Central Mountains/Sonoran Region
Bighorns On the Brink
Today, less than 100 desert bighorn sheep occupy Ironwood Forest National Monument. Human impacts have rendered this herd the last Bighorn sheep population in Pima County, where they live virtually isolated in a sea of human development. This, in itself, is an impediment to long-term viability of this herd, as there are no nearby populations with which to exchange genetic material or to re-colonize habitat in the event of a die off from disease or other factors.
Bighorn sheep require large areas to roam for food, water, repro-duction and raising of lambs, and to avoid predators. As their range becomes fragmented, sheep numbers dwindle. Without access to habitat outside the monument, Ironwood Forest must meet these needs unto itself. And while these habitat requirements may have been satisfied by the monument in its once-pristine state, increasing recreational use and the proliferation of roads—capable of providing access to previously undisturbed areas—are likely over time to create barriers to sheep movement and threaten their longterm survival.
Ewes tend to be more sedentary than rams, often remaining on a single mountain and only moving to lambing grounds to give birth. Lambing grounds, which may be used year after year, are selected based on the need to protect offspring. Rams, on the other hand, outside the breeding season, tend to prefer lower, flatter areas further from escape terrain but with high quality, abundant forage. Young rams follow the largest rams to such sites. If these sites are lost, either to habitat destruction or to human activities, rams may be slow establishing new ones.
Rams tend to migrate more frequently than ewes—one has even been documented moving between the Silverbell, Waterman, and Roskruge Mountains. The Roskruge Mountains, unable to support a resident bighorn population, may serve as a corridor between the Silverbells and the Baboquivari/Coyote Mountains.
Although we are unaware of evidence indicating recent or regular movement to the Sawtooth Mountains, bighorn have occurred there in the past and might be considered for a transplant if some roads were closed and connections to the Silverbells were protected. Expansion on to the Tohono O’odham reservation is also possible. The Bureau of Land Management must strongly consider the acquisition of state and private lands where development may be impeding their movement.
While bighorn sheep do not usually require Wilderness to survive, they are clearly vulnerable to the presence of people and roads. Given the relatively small size of Ironwood Forest National Monument, the rarity of lambing sites, and the lack of nearby populations or even accessible habitat, it is essential that habitat in the Monument be managed to prevent bighorn habitat fragmentation.
Wilderness designation for the Silverbell, West Silverbell, Ragged Top, and the Sawtooth Mountains represent a necessary first step, as it will curtail easy access to the core bighorn habitats—especially if most roads approaching these mountains are also eliminated. While necessary, Wilderness designation is not likely to be sufficient to protect bighorns in the long run. This will require the identification and protection of movement corridors between ranges that will allow rams unrestricted access to foraging areas and provide ewes with sufficient lambing grounds. It will also require closing such sites to human visitation, at least seasonally.