report, the five UK researchers determined that 20 of the 27 species of butterflies they studied "showed long-term contractions in their habitat associations, despite some short-term expansions in habitat breadth in warmer-than-usual years," when the butterflies they studied spread out from their primary habitat to occupy still other sites.

What it means
The findings of Oliver et al. suggest to them that some non-climatic driver must be responsible for most of the habitat contractions of British butterflies over the last three decades, for they note in this regard that "butterfly population declines in the past century have been primarily driven by habitat destruction and degradation, particularly in relation to agricultural intensification and abandonment," citing Asher et al. (2001) and Warren et al. (2001). Therefore, they lament the likelihood that these other anthropogenic-induced constraints "appear to be out-weighing the positive effects of a warming climate on habitat breadth." And in the final sentence of their paper, they thus suggest that "only if other non-climatic drivers can be reduced or reversed will species be able to fully exploit any emerging opportunities provided by climate warming," which is here shown to be a very positive phenomenon for British butterflies and, by analogy, is implied to be similarly beneficial for many other species of animal life in many other lands.

References
Asher, J., Warren, M., Fox, R., Harding, P., Jeffcoate, G. and Jeffcoate, S. 2001. The Millennium Atlas of Butterflies in Britain and Ireland. Oxford University Press, Oxford, United Kingdom.

Warren, M.S., Hill, J.K., Thomas, J.A., Asher, J., Fox, R., Huntley, B., Roy, D.B., Telfer, M.G., Jeffcoate, S., Harding, .P, Jeffcoate, G., Willis, S.G., Greatorex-Davies, J.N., Moss, D. and Thomas, C.D. 2001. Rapid responses of British butterflies to opposing forces of climate and habitat change. Nature 414: 65-69.