When I was very young I used to spend a lot of time in my uncle's house. He kept caged songbirds. One day I opened a cage to get a closer look at a bird. The bird flew out and escaped into the garden through the open back door. We never saw him again.
My uncle was annoyed when he discovered what happened, but he wasn't half as upset as I was. It occurred to me the bird had spent so long in captivity that it had forgotten how to survive in the wild. I envisioned miserable and tragic scenarios for the escaped bird. Eventually, I reluctantly allowed my uncle to comfort me with stories of how the little bird would re-unite with family and friends and lead a blissfully happy free life.
I now know my childish forebodings predicted reality more accurately than my uncle's rosy predictions. Returning captive animals to the wild is a favoured approach in programmes aimed at reviving the fortunes of endangered species. Time and again this tactic runs into severe difficulties.
As a general rule, most "higher" wild animals (i.e. animals with well-developed nervous systems) captured and held in captivity for a prolonged period lose essential skills necessary for them to live in the wild. Endangered higher species that are bred and reared in captivity never learn the skills of the wild to begin with.
It is estimated that the human population of the world by 1600 was about 500 million. From about that time, world population began to increase dramatically, until today it exceeds six billion. Along the way most wilderness areas were domesticated, thus placing the natural diversity of flora and fauna under pressure. The situation has worsened and today the world is experiencing a massive loss of species.
MANY biologists have cherished the hope that endangered species could be taken from the wild, placed in zoos, encouraged to breed and, when numbers had risen high enough, released back into the wild. This policy works well with certain species such as snakes or butterflies that are born "biologically hardwired" with all the instructions they need for survival in the wild. However, serious problems present themselves when trying to apply this rescue policy to endangered species that are not endowed with this hardwiring.
Many animals have to learn complex behaviours if they are to forage for food successfully and to avoid being killed by predators. They mostly learn this behaviour from older, experienced members of their own species. Young animals taken from the wild and reared in captivity are at a major disadvantage - they never learn essential skills of the wild.
Human handlers have tried to teach these skills to animals but the results are usually very poor. As a result, when animals reared in captivity are released into the wild it is not uncommon for up to 80 per cent of them to be killed by predators. Sometimes they are all killed.
If a species is to have a secure future its numbers must be maintained above a certain level. If the numbers fall below this level the species is classed as endangered. One of the problems posed by small numbers is genetic drift. This results in an inexorable trend, other things remaining equal, towards genetic uniformity.
Genetic drift results from random effects of the environment that decrease the frequency of some genes and increase the frequency of others. This is different to the well-known evolutionary mechanism of natural selection, where environmental changes automatically select for genes best fitted to allow the species to adapt to the changed environment. The changes effected by genetic drift are neutral and have no adaptive value.
CONSIDER a small cow pasture containing a population of snails of two colours, yellow and brown. Let us say 10 per cent of the snails are yellow and 90 per cent are brown. Let us further say that in one generation a greater proportion of yellow snails are crushed by cows' hooves than brown snails.
The snails breed and the next generation will have less than 10 per cent yellow snails. Now say that in the next generation, the cows crush a greater proportion of brown than yellow snails. This will again change the proportion of brown versus yellow snails in the second generation. These random fluctuations in the percentage of the two types of snails is genetic drift. Sewall Wright (1889-1998) the American mathematician proved that eventually, if no other factors intervene, these fluctuations will bring the snail population to 100 per cent yellow or 100 per cent brown, purely by chance.
While genetic drift occurs in all wild populations, when large numbers of animals are involved the effect is muted. It is much outweighed by the effects of natural selection, and doesn't lead to genetic uniformity. When a wild species is reduced to small numbers it becomes very susceptible to being wiped out by some severe negative change in the environment, e.g. a particularly harsh winter, a disease, etc. Extinction becomes pretty much certain once genetic drift eliminates the genetic diversity of a small group.
Genetic diversity provides essential protection for a species by ensuring at least some individuals are genetically endowed with characteristics that allow them to withstand an environmental change which is generally very bad for the species.
Because of the difficulties inherent in successfully releasing captive animals back into the wild, biologists are now coming to the opinion that the only appropriate approach is the preservation of endangered wild habitats or the restoration of damaged habitats. Using this approach, endangered species would only be taken into captive protection programmes in the small minority of cases where it is clear that the animals are doomed if they remain in the wild.
William Reville is a senior lecturer and Director of Microscopy at UCC