Sheep Populations Increased Significantly Following Removal of Domestic Cattle
by Jeff Aardahl, Senior CA Representative, Defenders of Wildlife
Desert bighorn sheep occupy approximately 50 ranges in the Mojave Desert (Figure 1) with a total estimated population of 3,778 individuals, with populations in each range varying in size from less than 25 to 400. Most of these ranges are in public ownership, located within the California Desert Conservation Area (CDCA) and managed by the Bureau of Land Management (BLM).
Figure 1. Map of desert bighorn sheep populations in southeastern California.
Source: California Department of Fish and Wildlife.
Four mountain ranges occupied by desert bighorn were selected to show the relationship between bighorn sheep population, size and trend, and presence or absence of cattle. Those four ranges are shown in Table 1 along with desert bighorn population estimate, grazing allotment, and the presence or absence of cattle.
Table 1. Desert bighorn sheep ranges, population estimates, grazing allotments and number of cattle.
Figure 2. Cattle grazing allotments that overlap with bighorn sheep populations.
The Afton Canyon, Ord Mountain and Lazy Daisy allotments overlapping mountain ranges occupied by desert bighorn sheep, are shown on the this map above.
The CDCA was designated in 1976 under Section 601 of the Federal Land Policy and Management Act “...to provide for the immediate and future protection and administration of the public lands in the California desert within the framework of a program of multiple use and sustained yield, and the maintenance of environmental quality.” The CDCA Plan was approved in 1980 by the BLM State Director, BLM Director, and the Secretary of the Interior. It included numerous prescriptions to avoid or minimize impacts to desert bighorn sheep from livestock grazing.
Livestock grazing allotments in fair or poor condition due to excessive livestock grazing had livestock forage allocations (Animal Unit Months or AUMs) reduced by 25 and 50 percent, respectively, which reduced the number of livestock using the allotments. The CDCA Plan also included numerous provisions for conservation of bighorn sheep and its habitat throughout the CDCA:
- No domestic sheep grazing in desert bighorn sheep habitats;
- No AUM allocation for livestock within Crucial Desert Bighorn Sheep Habitat;
- Eliminate livestock grazing within desert bighorn sheep ranges south of Interstate 40;
- Fence water sources to eliminate competition and stress with desert bighorn sheep;
- Initiate a stewardship program under the guidance of the multiple use advisory council to focus on management;
- Begin a major research program on impacts;
- A specific effort should be made to mitigate the effect of these decisions on any livestock operator who’s (sic) operation will suffer major detriment.
The CDCA Plan clarified these final decisions in the Wildlife Element:
BLM will study the effects of livestock grazing on desert bighorn sheep in the eastern Mojave Desert (i.e., Kingston, Clark, New York, Providence, Granite, Piute, Woods, Hackberry, Kelso, Old Dad, Ivanpah, and Mescal Mountains, Midhills, and Castle Peaks), to determine 1) the number of bighorn sheep in each mountain range; 2) the health, condition, and population trends in each herd; and 3) the effects of livestock grazing on concentration areas and permanent and seasonal ranges. These studies will be initiated as soon as possible and might require 10 years to complete. If livestock grazing is found to have negative impacts on the bighorn sheep and grazing threatens the health and viability of herds, then changes will be made in grazing allotments so that healthy, viable herds of bighorn can continue to exist in this region.
The CDCA Plan provided a number of additional management prescriptions in the Livestock Grazing Element. However, the Plan had a short life due to numerous amendments approved by BLM between 1981 to 1983 that expanded grazing allotments, designated new allotments, and increased forage allocated to livestock. Those amendments and other relevant information are described below.
Afton Canyon Allotment: Under the CDCA Plan, 120 cattle were authorized to use the 41,825 acre Afton Canyon Allotment during the spring season. In 1983, BLM approved amendment #9 which permitted year-long use and increased the number of cattle from 120 to 171. Forage was allocated to support 15 desert bighorn sheep. The forage allocation for cattle was a six-fold increase over that in the original CDCA Plan.
As part of the mitigation measures to offset impacts to the desert tortoise from the expansion of the Army’s National Training Center at Fort Irwin, the Army acquired the grazing permit for the Afton Canyon Allotment in 2004, relinquished the grazing permit to BLM, and requested that livestock grazing and the allotment be permanently retired. All cattle were removed in 2005 and BLM permanently retired the allotment in 2019. Elimination of the Afton Canyon Allotment had the unintended benefit of a dramatic increase in the desert bighorn sheep population as shown in Table 1.
Ord Mountain Allotment: Under the CDCA Plan, 190 cattle used the 66,503 acre Ord Mountain Allotment yearlong. In 1983, BLM approved CDCA Plan amendment #11 to expand the allotment to 125,308 acres and increased the number of cattle to 302. California Department of Fish and Wildlife (CDFW) staff found no conflict with desert bighorn sheep because the expanded allotment did not overlap with the newly discovered desert bighorn sheep herd in the Newberry Mountains. No desert bighorn sheep were known to occur in the Ord Mountains at that time.
Beginning in 2006, the number of cattle on the allotment was reduced by the rancher from 302 to less than 25 and has remained at that level. This significant reduction resulted in the significant increase in the desert bighorn sheep population as shown in Table 1.
