The most endangered mammal in North America, the black-footed ferret, has taken off like a weed in southeastern Wyoming, US. This unexpected success story suggests that the best conservation strategy for reintroducing the ferret – and perhaps some other endangered species – might be to release more animals and worry less about them.

“Most managers still coddle them one by one. I say shovel ’em out of the back of pickup trucks,” says David McDonald, a behavioural ecologist at the University of Wyoming in Laramie, only half facetiously. “Some reintroductions won’t take, but we need to keep a mosaic of weed patches running.”

In Wyoming’s Shirley Basin, for example, biologists reintroduced 228 captive-bred ferrets (Mustella nigripes) in the early 1990s, only to watch their numbers dwindle to just five individuals by 1997 – despite intensive monitoring.

Booming population

Then, suddenly, the tide turned. In 2003, biologists found 52 animals and by 2006 the population numbered several hundred.


Most likely, a booming population of prairie dogs, the ferrets’ prey, and a reduced incidence of plague and other diseases allowed the ferrets to increase – though their rapid reproductive rate took experts by surprise even so, says Martin Grenier, who manages the ferret programme for the Wyoming Game and Fish Department in Lander.

A closer analysis by Grenier, McDonald and their colleague Steven Buskirk showed that the success or failure of a ferret population hinges on the reproductive success of young animals, rather than their longevity.

That means that adverse conditions in any one year can drastically affect the size of the ferret population, and that the success of any particular reintroduction is difficult to predict or ensure.

Roll the dice

Faced with such uncertain prospects, the best strategy for conservationists may be to roll the dice as many times as they can, hoping to seed enough populations for a few to succeed. “Rather than spend a large amount of resources in those first couple of years after reintroduction, let’s step back. Put the animals on the ground, then put additional resources into more sites,” says Grenier. A similar strategy may prove useful for other imperilled species such as Swift foxes (Vulpes velox), he notes.

Fisheries managers, too, sometimes go the cheap-and-abundant route. In Minnesota, biologists now stock lakes with millions of three-day-old walleye larvae (Sander vitreus vitreus) instead of rearing the hatchlings for a year in protected basins and then stocking fewer, larger fish. “It’s hugely successful,” says Donald Pereira, a fisheries biologist with the Minnesota Department of Natural Resources in St Paul.

This scattergun approach would only work for species that can be bred easily in captivity, cautions David Wilcove, a conservation biologist at Princeton University in New Jersey. “You wouldn’t try it with something like a California condor or whooping crane, because they don’t breed that easily in captivity and you don’t have that large a population,” he says.

However, conservationists may be able to get away with less care in other cases. For example, he notes, wolf biologists have sometimes coddled reintroduced animals and other times left them to fend for themselves, and both strategies have been successful.

Journal reference: Science, vol 317, p 779, (DOI: 10.1126/science.1144648)