Northern Prairie Wildlife Research Center
When Rachel Carson wrote Silent Spring2 her primary concern was over the widespread use of chemical pesticides in the environment. Her concern was translated into efforts that clearly tied the use of at least one pesticide (DDT) to reproductive failures in many bird species including our national symbol, the Bald Eagle. Because of Carson's warning, we were alerted to potential large-scale losses of birds, and perhaps other animals, by this chemical. We became conscious of the possible effects of persistent pesticides in the environment.
The "silent spring" that Dr. John Terborgh warns us about has different causes and acts much more insidiously. It has crept up slowly on wildlife researchers, managers, and birdwatchers. To the casual observer the change may not be obvious until he or she is asked how the numbers of Ovenbirds, or any of several other bird species, today compares with numbers 10 or 20 years ago.
The numbers of many songbird species are declining, some at alarming rates (Table 1). For many species these population declines are not restricted to Illinois, but are widespread throughout eastern North America. Their population declines require the cooperative attention of land managers and owners if efforts to conserve these species are to succeed.
|Table 1. Estimated population trends for selected forest and grassland bird species in Illinois. Estimates are based on unpublished data from the United States Fish and Wildlife Services Breeding Bird Survey (1966-1991).|
|Forest Species||% Change||Grassland Species||% Change|
|Acadian Flycatcher||-66.7||Western Meadowlark||-86.1|
|Yellow-billed Cuckoo||-56.8||Grasshopper Sparrow||-85.4|
|Ruby-throated Hummingbird||-48.2||Savannah Sparrow||-63.0|
|Eastern Wood-pewee||-36.5||Eastern Meadowlark||-61.0|
|Red-eyed Vireo||-36.5||Field Sparrow||-57.9|
|Great Crested Flycatcher||-34.9||Northern Bobwhite||-56.8|
Loss of habitat is usually considered to be the major factor contributing to wildlife population declines and is generally considered to be the greatest threat to present day wildlife populations.3 If wildlife has no place to raise young successfully, feed, and survive hardships, their populations cannot be maintained. For several species of birds, habitat loss is widely suspected to be a major factor behind current population declines.
Yet, the simple loss of habitat does not explain the entire problem. There are currently more acres of forest in Illinois than at any time since at least 1924.4,5 Despite this increase in forest acreage, populations of many species of forest birds continue to decline (Table 1). For grassland bird species, the Conservation Reserve Program has resulted in the setting aside of more than 600,000 acres of highly erodible agricultural land in Illinois since 1985, much of it planted to grassland habitat. Still, populations of many species of grassland birds have continued to decline over the last 10 years (Bobolinks as high as 20% per year)6 - including the very time these habitat increases have been occurring. Clearly, the problem is more than just the acreage of habitat available.
Contemporary wildlife research now shows that many species of forest and grassland birds require large blocks of habitat, avoid habitat edges, or do not nest successfully near edges. Populations of these species generally do poorly in areas where habitat is broken into small, isolated blocks, a process called "habitat fragmentation" (Figure 1). These birds are often called "area-sensitive" species. The results of this research indicate that new perspectives on land management are warranted.
Figure 1. Forest fragmentation and loss in Saline and Gallatin counties, southern Illinois. Prior to 1820 both of these counties were completely forested. By 1980 most of the forest habitat in these counties had been converted to agriculture and other purposes. Instead of one large contiguous forest area, the landscape now consists of several isolated, mostly small, forested blocks. Forest has survived best in the Shawnee Hills along the southern part of the counties and in the lowlands along the Wabash and Ohio rivers (Figure adapted from Iverson and Joselyn65).