Northern Prairie Wildlife Research Center
Status Survey and Conservation Action Plan
Threats: Biological Factors
Human activities have under some circumstances increased predation pressures on
cranes. For example, crow and raven predation of crane nests has increased in
areas where garbage dumps have led to high corvid numbers and human disturbance
frightens nesting cranes (Archibald 1987). Such changes in predation dynamics
often involve other changes in habitat conditions. In particular, alterations
of the natural hydrologic regimes of wetlands can result in generally or periodically
drier habitats, opening opportunities for predators. Drier habitat conditions
have magnified the effects of recent coyote colonization in the range of the Mississippi
Sandhill Crane (S. Hereford pers. comm.). In several cases, “predator release”—the
halting of systematic predator persecution—has had impacts on cranes in protected
areas (Littlefield and Thompson 1987).
The introduction or unanticipated spread of exotic species due to human activity
has occasionally affected cranes and their habitats. Throughout the world, both
wetland and grassland crane habitats have been altered by the spread of exotic
species. Although most of these alterations have had only minor effects on cranes,
exotic plant invasions have had detrimental effects in many wetland areas around
the world. Instances of animal invasions affecting cranes are rare, but not unknown.
For example, minks (Mustela vison) that have escaped from captivity have
established themselves in the marshlands of Hokkaido, and may be important predators
of the resident Red-crowned Crane (Archibald 1987; H. Masatomi pers. comm.).
Genetic and demographic problems of small populations
In several cases, the decline and fragmentation of crane populations has rendered
them susceptible to genetic and demographic problems associated with small populations,
including decreased resistance to disease, skewed sex ratios and age distributions,
and susceptibility to storms, disease outbreaks, and other catastrophic events.
These threats are most serious for the Whooping Crane, the Mississippi and Cuban
Sandhill Cranes, and the Western and Central populations of the Siberian Crane.
Wild and captive cranes are vulnerable to a variety of infectious and parasitic
diseases, including salmonellosis, avian tuberculosis, avian cholera, inclusion
body disease of cranes (IBDC), crane herpes virus, eastern equine encephalitis,
coccidiosis, avian pox, and Newcastle’s disease (Docherty and Romaine 1983, Carpenter
and Derrickson 1987, Dein and Langenberg and Dein 1992). Relatively little research,
however, has been done on the incidence of these and other diseases; most of the
available information comes from studies of Sandhill and Whooping Cranes in North
America or from studies of captive cranes (Forrester et al. 1976, Carpenter and
Derrickson 1987, Mirande 1991). In general, pathogens seldom pose a serious threat
to wild crane populations, and then only during periods of high population density
or environmental stress, or when there is contact with domestic poultry (Carpenter
and Derrickson 1987, J. Langenberg pers. comm.). The risk of disease outbreaks
may be increasing among artificially concentrated populations, including the Sandhill,
White-naped, Hooded, and Red-crowned Cranes (Archibald et al. 1981, Eguchi et
al. 1991, Kawamura 1991). This potential is likely to increase further as the
loss of habitat continues and cranes are forced to concentrate on smaller breeding,
staging, and wintering areas.
Disease among captive cranes is also of concern from a conservation perspective.
Disease outbreaks can set back captive propagation programs for the endangered
species and subspecies. In addition to the loss of the individual birds themselves,
outbreaks can disrupt long-term plans for sound genetic management of captive
populations. Captive management and husbandry techniques have reduced these
risks, but constant monitoring is essential to minimize the potential for outbreaks.
This is especially important as release programs expand and increase the possibility
of introducing disease into wild populations.
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