Northern Prairie Wildlife Research Center
The Cranes
Status Survey and Conservation Action Plan
Threats: Habitat Loss and Degradation
Habitat loss and degradation is the most important class of threats to the world’s
cranes (Archibald et al. 1981, Harris 1994a). Declines in habitat availability
and quality affect the distribution, movement, and breeding success of cranes,
and involve all habitat types—breeding grounds, migration stopover points and
staging areas, wintering grounds, resident habitats, and roosting areas. Major
forms of habitat loss and degradation affecting cranes include the following:
Conversion of wetlands
Conversion of wetlands for agricultural and non-agricultural purposes (including
urban, commercial, and recreational development, oil exploration, and road construction)
is the most significant factor affecting cranes and their habitats around the
world (see Harris 1994a, Hussain 1994 Zhang 1994),. The process of converting
wetlands usually involves extensive clearing of natural vegetation, draining and
other alterations of hydrological processes, burning, cultivation, and other activities
that render former crane habitats unsuitable for nesting, feeding, roosting, and
stopping during migration. All of the wetland-dependent crane species have been
affected by this process to a greater or lesser degree.
Over-exploitation of wetland resources
Cranes and people have long coexisted successfully in many wetland areas, but
increasing pressures related to human population growth and economic constraints
have in some areas overtaxed wetland ecosystems and the resources they provide.
Overexploitation of the plant, animal (especially fish), and water resources of
these wetlands can have negative impacts upon cranes and other species (e.g.,
Ma and Su 1991,Wang Q. 1991, Harris 1992a, Su 1992). This problem is perhaps greatest
in areas of Africa and Asia where population pressures are greatest and wetland
resources are used to meet expanding subsistence needs.
Conversion of grasslands
Agricultural expansion into grasslands has also appropriated crane breeding and
foraging habitat, especially in sub-Saharan Africa, parts of South Africa, the
steppe regions of Eurasia, and savannas and prairies in North America (Vernon
et al. 1992, Allan and Nuttall 1995, Winter et al. 1995, Allen 1952).
Changes in agricultural land use
In areas where cranes have adapted to feeding and breeding in traditional agricultural
lands, changes in the choice of crops, fallow and rotation schedules, cropping
patterns, field preparation, and cultivation and harvesting practices can have
subtle but important effects (e.g., Bishop 1991, Khachar et al. 1991, Mafabi 1991,
Winter et al. 1995).
Other agricultural impacts
In many landscapes, agricultural practices in upland areas have brought about
increased rates of soil erosion. The resulting sediment loads alter the hydrological
processes and vegetation in downslope wetlands. This has had direct impacts on
crane habitat in Japan, China, Australia, and other areas (Archibald 1987, Li
F. and Li M. 1991, A. Haffenden pers. comm.). Intensified livestock grazing has
resulted in the degradation of crane habitat through disturbance, alterations
in wetland hydrology, and trampling of wetland vegetation (including crane nests
and cover vegetation) (e.g., Duc 1991, Gole 1991a, Urban and Gichuki 1991, Mustafa
and Durbunde 1992). Increased use of pesticides and fertilizers in agricultural
systems impacts cranes directly through exposure and ingestion and indirectly
through eutrophication and contamination of wetlands, accumulation in food sources,
and changes in the trophic structure of ecosystems (Gole 1989b, 1991b; Kawamura
1991; Muralidharan 1992).
Dams and water diversion
Construction of dams and irrigation systems has profoundly altered the hydrological
regimes and stream channel characteristics of river systems and associated wetlands
critical to many crane species. Dam construction and water diversion has already
affected large river and wetland systems such as the Senegal, Zambezi, Lake Chad,
Danube, and Platte, as well as many smaller streams and wetlands. Other major
river systems that are critically important for cranes—including the Mekong, Yangtze,
and Amur Rivers, the Okavango system, and the Sudd wetlands—are currently under
threat from major dam and diversion projects (Su 1992, Pearce 1993, Smirenski
1995, Topping 1995, Garba in press).
Urban expansion and land development
The development of wetlands and other crane habitat for human habitation has been
an important factor in the restriction of the ranges of several species, including
the Whooping Crane, Red-crowned Crane, and White-naped Crane. Land subdivision
and development also lead to habitat fragmentation. As large properties are subdivided,
previously expansive fields, pastures, meadows, and wetlands are reduced in size
and often fenced. The effective reduction in foraging range can pose problems
to cranes that have traditionally taken advantage of these lands, and especially
to those with flightless young. Unable to follow their parents to food sources,
the young can become tangled in fences or even starve within these enclosed areas.
Over the last several decades, Red-crowned Cranes and Brolgas in particular have
been affected by this form of fragmentation (Archibald 1987, P. Du Guesclin pers.
comm.). Urban expansion and other forms of land development also affect cranes
by increasing the exposure of cranes to human disturbance.
Deforestation
Because few crane species depend heavily upon forested habitats, deforestation
does not usually pose a serious direct threat to cranes. However, because deforestation
has long-term and far-reaching impacts upon watershed hydrology, and hence on
wetlands, deforestation can be considered a significant indirect threat (Archibald
1987, Li F. and Li M. 1991). The problem is perhaps most serious in the savannahs
of Africa and in key watersheds of China and other parts of East Asia. In areas
where deforestation contributes to disrupted flooding cycles, it feeds the demand
for dams and other flood control structures.
