|Grassland Birds: An Overview of
Threats and Recommended Management Strategies
Peter D. Vickery1, James R. Herkert2, Fritz L. Knopf3, Janet Ruth3, and Cherry E. Keller4
Grasslands are ecosystems that have evolved with frequent disturbances. Historically, the agents principally responsible for maintaining grassland habitats were drought, grazing by native herbivores, and fire. Generally, a west-to-east disturbance continuum exists in North America; grazing has been and continues to be the primary ecological force on western shortgrass areas, whereas fire was the prominent disturbance factor to the east.
Grassland habitats are occupied by a small number of uniquely adapted bird species. Most species select a particular suite of habitat features. Mountain Plovers (Charadrius montanus) and Horned Larks (Eremophila alpestris), for example, require short, sparse vegetation, whereas Henslow's Sparrows (Ammodramus henslowii) and Sedge Wrens (Cistothorus platensis) require taller, more dense vegetation. Some grassland species are more abundant on recently burned or grazed grasslands, whereas others are more abundant on undisturbed or idle grasslands. Also, habitat preferences can vary throughout the range of a species. Because of these differences in habitat preferences, and because of regional differences in soils and floristics, the response of a particular species to a specific grassland management prescription may be variable.
During the past quarter century, grassland birds have experienced steeper, more consistent, and more widespread population declines than any other avian guild in North America. While some grassland species are Neotropical migrants, most are short-distance migrants that winter primarily in the southern U.S. and northern Mexico. Thus, opportunities for conservation exist on both breeding and wintering grounds. The winter ecology of most grassland birds is poorly known; winter survivorship could be a critically important factor in the long-term declines that some species have experienced. Therefore, grassland bird conservation programs within North America need to address both breeding and wintering ecology.
This discussion is focused on threats to grasslands east of the Rocky Mountains; grasslands farther west (e.g., California grasslands, palouse prairie) are beyond the scope of this presentation.
Because most grassland ecosystems have been altered profoundly within the past two centuries, many are now among North America's most endangered ecosystems. Effective grassland bird conservation and management must recognize the historical dynamics under which these habitats have evolved and, where feasible, incorporate the ecological processes that have generated and maintained these distinctive grassland biotas.
Western Shortgrass Prairie
Western prairies are characterized by short (<10 cm), warm-season grasses on shallow, poor soils, and occur from the Rocky Mountains eastward to the central Great Plains. Predominant ecological forces on these prairies have been seasonal drought and intensive grazing by native herbivores including prairie dogs, bison, and pronghorn. Dominant grasses include buffalo grass (Buchloe dactyloides), blue gramma (Bouteloua gracilis), and some wheatgrasses; these grasses requireand benefitfrom intensive grazing, which prevents this habitat from growing into a shrub- or forb-dominated landscape.
Midwestern Tallgrass Prairie
Historically, the tallgrass prairie extended from the central Great Plains to the Midwest, and was characterized by taller grasses, deeper soils, and more precipitation than shortgrass areas. These ecosystems were maintained primarily by prairie fires, set both by lightning and by Native Americans. Characteristic species include big bluestem (Andropogon gerardii), little bluestem (Schizachyrium scoparium), Indian grass (Sorghastrum nutans), and cord grass (Spartina pectinata).
During the past 150 years, agricultural grasslands (hayfields and pastures) have replaced the native prairies and are currently the dominant grassland habitat throughout much of the tallgrass region. In fact, the decline of tallgrass prairie, estimated to be 88% to 99%, exceeds that reported for any other major ecosystem in North America. However, the acute population declines that could have resulted from such extensive loss of native tallgrass habitat have been buffered somewhat by expansion of agricultural grasslands into previously forested areas in the eastern U.S., which has allowed many species of grassland birds to expand their historic ranges.
Native grasslands in the East include sites from Maine to Florida and west to coastal Texas. These grasslands range from poor, xeric soils in New England, to mountain balds in parts of the Appalachian range, to wet prairie in Florida, to transitional grasslands emerging from tallgrass prairie and longleaf pine ecosystems into coastal marshes in Texas. Historically, eastern grasslands resulted primarily from natural and Native American-induced fires, usually in xeric habitats and poor soils. Some of these sites were extensive, for example the Hempstead Plains on Long Island, New York, covered more than 24,000 ha. In Florida, lightning ignitions and hydrology were primarily responsible for maintaining prairie habitat. These prairies provide important breeding habitat for grassland birds, e.g., Florida Grasshopper Sparrow (Ammodramus savannarum) and Bachman's Sparrow (Aimophila aestivalis); they also provide critical wintering habitat for many species. Primary grass species include little bluestem, poverty grass (Danthonia spicata), and in Florida, wiregrass (Aristida beyrichiana).
MAJOR THREATS TO GRASSLAND ECOSYSTEMS AND RECOMMENDATIONS FOR GRASSLAND HABITAT CONSERVATION
Three issues are critical to preservation of grassland habitats throughout North America.
First, all native temperate grasslands have experienced major, sometimes profound, losses of habitat from agriculture, range management, and urban development. In addition, habitat fragmentation and degradation have been severe. Habitat loss is most frequently viewed when grassland is converted to cropland or other uses, but loss of habitat also includes more subtle degradation such as unnatural grazing regimes, planting of exotic grasses, and the succession of grassland to shrubland. These less-obvious changes have resulted in the extirpations of many local bird populations. The highest priority should therefore be preservation and appropriate management of the largest tracts of existing native grasslands to avoid habitat fragmentation and degradation.
