Microbes replace wolves in culling herds
By Dan Whipple
UPI Science News
Published 9/1/2003 12:00 AM
BOULDER, Colo., Sept. 1 (UPI) — As more and more predators are eliminated
from ecosystems, nature is providing replacements to perform their
evolutionary function of culling herds — and in the process is creating
new dangers for the human race.
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.