For the cranes that either require or tolerate trees in their habitats, deforestation
poses a more direct threat. Deforestation may reduce the security of roost sites
for the Crowned Cranes (R. Beilfuss pers. comm.). For some species in certain
portions of their range—for example, Red-crowned Cranes in Hokkaido—deforestation
has also had direct impacts (Archibald 1987). Deforestation is of increasing
concern in previously unexploited crane habitats that are now subject to development.
This includes, for example, the breeding grounds of the Hooded Crane in the
boreal forests of eastern Russia (Smirenski 1989).
Afforestation
Afforestation threatens both grassland and wetland crane habitats. Conversion
of grasslands to tree (usually pine) plantations impairs visibility, reduces the
availability of nesting and foraging areas, and subjects nearby wetlands to reduced
run-off and desiccation. These impacts can leave the wetland-upland habitat complex
unsuitable for cranes. Afforestation has affected extensive areas of Blue and
Wattled Crane habitat in South Africa (Johnson 1992a, 1992b; Tarboton 1984, 1992b;
Allan 1994). Mississippi Sandhill Cranes and Hooded Cranes are also affected by
afforestation (Smith and Valentine 1987, Valentine 1987, Kawamura 1991).
Other changes in vegetation
In many parts of the world, the quality of crane habitat is threatened by changes
in plant community composition and processes. These changes are often induced,
in turn, by broad anthropogenic changes in fire, grazing, and hydrological regimes.
A well known example is that of the Platte River in North America, where dams
in the upper stretches of the river have moderated the flooding cycle, allowing
woody vegetation to take hold in the formerly open riparian zone (Currier et al.
1985, Currier 1991). This has reduced the availability of prime roosting sites
for migrating Sandhill and Whooping Cranes.
Marsh and shoreline erosion
Erosion of coastal marshes and shorelines is a major threat in several key crane
habitats. In these areas, the dredging of channels and the subsequent wave action
of boat and barge traffic gradually erodes shoreline soils and vegetation. This
is a significant problem on the wintering grounds of the Whooping Crane at Aransas
National Wildlife Refuge in the United States (Halpern 1992, Sherrod and Medina
1992, Zang et al. 1993).
Pollution and environmental contamination
Crane habitats around the world are threatened by many forms of household, agricultural,
and industrial pollution. Some pollutants, such as chemicals and organic wastes,
constitute long-term threats to habitat (primarily water) quality, with attendant
effects on crane physiology and reproductive success and on ecological conditions
(including crane food sources) within their wetland habitats (Wang Q. 1991, Kawamura
1991, Zhang 1994). Pollution also poses an acute threat to cranes through the
potential for catastrophic spills and other accidents. The presence of chemical
transportation and production facilities along coastlines and major rivers makes
wetlands in these areas especially vulnerable to such accidents. For example,
barges loaded with benzene, xylene, and other toxic substances traverse the Gulf
Intracoastal Waterway near the Aransas National Wildlife Refuge on a daily basis;
even one spill in this area could have a devastating impact on the Whooping Cranes
and their habitat (Lewis et al. 1992a, Lewis 1995b).
Oil development
Oil exploration, drilling, extraction, transport, and processing constitute a
special class of pollution threats (e.g., Liu et al. 1991, Su 1992, Kanai et al.
1993, Dai and Qi 1994, Ojok in press). Many important crane habitats around the
world contain known or suspected oil reserves. Oil development activities take
place within the Aransas National Wildlife Refuge, but are prohibited when the
Whooping Cranes are present. An accident at other times of the year could have
long-term impacts on environmental quality throughout the refuge. Chronic low
level discharges related to oil production and transportation are also of concern
(Robertson et al. 1993, Lewis 1995b).
Oil has been discovered in and near the breeding grounds of Siberian Cranes
in eastern Russia. Drilling rights have been granted on the Yamal Peninsula
near the Kunovat Nature Reserve, where the small Central population of Siberian
Cranes breeds (Archibald 1990). Oil exploration and oil field development have
taken place within several key crane reserves in China (Dalainor, Panjin, Shuangtaizi,
and Poyang Lake Nature Reserves) (Liu et al. 1991, Su 1992).
Collision with utility lines
Accidental collision with utility lines has become a significant source of mortality
in several species of cranes. This has been most carefully studied and documented
in the case of the Sandhill Crane in western North America, but accidental crippling
and mortality have been studied or reported in several other species (e.g., Kyu
and Oesting 1981, Archibald 1987, Brown et al. 1987, Howard et al. 1987, Neumann
1987, Goldstraw and Du Guesclin 1991, Masatomi 1991, Faanes and Johnson 1992,
Morkill and Anderson 1992, Ward and Anderson 1992). Although collision with utility
lines does not pose a major threat to the existence of the more abundant species
(such as the Sandhill Crane), it is a significant consideration in efforts to
maintain and recover the more threatened species, especially the Whooping and
Red-crowned Cranes (Akiyama 1981, Brown et al. 1987, Kuyt 1987, Masatomi 1991,
Faanes and Johnson 1992, Lewis et al. 1992b). The threat, however, can be successfully
mitigated by moving, removing, burying, or marking the utility lines (Morkill
and Anderson 1993, Alonso et al. 1994, Brown and Drewien 1995). Other structures
on the landscape, such as fences, can also result in accidental injury and death
in cranes (White 1987, Allen and Ramirez 1990, Filmer and Holtshausen 1992).
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