Lazy Daisy Allotment: Under the 1980 CDCA Plan, up to 266 cattle used the 451,617 acre Lazy Daisy Allotment only during the spring season if sufficient annual forbs and grasses were available. No cattle were allowed in the Old Woman Mountains to eliminate conflict with desert bighorn sheep. In 1982, BLM approved amendment #21 that reclassified the allotment from spring season to yearlong use. The amendment also removed the 1980 CDCA Plan decision that excluded cattle from the Old Woman Mountains. It is noteworthy that BLM’s justification for approving the amendment stated, “The [CDCA] Plan’s recommendation to exclude livestock from the [Old Woman Mountains] was based on the fallacious premise that bighorn sheep occupied this area.” The desert bighorn sheep population in the Old Woman Mountains has remained low despite CDFW describing the range as having some of the best habitat for desert bighorn sheep in the central Mojave Desert.
Based on existing information in published literature, wildlife agency reports, and studies, the primary impacts of cattle on desert bighorn sheep include effects of livestock diseases and competition for forage and space. These factors are described in more detail below.
Livestock Diseases: Bluetongue (BT) and Epizootic Hemorrhagic Disease (EHD) are two livestock diseases implicated in depressed desert bighorn sheep populations. BT and EHD are both caused by viruses transmitted from cattle to bighorn sheep by biting gnats of the genus Culicoides, which occur naturally in suitable habitats (wetlands and marshes) in the western U.S., including the Mojave Desert, and are attracted to water polluted with livestock feces and urine. Domestic cattle are a natural reservoir host for these viruses because they remain infected for long periods without suffering deleterious effects.
Desert bighorn sheep populations exposed to BT and EHD are typically depressed due to high lamb mortality. These viruses result in similar clinical characteristics that include lesions in the mouth and digestive system of domestic livestock and wild ruminants, including bighorn sheep. Infected wild ruminants typically exhibit weakness, labored breathing, excessive salivation, difficulty standing, low hanging head and arched back.
Field studies by CDFW have documented that desert bighorn sheep populations in the Mojave Desert have been exposed to BT and EHD based on laboratory analyses of blood samples. Table 2 shows the results for these viral diseases in the Cady, Ord and Newberry, Old Woman and Marble mountains, and presence or absence of cattle.
Table 2. Viral disease exposure in four desert bighorn sheep ranges and number of domestic cattle present.
Desert bighorn sheep exposure to BT and EHD in these four ranges varied from high and very high when significant numbers of cattle were present, to none or low when cattle were absent or relatively few in number. Bighorn sheep research biologists studying population structure in the Old Woman Mountains reported that preliminary results of their study implicated BT and EHD as the cause of the depressed desert bighorn sheep population since BLM authorized cattle grazing throughout the range in an amendment to the CDCA Plan in 1982.
Competition for Forage and Space: Competition is more likely to occur in arid desert environments where forage is dependent on precipitation that varies from year to year. The effects of climate change are expected to result in extended periods of below normal precipitation, leading to increased competition between bighorn sheep and cattle for scarce forage.
Bighorn sheep populations typically decrease when cattle are present and increase when they are absent due to both competition for forage and bighorn intolerance of cattle. This occurred in the Cady Mountains following the complete removal of cattle in 2005, resulting in a ten-fold increase in the desert bighorn sheep population by 2010. In the Old Woman Mountains, bighorn sheep research biologists documented abandonment of a prominent spring by desert bighorn once cattle began using it, resulting in vegetation loss around the spring and contamination of the surface water with feces and urine.
The CDCA Plan included many provisions for the conservation of desert bighorn sheep and its habitat. Had the original plan been fully implemented and not amended to allow increased domestic cattle grazing, desert bighorn sheep populations in mountain ranges used by cattle would likely be significantly larger today. Unfortunately, BLM approved many amendments that eliminated the conservation provisions by expanding grazing allotments, increasing the number of cattle, and changing the use season to yearlong. The Lazy Daisy Allotment is an example of the adverse impact of the CDCA Plan amendments on desert bighorn sheep and how BLM has abandoned its commitment to protect bighorn sheep and its habitat from the impacts of cattle.
Impacts of cattle on desert bighorn sheep include viral diseases transmitted by biting gnats, competition for forage, water, and space, or a combination of these factors. In ranges that have suitable habitat for biting gnats, the effects of BT and EHD viruses on desert bighorn sheep populations are likely more detrimental than competition. BLM has the responsibility for managing the resources in the CDCA, including bighorn sheep habitat, that provides for multiple land uses in a sustained yield manner and that maintains overall environmental quality. Members of the public and conservation organizations interested in conserving desert bighorn sheep and protecting its habitat in the CDCA could submit a proposal to BLM to amend the CDCA Plan by restoring the desert bighorn sheep conservation provisions that were included in the 1980 CDCA Plan
Formerly a biologist with the Bureau of Land Management and the National Park Service, Jeff Aardahl now works with Defenders of Wildlife focused on wildlife and habitat conservation, endangered species recovery, land use planning, and similar conservation matters.