In order to have a strong and surviving population, a species must have numbers on their side. The larger the population, the better odds that the species can thrive. This makes sense, because the more individuals there are, the better the chance that at least some in the population will survive no matter what happens to the group as a whole. For example, if a disease spreads throughout a population, and kills many in the group, then the ones who survive obviously have some ability to withstand that specific illness. As a result, the offspring of these surviving individuals will have better immunity to this disease. However, if a species does not have a large enough gene pool with enough variation then the population is weakened. A species with this problem will have a much more difficult time surviving if something bad happens.
Knowing this, many people and animal rights groups are inclined to argue against zoos because endangered species bred in captivity are coming from such a small gene pool. Zoos often try to defend there existence by saying that they are aiding in the preservation of endangered animals.
While this may be true, it doesn't solve the population crisis. We know that species need a large and diverse gene pool to really thrive and this is simply not available in the environment of a zoo. This is really a difficult issue because it is hard to find a suitable solution. On the one hand we all want to help endangered species, and on the other we must ask ourselves if it is ethical to breed animals in captivity. Is it really our place to meddle with nature? Is there a better way to preserve species?
Photo source: http://www.flickr.com/search/?q=Endangered+animals