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Transcript
Grassland Birds: An Overview of Threats
and Recommended Management Strategies
Peter D. Vickery
James R. Herkert
Fritz L. Knopf
Janet Ruth
Cherry E. Keller
Abstract—Grassland ecosystems are dependent on periodic disturbance for habitat maintenance. Historically, grazing by native
herbivores and prairie fires were the agents principally responsible
for maintaining grassland areas. However, elimination of native
herbivores, widespread fire suppression, and conversion for agriculture have greatly altered grasslands in the United States and
Canada. Because of these landscape changes, many grassland birds
are increasingly dependent on land managers for habitat creation,
maintenance, and health. Grazing, prescribed burning, and mowing/haying are the most frequently used, and versatile, grassland
management techniques. Grassland birds prefer a wide range of
grass heights and densities, with some species preferring short
sparse vegetation, and others preferring taller, more dense vegetation. Due to differences in species habitat preferences and regional
differences in soils and floristics, the responses of individual grassland species to specific grassland management practices can be
variable and often are regionally dependent. As a result, management of grassland areas is best directed toward the creation of a
mosaic of grassland habitat types. This habitat mosaic is probably
best maintained through some type of rotational management
system in which sections of large grassland areas receive management on a regular schedule. Such a rotational system would provide
a variety of habitat types in every year, would ensure the availability of suitable habitat for birds at either end of the grassland
management spectrum, and also would provide habitat for birds
whose preferences lie between these extremes.
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-toeast 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.
Historical Perspectives ___________
In: Bonney, Rick; Pashley, David N.; Cooper, Robert J.; Niles, Larry,
eds. 2000. Strategies for bird conservation: The Partners in Flight planning process; Proceedings of the 3rd Partners in Flight Workshop; 1995
October 1-5; Cape May, NJ. Proceedings RMRS-P-16. Ogden, UT: U.S.
Department of Agriculture, Forest Service, Rocky Mountain Research
Station.
Peter D. Vickery, Center for Biological Conservation, Massachusetts
Audubon Society, Lincoln, MA 01773. James R. Herkert, Illinois Endangered
Species Protection Board, Springfield, IL 62701. Fritz L. Knopf and Janet
Ruth, U.S. Geological Survey, Fort Collins, CO 80525. Cherry E. Keller, U.S.
Geological Survey, Annapolis, MD 21401.
74
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.
USDA Forest Service Proceedings RMRS-P-16. 2000
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 require—and benefit—from intensive grazing, which
prevents this habitat from growing into a shrub- or forbdominated 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.
Eastern Grasslands
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).
USDA Forest Service Proceedings RMRS-P-16. 2000
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
75
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.
Recommendations
• 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.
• 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.
Suggested Reading ______________
The Ecology and Conservation of Grassland Birds of the
Western Hemisphere, published in 1999 by the Cooper Ornithological Society (Studies in Avian Biology Series), provides
the most comprehensive background in grassland bird ecology and conservation for North and South America.
Recommendations
• 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 areasensitive, 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.
76
Shortgrass Prairie
Knopf, F. L. 1994. Avian assemblages on altered grasslands. Studies in Avian Biology, No. 15.
Samson, F. B. and F. L. Knopf. 1994. Prairie conservation in North
America. Bioscience 44:418-421.
Samson, F. B. and F. L. Knopf. 1996. Prairie conservation: Preserving North America’s most endangered ecosystem. Island Press,
Covelo, CA.
USDA Forest Service Proceedings RMRS-P-16. 2000
Tallgrass Prairie
Askins, R. A. 1993. Population trends in grassland, shrubland, and
forest birds in eastern North America. Current Ornithology 11:1-34.
Bock, C. E. and J. H. Bock. 1988. Grassland birds in southeastern
Arizona: Impacts of fire, grazing, and alien vegetation. Pages
43-58 in Goriup, P. D. (ed.), Ecology and conservation of grassland birds. Tech. Publ. No. 7. International Council For Bird
Preservation.
Bollinger, E. K. 1991. Conservation of grassland birds in agricultural areas. Pages 279-287 in Decker, D. J., M. E. Krasny, G. R.
Goff, C. R. Smith, and D. W. Gross (eds.), Challenges in the
conservation of biological resources: A practitioner’s guide.
Westview Press, Boulder, CO.
Dobkin, D. S. 1992. Neotropical migrant landbirds in the Northern
Rockies and Great Plains: A handbook for conservation and management. Publ. No. R1-93-34. USDA Forest Service, Missoula, MT.
Herkert, J. R. 1994. The influence of habitat fragmentation on
Midwestern grassland bird communities. Ecological Applications
4:461-471.
Herkert, J. R., R. E. Szafoni, V. M. Kleen, and J. E. Schwegman.
1993. Habitat establishment, enhancement, and management
for forest and grassland birds in Illinois. Tech. Publ. No. 1. Illinois
Department of Conservation, Natural Heritage. 20 pp.
Hunter, W. C. 1990. Handbook for nongame bird management and
monitoring in the southeast region. U.S. Fish and Wildlife Service, Atlanta, GA. 198 pp.
USDA Forest Service Proceedings RMRS-P-16. 2000
Skinner, R. M., T. S. Baskett, and M. D. Blenden. 1984. Bird habitat
on Missouri prairies. Terrestrial Series No. 14. Missouri Department of Conservation, Jefferson City. 42 pp.
Eastern Grasslands
Askins R. A. 1996. History and conservation of grassland birds in
the northeastern United States. In Vickery, P. D., P. Dunwiddie,
and C. Griffin, (eds.), The ecology and conservation of grasslands
and heathlands in the northeastern United States. Massachusetts Audubon Society, Lincoln.
Vickery, P. D. 1992. A regional analysis of endangered, threatened,
and special concern birds in the northeastern United States.
Pages 1-10 in DeGraff, R. M. (ed.), Transactions of the Northeast
section of the Wildlife Society 48. Amherst, MA.
Vickery, P. D., M. L. Hunter, Jr., and S. M. Melvin. 1994. Effects of
habitat area on the distribution of grassland birds in Maine.
Conservation Biology 8:1087-1097.
Vickery, P. D. and J. R. Herkert (eds.) 1999. Ecology and conservation of grassland birds of the Western Hemisphere; proceedings
of a conference; 1995 October; Tulsa, OK. Studies in Avian
Biology No. 19. Cooper Ornithological Society, c/o Western Foundation of Vertebrate Zoology, Camarillo, CA. 299 pp.
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