How Do You Put an Animal Back? Returning a Species to Where It Was
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
Releasing animals into places they once lived is one of the more dramatic conservation interventions, and it succeeds less often than the well-known cases suggest. The failures are as instructive as the successes.
What it involves
A reintroduction is the deliberate release of a species into part of its former range where it has been lost, with the aim of establishing a self-sustaining population. It is distinguished from reinforcement, which adds individuals to an existing population, and from introduction outside the historical range, which is a different and more controversial operation. The formal guidelines require that the original cause of the loss be identified and removed or reduced, that suitable habitat exist in sufficient quantity, that the released animals be genetically and behaviourally appropriate, and that the project have the support of the people who will live alongside the animals. That last requirement is frequently the binding one, and projects have failed on it while satisfying everything else.
What determines success
Reviews of many projects identify consistent factors:
- •Whether the original threat has actually been removed, which is the single strongest predictor
- •The number of individuals released and over how many separate releases
- •Whether the animals were wild-caught or captive-bred, with wild-caught doing considerably better
- •Whether released animals have the skills to find food and avoid predators
- •Habitat quality and connectivity to other suitable areas
- •Local human attitudes, particularly where the species conflicts with farming
- •Long-term monitoring and the willingness to intervene again
The captive-breeding problem
Animals raised in captivity frequently fail after release for reasons that took time to recognise. They may not recognise natural food, may not know how to hunt or forage efficiently, may lack fear of predators or of people, and may not have learned migration routes or the locations of resources that wild animals learn from their parents. Genetic adaptation to captivity occurs within a few generations, selecting for traits that suit a cage and not a landscape. The responses include pre-release training, soft release into enclosures at the site allowing gradual adjustment, releasing family groups rather than individuals, and in some cases teaching migration routes directly, which has been done for cranes by leading them with aircraft. Wild-to-wild translocation avoids the problem entirely and is preferred wherever a source population can spare individuals.
The cases usually cited
A few projects are quoted constantly and are worth stating accurately. Wolves were returned to Yellowstone in 1995 after seventy years absent, and the widely repeated account of the whole landscape being transformed through effects on elk and then on vegetation and rivers is a simplification that has been challenged in detail by ecologists, who point to concurrent changes in other predators, in climate and in elk management. Beavers have been reintroduced across Europe with clear and measurable effects on water retention and flood attenuation, which is among the better-documented successes. Condors in California survive only through continuing intensive intervention, since the lead poisoning that nearly eliminated them has not been resolved. Przewalski's horses returned to Mongolia from captive stock and now breed in the wild, which is the clearest case of a species recovered from extinction outside captivity.
The arguments it provokes
Returning a species is rarely uncontroversial and the objections are frequently reasonable. Predators conflict with livestock farming, and compensation schemes address the financial loss without addressing the disruption, which is why these projects generate the most opposition. Large herbivores affect forestry and crops. The landscape has changed since the species was lost, so the ecological role it once played may no longer exist or may now be filled by something else. Costs are high and the same money might protect more species elsewhere, which is a genuine allocation argument. And the choice of which era to restore to is a value judgement rather than a scientific one, since no baseline is uniquely natural. The projects that work tend to be those that treated these as questions to negotiate rather than obstacles to overcome.
The takeaway
The guidelines require that the original cause of loss be removed, that habitat exist, that the animals be suitable and that local people support it, and the last is frequently what decides the outcome. Wild-caught animals do considerably better than captive-bred ones, which may not recognise food or fear predators. Choosing which era to restore to is a value judgement rather than a scientific one.