Oh give me a home …

时间:2019-03-08 07:19:02166网络整理admin

By Kurt Kleiner IN 1916 Yellowstone National Park had only 25 bison. Now there are more than 2000, the largest wild population left in the US. But the careful nurturing of the herd could be in vain as animals straying from the park continue to be shot on the off-chance they might spread brucellosis to cattle. Despite scant evidence of a significant risk to livestock, Montana state officials say this culling is only common sense. But conservationists warn that in the long term crucial genetic diversity in Yellowstone’s wild bison herd could be lost. Last month a federal court upheld the right of officials from the park and the state of Montana to continue shooting bison outside park boundaries. About 100 animals were killed this winter and spring as hunger forced them to search farther afield for food. Some animals were killed while still inside the park because officials believed they were about to play truant. Conservation groups such as the Fund for Animals, a New York-based animal welfare organisation, have been campaigning for an end to the cull since the winter of 1996-97, when severe weather forced many bison out of the park and more than 1000 out of a herd of about 3500 were shot. “I still firmly believe that the bison management plan is illegal,” says Don Schubert, a wildlife biologist with the organisation. “The onus of the burden has always been placed on the bison. There hasn’t been a burden on the cattle.” At one time, about 60 million bison, or American buffalo (Bison bison), roamed the American prairies. But in the 19th century the animals were hunted to the point of extinction. At one stage, there may have been only 1000 left. Yellowstone National Park was down to about 25 bison as recently as 1902. But active management of the herd increased numbers to several thousand. Today, some 2200 bison live in Yellowstone. About half of them carry the infection Brucella abortus but few are seriously affected by it. However, in domestic cattle, the infection frequently causes spontaneous abortion, infertility and slow growth. Cattle probably passed the disease to bison early in the century. Since that time, Montana cattle ranchers have spent tens of millions of dollars eradicating brucellosis from their herds. Now they are concerned that bison from the park might pass the disease back to cattle. As a precaution, the Montana Department of Livestock and the Yellowstone authorities recently began to trap bison leaving the park to forage in surrounding pastures, something they normally do in the winter and early spring when food is scarce in the higher elevations of the park itself. They kill any animals testing positive for the disease. About 2000 head of cattle graze on the same pastures, which are mostly federally owned. Some livestock graze there all year round; some just in the spring and summer months. Ranchers worry that their cattle might pick up brucellosis through contact with an aborted fetus or birth material. Even in pastures where cattle don’t arrive until after bison have gone back to the park, they are concerned that the bacteria could survive on the grasslands and infect cattle when they graze there later. “I can’t understand why anybody in the world would like to propagate disease in the oldest national park in the United States of America,” says Marc Bridges, acting executive officer at the Montana Department of Livestock, which is largely responsible for the bison control measures. But it’s not clear how great the risk is. Fears are based largely on six cases of brucellosis in the Yellowstone area between 1961 and 1989 in cattle herds considered free of the disease. Many thought bison were the most likely source of infection. But a report published in 1998 by the National Academy of Sciences noted that the disease sometimes recurs in herds simply because it has not been completely eradicated. Although the report concluded that transmission from bison to cattle in the wild was possible, it also said bison could not definitely be blamed for those cases. “We in science never say never,” says Paul Nicoletti of the University of Florida in Gainesville, who is a brucellosis expert. “But under natural conditions, the bison do seem to be a minimal risk to the cattle.” There are several reasons why experts consider the risk to be very low. The most serious risk of contagion comes from contact with an aborted fetus. But in bison, brucellosis rarely causes abortion. Other birth material such as the placenta is also infectious, but after a normal birth bison tend to eat such material. Finally, the days or weeks between the time the bison leave the grasslands and the cattle return give the bacteria time to die. Nicoletti also complains that officials are killing not just bison cows, but also infected bulls that wander off the park lands—even though there is no known way for a bull to pass the infection on. “This thing is a tempest in a teapot. There are very few cattle. They’d be better off to buy the cows and pay the ranchers to move to Florida,” he says. Schubert suggests keeping cattle off the pastures an extra month to reduce the risk of infection even further. But state and park officials are pinning most of their hopes on finding a way to vaccinate the bison—given that the political will to find more effective vaccines for cattle seems to be lacking. The existing vaccine is only partially effective in cattle and bison—and park officials can’t distinguish between vaccinated and infected animals. Wayne Brewster, a biologist at Yellowstone, says the best bet is a new vaccine called RB51. But researchers are still several years from figuring out if it is safe and effective enough to use. They also need to devise practical ways to administer it. Even then, vaccinations are unlikely to be 100 per cent effective. Officials still plan to test bison for brucellosis and slaughter animals found to be infected, in an effort to eradicate the disease. Officials at Yellowstone say they can continue to cull the bison and still keep the number of animals in the herd at between 1700 and 2500, the figure they propose in what they call their “preferred alternative” plan. This consists of killing infected animals and eventually employing an effective vaccine. But some critics are asking how small the herd can get without a serious loss of genetic variability—especially since so much diversity was probably eliminated in the last century. “Bison are remarkably similar genetically. They’ve already been through one genetic bottleneck. We should be improving our protection of what diversity is left,” Schubert says. James Derr, a geneticist in the College of Veterinary Medicine at Texas A&M University in College Station, doesn’t think absolute numbers are the problem. But the way the animals are selected for killing could be wiping out family groups of bison, and eliminating diversity that way. Derr explains that bison tend to move in matriarchal family groupings—perhaps three generations of females, along with young bulls. This could mean that entire female lineages are being wiped out as they cross the park borders. “It’s so non-random,” he says. “The possibility of impacting genetic variability is much greater.” Schubert thinks the ranchers’ stance is political. “They feel if they allow the bison to range outside of the park, that’s a de facto expansion of the Yellowstone borders,” he claims. At the Montana Department of Livestock, Bridges sees it differently. “It isn’t a bison versus cattle issue,