Changes in the Ranges of European Wading Birds

In a revealing study published in Global Change Biology, Maclean et al. analyzed counts of seven wading bird species -- the Eurasian oystercatcher (Haematopus ostralegus), grey plover (Pluvialis squatarola), red knot (Calidris canutus), dunlin (Calidris alpina), bar-tailed godwit (Limosa lapponica), Eurasian curlew (Numenius arquata) and common redshank (Tringa tetanus) -- made at approximately 3500 different sites in Belgium, Denmark, France, Germany, Ireland, the Netherlands and the United Kingdom on at least an annual basis since the late 1970s. This they did in order to determine what range adjustments the waders may have made in response to concomitant regional warming, calculating the weighted (by count) geographical centroids of the bird populations for all sites with complete coverage for every year between 1981 and 2000.

This work revealed, in the words of the seven scientists, that "the weighted geographical centroid of the overwintering population of the majority of species has shifted in a northeasterly direction, perpendicular to winter isotherms," with overall 20-year shifts ranging from 30 to 119 km. In addition, they report that "when the dataset for each species was split into 10 parts, according to the mean temperature of the sites, responses are much stronger at the colder extremities of species ranges." In fact, they found that "at warmer sites, there was no palpable relationship [italics added] between changes in bird numbers and changes in temperature." Hence, they concluded that "range expansions rather than shifts [italics added] are occurring" as the planet warms.

In discussing the significance of their findings, the members of the international research team say that the commonly used climate-envelope approach to predicting warming-induced species migrations -- which is the one employed by the IPCC -- "essentially assumes that as climate alters, changes at one margin of a species' range are mirrored by those at the other, such that approximately the same 'climate space' is occupied regardless of actual climate," but that their work suggests "that this may not be the case: climate space can also change."

In further discussing their important finding, Maclean et al. write that "it is actually not surprising that responses to temperature appear only to be occurring at the colder extremities of species ranges," for they note that "it has long been known that it is common for species to be limited by environmental factors at one extremity, but by biological interactions at the other," citing the work of Connell (1983) and Begon et al. (2005). Hence, they conclude that it is likely that "the warmer extremities of the species ranges examined in this study are controlled primarily by biotic interactions, whereas the colder margins are dependent on temperature." Considered in this light, global warming -- CO₂-induced or natural -- provides an opportunity for species to expand their ranges either poleward in latitude or upward in altitude at their cold-limited boundaries, while largely maintaining the locations of their lower-elevation or equator-facing boundaries, and this phenomenon leads to increases in local species richness over most of the planet, a phenomenon most would likely welcome.

Additional References
Begon, M., Townsend, C. and Harper, J. 2005. Ecology: From Individuals to Ecosystems. Blackwell, Oxford, UK.

Connell, J.H. 1983. On the prevalence and relative importance of interspecific competition: evidence from field experiments. The American Naturalist 122: 661-696.