Habitat Fragmentation
and Desert Ungulates
--From Nabhan, Gary Paul and Andrew R. Holdsworth.1998.
State of the Sonoran Desert Biome: Uniqueness, Biodiversity,
Threats and the Adequacy of Protection in the Sonoran Bioregion.
Sponsored by The Wildlands Project.
Desert bighom sheep and Sonoran pronghom antelope once inhabited
most of the Sonoran Desert bioregion. The pronghom and bighorn
of the Pinacate area of northwestem Sonora were once so plentiful
as to seem limitless. However, the decades following the publication
of William T. Homaday's Campfires on Desert and Lava (1908)
brought more and more hunting expeditions to the Pinecate
area in search of desert trophies. Now the Sonoran pronghom
antelope (Antilocapra americana sonoriensis) population is
extremely small with only 350 counted in 1993 (Arizona Game
and Fish 1995). The desert bighom (Ovis canadensis mexicana,
O.c. califomia, O.c. nelsonii, O.c. Cremnobates) that once
may have numbered several hundred thousand in the deserts
of the southwestem United States numbered about 25,000 in
1991 (Krausman 1997). The total numbers of bighorn have increased
some, but the remaining populations are often small and isolated
from each other. Only 7 of 59 populations in Arizona and only
11 of 77 populations in California have more than 100 individuals
(Krausman 1997). While there is some dispute over the minimum
population size necessary to maintain a population of desert
bighorn, the noted wildlife biologist Paul Krausman (1997)
aptly said "it appears that most populations of desert
bighorn sheep are at, below, or are approaching what many
researchers would consider minimum viable population levels."
The particular suite of causes for the small populations
of desert bighorn depends on the area, but overall habitat
fragmentation due to agriculture, livestock grazing (including
its associated spread of disease to bighorns), road building,
housing developments, and fire suppression is the major problem.
Managing bighorn on a landscape scale so as to protect the
connectivity between mountain ranges that are used for lambing
and foraging is a critical addition to the traditional management
that has occurred almost entirely at the local scale. While
overhunting could be a problem for some small populations
of desert bighorn, the extraordinarily high trophy value of
desert bighorn could be the best incentive for their protection.
For example, their trophy value is driving efforts to sustainably
hunt the bighorn population in the Sierra Bacha of Sonora.
Sources
William T. Homaday's Campfires on Desert and Lava (1908)
(Arizona Game and Fish 1995).
(Krausman 1997).
--Paul Krausman with quote by V.C. Bleich et al. 1990
The Influence of Scale on the Management of Desert Bighorn
Sheep (1997)
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