Second, conservation planning at the regional level is important for all grassland systems; only by cooperatively managing all grassland habitats on a landscape or regional level can the complete range of grassland bird habitats be addressed. Conservation goals should not include management for the greatest number of grassland bird species at each grassland site; such management is neither necessary nor practical, and is likely to be counterproductive to overall conservation goals. Individual sites are usually best suited for management for a particular subset of grassland birds, e.g., Le Conte's Sparrows (Ammodramus leconteii) and Sedge Wrens in sedge meadow habitats, within the context of larger regional goals.
Third, effective management of grassland landscapes will require involvement of a diverse group of natural resource professionals such as range managers, game and nongame biologists, soil conservationists, agronomists, farmers, and ranchers. In many areas, grassland management historically has emphasized soil conservation, and it will be important to combine the goals of avian habitat conservation with soil conservation and agricultural goals.
THREATS TO SHORTGRASS PRAIRIE AND MANAGEMENT RECOMMENDATIONS
Shortgrass prairies evolved under intense grazing by prairie dogs and bison. Consequently the shortgrass prairie bird fauna evolved to select a variety of different site characteristics, created within landscapes receiving grazing pressure ranging from light to severe. Unfortunately, current range management practices strive to graze rangelands uniformly. These practices remove or inhibit heterogeneous grazing impacts across landscapes, and do not favor the specific habitat requirements of many species. For example, Mountain Plovers require heavily grazed sites for breeding, but Lark Buntings (Calamospiza melanocorys) prefer denser vegetation. Thus, moderate grazing everywhere is unlikely to result in suitable habitat for either species. In many locales, insufficient grazing has led to the invasion of grasslands by shrubs and forbs. Rather than opposing grazing as a management tool in all grasslands, conservation groups should encourage grazing that imitates natural conditions as closely as possible.
Also, native grazers, e. g., bison and prairie dogs, should be encouraged; by cropping vegetation more closely than domestic livestock, native grazers create patchier, more heterogeneous landscapes. Prairie dog populations are still aggressively poisoned, and presently occupy only 2% of their native range.
In addition, planting of non-native grass species and invasion by exotic species also results in grassland degradation.
*Implement practices that restore the inherent heterogeneity of native grazing communities, such as encouraging larger grazing allotments, and grazing more intensively in certain areas.
*Discontinue prairie dog control on public lands.
*Encourage conversion of livestock operations to bison grazing to create more varied habitat mosaics preferred by some grassland specialists, e.g., Mountain Plover.
*Encourage use of seedings native to the shortgrass region. Such grasses require intensive grazing to maintain vigor.
THREATS TO TALLGRASS PRAIRIE AND EASTERN GRASSLANDS AND MANAGEMENT RECOMMENDATIONS
Habitat loss from human development and forest succession, a consequence of fire suppression as well as human development, has led to severely fragmented and isolated grasslands in the East and Midwest. Fire suppression and the resultant encroachment of woody vegetation are major management problems for tallgrass and eastern grasslands, influencing grassland bird distribution patterns and nesting success. In addition, early- and mid-season cutting of agricultural grasslands has catastrophic impacts on nesting success of birds using these habitats. In tallgrass prairie, agricultural conversion of pastures and hayfields into rowcrops also is a major threat to grassland birds. In Florida, loss of native prairie has resulted from agricultural conversion to tame pastures and citrus groves.
*Identify and protect large grassland sites (>100 ha). Avoid further loss and fragmentation of existing grassland habitats. The protection and proper management of these habitats, especially those used by area-sensitive, rare, or declining species, is the most effective means of conserving grassland bird populations.
*Actively manage grasslands to control woody encroachment through the use of fire, grazing, and mowing.
*Within a regional context, protect and manage enough sites to provide sufficient diversity of grassland habitats ranging from wet sedge meadows to dry, xeric grasslands.
*At the local level, large grassland sites should usually be managed to include a mosaic of management prescriptions, including both recently disturbed (i.e. burned, grazed, mowed) and undisturbed grassland areas. This mosaic is probably best maintained through a rotational management system, e.g., on a site with a five year rotation, approximately 20% should be burned each year.
*Mowing and haying on public grassland areas should not be conducted during the height of the breeding season (May 1 - July 15). On private lands, efforts should be made to provide incentives for deferred mowing.
IMPORTANT RESEARCH NEEDS
As a result of extensive research during the past 20 years, habitat requirements for many species of grassland birds during the breeding season are generally well known. These recommendations highlight important research needs where information is nonexistent or inadequate.
*Improve knowledge of the winter distribution, winter ecology, and winter habitat requirements of many grassland birds. In general, information is poor or completely lacking. This is the primary research need for grassland bird conservation.
*Monitor and document the efficacy of grassland management practices, including measuring the long-term responses of birds to management.
*Develop cost-effective methods of monitoring reproductive success.
*Develop techniques for grassland creation, restoration, and enhancement which benefit grassland birds. In the past, development of restoration techniques has focused on plant ecology, failing to recognize the needs and contributions of birds and other wildlife to the restoration of a functioning ecosystem.
The Ecology and Conservation of Grassland Birds in the Western Hemisphere, to be published in 1999 by the Cooper Ornithological Society (Studies in Avian Biology Series), will provide the most comprehensive background in grassland bird ecology and conservation for North and South America.
1 Center for Biological Conservation
2 Illinois Endangered Species Protection
3 U. S. Geological Survey
4 U. S. Geological Survey