Can't Rely on Captive Breeding to Save Species

September 26, 2007

By David Suzuki with Faisal Moola

One of the methods often proposed to save endangered species is to breed the threatened animals in captivity and then release their offspring back into the wild at a later time. The theory is that by giving the animals a safe place to breed and rear their young, you're making it more likely that the species will be able to successfully survive in their natural habitats.

It seems simple enough. Of course, in nature things are never quite that simple. And recent research has found that breeding wild animals in captivity actually has consequences at a genetic level that make them less likely to be able to survive in the wild.

The problem is natural selection. As Charles Darwin described way back in his 1859 treatise On the Origin of Species: By Means of Natural Selection, heritable genetic traits that are favourable to a species in a given environment become more common over successive generations. Unfavourable traits become less common. That's natural selection.

If you take an animal (or any living organism, for that matter) out of its natural habitat and introduce it to someplace new, natural selection takes over and traits that are favourable to the new location — in this case captivity — become more and more common in subsequent generations.

This poses a problem for animals in captivity because, try as we might, it's virtually impossible to replicate natural conditions in a zoo or recovery centre. Space is one factor. Many animals require large amounts of space in which to roam, swim or fly. Zoos can't be that large. In captive spaces the animals are also always safe, well fed and made comfortable. Such "easy" living may make for physically healthy animals that are able to reproduce more often, but it doesn't necessarily make them genetically fitter.

In fact, quite the opposite. In a recent edition of the journal Molecular Ecology, Dr. Richard Frankham of Australia's Macquarie University explains how rare, and often detrimental, genetic variations that are suppressed in the wild can become common in captive creatures. These genetic adaptations to captivity have been documented in mammals, fish, plants, insects and even bacteria. And while reproductive fitness often improves a great deal, Dr. Frankham points out that, "Characteristics selected for under-captive conditions are overwhelmingly disadvantageous in the natural environment."

From turkeys, to amphibians, fish, plants and insects, it seems that the longer an organism stays in captivity, the less fit it becomes for the great outdoors. Fish from salmon hatcheries, for example, evolve smaller eggs that are less likely to survive in the wild. This helps to explain why re-introducing captive-bred species to the wild often has such a low success rate.

It doesn't help that, while captive breeding is being conducted, the natural habitat of the captive species continues to be destroyed, or otherwise made less hospitable to its former resident. Recently, the B.C. government announced that it is going to start a captive breeding program for the province's critically endangered spotted owl. Yet logging will be allowed to continue in the owl's habitat.

As I recently discussed in an earlier essay on bush meat, the loss of species from an ecosystem can also trigger changes that makes the ecosystem itself less hospitable to the very species you are trying to recover. For example, certain large-seeded fruit trees rely on large primates to disperse their seeds, while the primates rely on the fruit for food. When primates are hunted to near extinction, the forest itself suffers and the fruit trees die off, so there's less food — making the re-introduction of large primates to the area even more difficult.

Saving a species from extinction may well require captive breeding. But it's really a Hail Mary pass to the future. Without habitat, a species is a mere caricature of what it is in the wild. The longer creatures are in captivity, the less likely they will be to ever survive on their own, even if we manage to stop destroying their homes. And that makes saving their homes all that more important in the first place.

Written for Science Matters blog