The American Great Plains have seen many changes during the last century and a half as agriculture and increased human settlement have changed a once common landscape into one of the most endangered ecosystems in North America. A team of ecologists set out to discover how changes to the region have affected birds living in the middle of the continent. The results of their work have been published in this month's Ecological Applications (volume 11 number 1).
The research team, led by Bryan R. Coppedge and David M. Engle of Oklahoma State University, examined data from breeding bird surveys conducted between 1965-1995 in the southern Great Plains of Oklahoma. The researchers compared the survey results to information regarding the changing landscapes in these same areas.
Like much of the land comprising the Great Plains of the United States, the areas examined in this study have seen drastic land use changes in the last several decades. Areas which were once open grasslands have become mosaics of cultivated croplands, prairie remnants, and expanding woodlands.
Fire, which once restricted the growth of trees, has been strictly controlled in many areas. This has increased the number of junipers and mesquite trees. Trees have also been planted throughout the area for use as windbreaks and shelterbelts.
Coppedge's team found that the increase of trees in the area was beneficial to certain kinds of neotropical migrant birds, specifically those which prefer woody habitat such as Yellow-billed Cuckoos, Great-crested Flycatchers, and Mississippi Kites.
Similarly, the number of temperate migrant and resident bird species in the study areas seemed to increase throughout much of the last two decades. From 1981 to 1995 Cattle Egrets, Eastern Bluebirds, Carolina Chickadees, Tufted Titmice, Bewick's Wrens, Northern Cardinals, Field Sparrows, and American crows all increased in the areas studied. These birds, Coppedge points out, are all associated with woody areas or are open-habitat generalists.
In sharp contrast, however, the team discovered that a number of birds which are endemic to grasslands declined in number in some of the study areas. Western Meadowlarks, for instance, consistently decreased in number during the study.
During some parts of the late 1980s these birds were completely absent from one of the areas monitored. Other decreasing grassland species included Lark Sparrows, Horned Larks, Cassins' Sparrows and Grasshopper Sparrows.
"These numbers are important because many birds that migrate to the neotropics are rapidly declining," says Coppedge. "But the summer breeding and nesting habitat for grassland birds is also shrinking, posing a management dilemma."
In reviewing their data, the researchers found that the size of available grasslands was only one important factor. The composition and structure of vegetation in the areas studied were also important.
Like many parts of the Great Plains, some of the areas included in this study were converted from cropland to agricultural grasslands under the federal Conservation Reserve Program (CRP). CRP lands are planted with perennial vegetation in order to curtail soil erosion and improve the soil and water quality in many areas of the West.
Much of the land in the CRP program has been planted with non-native grasses. Despite this fact, the researchers found that in some cases grassland birds can benefit from conversions. At one study site there was an increased abundance of Dickcissels, Grasshopper Sparrows, Field Sparrows, and Eastern Meadowlarks after the planting of CRP grasslands in the late 1980s.
The researchers note, however, that a similar response was not seen in the other study areas. CRP grasslands were most used by the birds in areas which were most affected by encroaching juniper trees and which had the least amount of intact native grassland nearby.
"CRP lands may be the lesser of two poor choices in habitat," says Coppedge. "Birds are probably using the most structurally suitable habitat available in landscapes where native grasslands are becoming more and more scarce."
Coppedge also notes that the Great Plains once served as a geographic barrier to woodland birds. As Eastern tree species have moved more and more westward, that barrier becomes easier and easier to overcome. Thus birds from the East are now more likely to mix with birds from the West. This mixing, he says, can cause a loss of genetic diversity which could be detrimental to the ultimate survival of some species.
"It is important that grassland degradation be prevented," says Coppedge. "Even partial restoration of some of the factors which historically controlled woody vegetation would be helpful to grassland birds."
Management options could include prescribed fire, and the reintroduction of grazing animals which are native to the area, such as bison, says the scientist.
"Grassland habitat management and set-aside programs should also be concentrated in areas where grasslands have been severely altered by woody vegetation or fragmented by farming," says Coppedge.
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