Scientists debate ‘magic number’ of wolves needed for species' survival
By ROB CHANEY of the Missoulian | Posted: Sunday, May 22, 2011 7:00 am
Conservation groups and the federal government continue to disagree how many gray wolves are needed in the Northern Rockies to ensure the species’ survival. National Park Service photo
One of the biggest arguments left unresolved by last year's wolf lawsuit was the most obvious: How many wolves are enough?
The U.S. Fish and Wildlife Service took the gray wolf off the endangered species list in 2009, with the caveat that at least 150 wolves and 15 breeding pairs endure in each of the three states in the northern Rocky Mountain population (Montana, Idaho and Wyoming).
Recent surveys found at least 1,700 wolves in that area – more than enough to justify delisting.
But a coalition of environmental groups sued the government, claiming those numbers were wrong. To survive and thrive, they argued, the population needed at least 2,000 and preferably 5,000 wolves.
FWS biologists said they used the best available science to pick their number. Coalition members cited the well-established rules of conservation biology to justify their threshold. While the scientists dueled, U.S. District Judge Donald Molloy decided the case on a technicality and Congress reversed him with a budget rider. Wolves in the Northern Rockies are now delisted, but almost nobody's happy.
Over the past decade, biologists have sought a "magic number" that would simplify endangered species debates. In 2010, an Australian team led by Lochran Traill of the University of Adelaide published a study declaring 5,000 was the population size required to prevent any species' extinction.
"We don't have the time and resources to attend to finding thresholds for all threatened species," Traill told Science Observer Magazine. "(T)hus the need for a generalization that can be implemented across taxa (classes of animals and plants) to prevent extinction."
But another group of U.S. Forest Service researchers along with American and British professors warn that a simple tool may be a flawed tool. While they agree that an easily understood standard helps persuade judges or members of Congress of the need for action, the 5,000 figure doesn't add up. Their paper will be published in the journal Trends in Ecology and Evolution.
"It's natural for any policy maker and practitioner to look for ways of simplifying the overwhelming process of endangered species management," said Greg Hayward of the Forest Service's Alaska Region Office. "If that worked, it would be a delightful world to live in. But if you're really going to do anything positive, in terms of turning around the situation for these species, going for that simple rule of thumb isn't going to help."
Both sides use a lot of math to make their points. Traill and company looked at 1,198 species with a computer model that calculated how many of each would be needed for the plant or animal to survive in the long term. In particular, the study looked at how many are needed to ensure a species doesn't in-breed itself into extinction.
That's key because one requirement to getting off the endangered species list is a population big enough to guarantee genetic diversity. Earthjustice attorney Doug Honnold relied on that in his argument to Molloy, to show why the wolf should remain a listed species.
"If you're talking about genetics, then there are some basic genetic principles that apply across all species," Honnold said. "It's been documented with every species that's been studied."
Honnold referred to what's called the "50-500 rule" which states you need at least 50 breeding-age females of a species for short-term survival or 500 for the long term. In the case of wolves, there's usually only one breeding female in a pack of four to 10 wolves, so the total population number balloons to 2,000-5,000.
The "magic number opponents" respond that genetics isn't everything. In the case of wolves, where might that 2,000-5,000 figure apply? Do we need a minimum viable population in the three states where wolves were reintroduced back in 1995? Or should the figure be spread across the six-state area now delisted by congressional fiat (adding Utah, Washington and Oregon to Montana, Idaho and Wyoming)? Does it count the Canadian wolves that have relations with American packs along the international border?
"Under the Endangered Species Act, we sort of ignore other segments of populations that are outside the United States," said Hayward's colleague, Steven Beissinger of the University of California-Berkeley. "In the case of the paper we did, one thing we found was, the particular technique people used to come up with this minimum number was very context-specific."
In other words, each animal needs its own formula. Passenger pigeons had different lifespans and breeding rates than wolves. They could fly across continents at will, while wolves may be stymied by freeways. Passenger pigeons were, in fact, the most abundant land bird in the continental United States – 3 billion to 5 billion individuals – before the population crashed between 1870 and 1890. [ note: Here I disagree with the reporter. The passenger pigeon population did not crash. It was deliberately exterminated, using the most atrocious means imaginable.]
Science rarely gets to be just science. Lots of scientific reasons justify the wolf's presence on the landscape: It reduces elk populations, which in turn improves the plant communities along streams, which brings back songbirds and beavers.
But reduced elk numbers aggravate a hunting community that's invested millions of dollars to improve elk habitat. Wolves also have proved a poster target for politicians who want to leash the Endangered Species Act.
Natural Resources Defense Council staff scientist Sylvia Fallon said the U.S. Fish and Wildlife Service knew it would face public resistance if it proposed reintroducing lots of wolves, so it picked a deliberately low 150-per-state figure to get the reintroduction in play.
"They (FWS biologists) say they came up with that number in consultation with scientists, but they never said who they were," Fallon said. "It was some guesswork factoring in social and political considerations at the time, what would be acceptable to the states and the public."
FWS attorneys rejected that claim in their court briefs, but they never got to have the argument in Molloy's courtroom. Without ever discussing what an appropriate number should be, the judge only said the federal government illegally used state boundaries to divide a natural population.
Beissinger suggested a better target in the search for the elusive magic number. Instead of a unified field theory of how many of a species is needed to survive, we humans should settle on what risk factor we're willing to work with, he said.
"In my profession, we don't have a single standard that's been set for what degree of risk we're willing to accept for a species to go extinct," he said. "I could make a calculation for a species and say nine times out of 10, it would be viable there, for 50 years. Would that be good enough, or would you want a 95 percent chance, or an 80 percent chance? But it's too naive to use just measures of population size and come up with some rule of thumb whether a population is safe or not."
Reporter Rob Chaney can be reached at 523-5382 or at [email protected].