Q: Why do some animals go extinct while other species regenerate?

— Aatman, 10, Chavez Elementary School, Madison

A: A lot depends on the circumstances in which animals have become rare in the first place. Some species are simply more rare than others and don’t occur in very large numbers. But usually there is some factor that has driven a population to even greater rarity.

It could be a big environmental change in the habitat, something affecting food, water, shelter or cover. There could be new factors in their environment like new competitors or an invasive species that is exploiting resources or taking resources away from the other animals.

Sometimes the cause is an aspect of the species biology in which there is an imbalance of animals in terms of their age or sex. There might not be enough females, or there might not be enough animals of the right breeding age available.

Factors leading to extinction are really a mix of natural environmental conditions and aspects of the animals’ population, whether they can successfully reproduce and increase their numbers.

A good example of a species that was able to regenerate its population is the whooping crane. The whooping crane fell to a low of only 15 or 16 birds in existence in 1941. There were only a few females that survived, and it’s believed that all whooping cranes alive today are the descendants of about four or five mothers.

The factor most at play in the decline of the whooping cranes was that they lost a lot of their habitat. This species bred in rich, productive wetlands that have essentially been transformed into farmland. They lost their home.

Refuges were then established to help protect areas that the cranes and other waterfowl used at different times of the year. This protected vital portions of their life cycle. Whooping cranes had a home again, a place where they could essentially recover.

Fortunately, because the gene pool was still rich enough, these birds have begun to slowly recover and replace their numbers.

[Editor's note: This story has been updated to reflect a correction. The photo that originally accompanied the story was of an East African Crowned Crane.]

Barry Hartup is the director of conservation medicine at the International Crane Foundation and a clinical instructor in the School of Veterinary Medicine at the University of Wisconsin-Madison.

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