Bird Case Study

If the rate at which humans are injecting greenhouse gases into the atmosphere is not greatly decreased, there is a significant chance that the Earth's climate will warm by several degrees Celsius by the year 2050 (Titus and Narayanan 1995, IPCC 2001a).With that in mind, Root (1988a) examined the biogeographic patterns of all wintering North American birds. She chose this group of species because birds are important parts of ecosystems and because of the availability of the necessary data. The National Audubon Society has volunteer forces amassed to aid in the collection of Christmas Bird Count data. By using these data, Root determined that for a large proportion of species, average distribution and abundance patterns are associated with various environmental factors (e.g., northern range limits of some species are apparently limited by average minimum January temperature [Root 1988a,b,c, 1989; Repasky 1991]).

The scaling question is, What mechanisms (such as competition or thermal stress) at small scales may have given rise to the large-scale associations? Root first tested the hypothesis that local physiological constraints may be causing most of the particular large-scale, temperature-range boundary associations. She used published, small-scale studies on the wintering physiology of key species to determine that about half of the songbird species wintering in North America extend their ranges no farther north than the regions where, to avoid hypothermia during winter nights, they need not increase their metabolic rates more than roughly 2.5 times their basal metabolic rate (Root 1988b). Root embarked on a larger, regional study to determine whether the longer nights, hence fewer hours of daylight available for foraging, or the colder temperatures in the more northerly locations are relatively more important. Preliminary results indicate that temperatures are more likely than day length to explain this effect (Root 2000). Thus global temperature changes would probably cause a rapid range and abundance shift, at least by selected bird species. Indeed, Root found significant year-to-year shifts in ranges and abundances; these shifts are apparently associated with year-to-year changes in winter temperatures (Root

1994). No claim is made at this point in the research for the generality of the preliminary results indicating strong and quantitative links between bird distributions and climate change. This example does permit, however, a clear demonstration of refined methods for cycling across scales to estimate ecological responses to climate change.

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