Thursday, January 08, 2009
Shaky prospects for grassland birds
A paper in the most recent issue of Biological Conservation lays out a grim picture for grassland birds. Landscape ecologist Kimberly With (Kansas State University) and her colleagues studied three grassland bird species (Eastern Meadowlark, Grasshopper Sparrow, and Dickcissel) in the Flint Hills of Kansas, one of the largest intact tallgrass prairies in the world and considered an important “core” region for grassland bird species. They modeled population viability and found that “none of these species is demographically viable at a regional scale under realistic assumptions, with estimated population declines of 3–29%/year and a likelihood of regional viability of 0–45% over the two years of study.” The paper sets off an alarm: huge grasslands that were thought to be productive, reliable "sources" for grassland bird populations cannot be counted on to save the species.
Population levels may be dropping despite conservation efforts in part because grassland birds are experiencing an “extinction debt.” Although the most drastic land use changes that affect grassland birds occurred 70 to 100 years ago, populations may take generations to respond (e.g., “learning” not to nest in degraded, low-productivity habitats). “If the North American grasslands have been reduced in area below some critical habitat threshold,” the authors contend, “theory predicts that it may take many decades for an extinction debt to pay out, especially if this coincides with the individual extinction thresholds of many species. If this is the case with grassland birds, then we may be witnessing an unfolding conservation crisis.”
The authors go on to discuss how climate change will likely exacerbate this grave situation: “Changes in precipitation patterns, with less frequent but more severe precipitation events, are an expected consequence of global climate change in the Great Plains. Droughts may therefore become more common, and coupled with current land-management practices, may further exceed the ability of birds to persist in the face of such widespread environmental changes.”