A new study of population trends among 46 ecologically diverse bird species in North America conducted by avian ecologist Joel Ralston and colleagues at the University of Massachusetts Amherst overturns a long-held assumption that the climate conditions occupied by a species do not change over time.
Instead, as the researchers report in the current early online issue of Global Ecology and Biogeography, birds that have increased in abundance over the last 30 years now occupy a wider range of climate conditions than they did 30 years ago, and declining species are occupying a smaller range of climate conditions than 30 years ago, Ralston says.
Species with relatively stable population trends maintained them. The authors believe this is the first study to investigate the relationship between population trend and the range of climatic conditions occupied, or "climate niche breadth" (CNB).
Ralston, now at Saint Mary's College, Notre Dame, Indiana, says, "It was previously thought that as species expand their ranges, they would do so while maintaining their climate niche. We show that as species become more abundant, they are actually moving into new climate conditions, and declining species are disappearing from some of the climate conditions they used to be found in. This makes theoretical sense but it counters the long-held assumption that climate niche breadth doesn't change in species."
The wood thrush is a declining species that some conservation biologists are concerned about. While one analysis shows a 2 percent per year decline in their numbers, a new model from UMass Amherst suggests this species had about a 7.5 percent decrease in its climate niche breadth over the study period. Credit: UMass Amherst/David King
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