In recent years, previously unknown diseases have begun taking an alarming toll on species. West Nile virus, mad cow disease and chronic wasting disease 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 people, 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."
Peter Daszak, director of the Consortium for Conservation Medicine in Palisades, N.Y., is less ambiguous. "Diseases are causing mass die-offs and extinctions," he 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. 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 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."
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.
The disease 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. It results in deteriorating coat and body, unusual behavior and death. CWD was first identified in captive muledeer in 1967. It was detected in free-ranging deer in the mid-1980s in Colorado and Wyoming. CWD apparently afflicts 4 percent to 8 percent of the deer in this region.
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 humans preferentially take the healthiest animals.
In the 1992 book "Natural Enemies," biologists Tim Caro of the University of California, Davis, and Clare Fitzgibbon of Cambridge University in Britain wrote, "Several sources of evidence have confirmed that much of the mortality in prey populations can be attributed to predation ... In the northern latitudes of North America, evidence suggests that predators used to keep ungulate populations at low densities before predators were persecuted following the arrival of European people."
The authors continued: "In some areas, approximately 70 to 85 percent of caribou, 94 percent of elk, 85 to 94 percent of moose and 75 percent of deer calf mortality is due to predation ... There are several anecdotal accounts which suggest that 'strange' animals are selected. Wildebeest that are infected by Gedoelstia flies can have uncoordinated gait and are quickly picked off by predators."
Caro and Fitzgibbon also wrote: "Parasites ... often exhibit a number of adaptations to increase the chances that an infected prey will be taken by the right predator."
So does CWD fit neatly into a scenario where it spreads through the population in partial response to a lack of natural predation on the target species? Perhaps not.
"It wouldn't really function like predation," Beth Williams, a wildlife veterinarian with the state of Wyoming, told UPI. "The age classes would be different from the ones taken out by predators ... CWD hits the prime, adult age class. ... With CWD, we have to wait for the long term so we can really evaluate it."
Williams said so far CWD has not had a significant impact on wild cervid populations, although some computer models that indicate it will.
Though Williams said CWD is not her favorite candidate to replace predators as a keystone species, 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."
With CWD, Williams explained, the latest research indicates "it is likely they are shedding the agent into the environment for a while before they show clinical signs. They can shed for a fair amount of time before a predator would be able to test the animal and figure out, 'This looks like dinner.'"
John Thompson, director of the STEPS Institute for Innovation and Environmental Research in Santa Cruz, Calif., and a professor of ecology and evolutionary biology at Washington State University in Pullman, thinks, "It is certainly true that under some kinds of conditions and hypothetical scenarios it would work. If you eliminate predators, population numbers could increase and, under high density conditions, a disease could be more prevalent."
Though the argument is plausible, there are alternatives. "You have this constantly changing geographic mosaic," Thompson told UPI. "It could be disease, it could be competition. Rather than one form of interaction like disease, there is a constantly changing mosaic."
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.