Eating Stone by Ellen Meloy Page B

genetic variation, a sign of inbreeding, and a history of fluctuations: a cycle of crashes to low numbers, followed by gradual increases.
    Although wildlife science has made vast leaps forward, population dynamics—the balance of birth and mortality, plus enough complex factors to incite nosebleeds—still evoke guesswork and differences of opinion. In general, herds expand when conditions are favorable. Mortality adjusts the population to a new situation, such as climate changes or food shortages.
    When mortality is high, a herd may reach a low point from which it cannot recover. (Although the number is controversial, some biologists set the “minimum viable population” at fifty animals.) Sadly, this may be the story of many of the Southwest's lost sheep. It could leave the Blue Door Band thirty animals away from a similar fate.
    In the mid-1970s, counts in the San Andres came in at around two hundred sheep. Ram hunts were allowed, controlled and sparingly, so that some animals could be taken without jeopardizing the herd. The last ram hunt on the refuge took place in 1978.
    One of the perils of island life is susceptibility to disease. With its high degree of sociability, a herd can be a petri dish for contagion. Serious disease predisposes animals to predation, to continued and new disease, to poor nutrition and reproduction. Too weak, a population may fall into a downward spiral. Too rare— a species low in density or geographically isolated—they face a thin threshold for collective disaster.
    In their holdout enclave, the San Andres bighorns were hit by a virulent outbreak of scabies, a disease caused by ectoparasitic mites. Scabies mites inflict a variety of disorders that can reduce an animal's health and vigor: hair loss, crazed itching, folding of the external ear, plugged ear canals, eardrum damage, loss of hearing, and upset equilibrium. A normally agile sheep with clinical (severe) scabies can lose its muscular coordination and accidentally fall off a cliff and break its legs or crack its skull.
    The San Andres scabies-mite infestation was sudden and severe. Sheep counts in 1979 revealed seventy-five to eighty ani-mals, down from two hundred in an alarming crash. Scabies may have predisposed them to another affliction, a virus called contagious ecthyma, which can lead to blindness, lesions, impaired feeding, and starvation. From 1979 into the next decade, 85 percent of the herd died.
    Domestic sheep, goats, and cattle, as well as wild ungulates, attract mites; the relationship between mites and host is murky. How Psoroptes spp. mites were originally passed to the San Andres ungulates was never clear, either, although many agree that the mites likely came from domestic livestock introduced to the region in the early 1900s. The parasites may have long been present, possibly cyclical, possibly dormant, perhaps residing in a reservoir host such as mule deer, then became manifest in the bighorn epidemic.
    In 1979, refuge managers began a salvage operation. They captured forty-nine sheep from the mountain and treated them for scabies. For some, intervention came too late. Accidents and disease-related losses in captivity further reduced their numbers. Only twelve survivors of the roundup would return home, there joining transplants brought in from another area. In 1980, the desert bighorn sheep was listed as an endangered species in New Mexico. In early 1981, the San Andres herd back in the wild numbered about forty sheep.
    Over the next fourteen years, annual refuge surveys counted a herd that stayed below forty but above twenty sheep. Disease continued to plague them. The local mule deer population declined and mountain lion predation intensified. Mutton was on the menu. When San Andres bighorns died, mountain lions were the proximate cause.
    By 1997, the desert bighorns native to these mountains were extinct, one ewe short of zero.
    In the many places across the American and Mexican deserts now empty of their