Microbes Replacing Wolves In Culling Herds
by Dan Whipple, UPI
November 5, 2003

Recent research suggests that as more and more predatory animals are eliminated from ecosystems, nature is providing dangerous replacements to perform their evolutionary function of culling herds.

In recent years, previously unknown diseases have begun taking an alarming toll on species. West Nile virus, mad cow disease and chronic wasting disease (CWD) are just a few names on a long and growing list of wildlife parasites and diseases that have been making headlines as they threaten to ravage animal populations - and infect humans, often with tragic consequences.

They have become substitute predators - natural adaptations that help regulate animals in areas where predators and other 'keystone species' have been eliminated.

But because these organisms do not behave exactly like the species they are replacing, their eventual impact on wildlife is uncertain.

"Carnivores are controversial - we're only now learning their roles in affecting the populations of animals," Joel Berger, a field biologist with the Wildlife Conservation Society, who studies predator-prey interactions, told United Press International.

"Ecological systems aren't simple," he pointed out.

Peter Daszak, director of the Consortium for Conservation Medicine in Palisades, N.Y., is less ambiguous.

"Diseases are causing mass die-offs and extinctions," Daszak told UPI.

Along with the better-known modern plagues, such as chronic wasting disease in deer and elk and West Nile in birds, are other, perhaps even more devastating illnesses.

Daszak cites chytrid fungus, which is causing a drop in amphibians worldwide. Berger offers brucellosis, which affects bison and elk, and rinderpest, another deadly livestock disease.

All appear to be acting as predators in areas where traditional predatory species have been severely reduced in number or eliminated entirely.

Much of the problem is due to the easy transfer of species in the modern world into new habitats, where they begin to compete with animals that have not evolved to deal with them.

For example, a recent paper in the journal Ecological Letters demonstrated how the North American grey squirrel is replacing England's native red squirrel, not through direct competition, but because the grey squirrel carries the parapoxvirus, to which it is immune, but which is deadly to the red squirrel.

"More generally," say the authors, "the fate of the red squirrel highlights the need for greater attention to the potential for disease to exacerbate the consequences of species invasions: another headache for conservationists."

Chronic wasting disease (CWD) may be getting to that point, Daszak said.

"CWD is hitting a huge number of deer, and it may act because we've lost the predators there. Nothing is out there regulating populations," he said.

CWD is causing serious concerns because there is large constituency of people - hunters - who have an interest in the deer and elk population.

But many nongame species facing an imminent threat of extinction - such as amphibians from chytrid fungus - lack the high profile of "charismatic megafauna," as biologists sometimes call them, with tongue half in cheek.

CWD resides among a family known as transmissible spongiform encephalopathies. It affects cervids - members of the deer family - causing a sponge-like degeneration of the brain. This results in deteriorating coat and body, unusual behavior and death.

Under normal conditions, predators - in this case probably wolves - would take the sick deer and thus limit the spread of the disease and at the same keep the herd population in check.

But in the Rockies, all of the deer's natural predators except humans have been eliminated - and human hunters preferentially take the healthiest animals.

Beth Williams, a wildlife veterinarian with the state of Wyoming, said CWD is not her favorite candidate to replace predators as a keystone species, but she does have her own example of an illness that may be gaining ground in the absence of predation.

"Bighorn sheep with lungworms, for instance," she said. "In those animals, there's been some thought that wolves would chase them, and if they had a heavy load of lungworm, they may get taken out preferentially by the wolves, since the sheep can't run as fast. When you don't have wolves testing the population, it might lead to a higher level of lungworm being present in the population."

Most scientists agree more research is needed. Berger said he attended a meeting recently in which a number of participants expressed interest in exploring the relationship between the loss of predators and the rise of wildlife diseases.

© 2003 United Press International

Sources:

UPI Science News
www.upi.com/view.cfm?StoryID=20030828-120110-5839r Microbes replace wolves in culling herds

http://www.anc.org/wildlife/wildlife_article.cfm?identifier=2003_1105_microbes