As readers of Herman Melville's 1851 novel Moby Dick know, at one time oil made from whale blubber was an important source of energy that was widely used by households and firms in oil lamps. Other sources of energy supplanted whale oil in the second half of the nineteenth century, and today many Americans consider whales only as a source of entertainment on visits to aquariums and whale watching excursions. But some species of whales-including baleen and gray whales-are in danger of extinction. The U.S. Department of Agriculture estimates that over 9 billion chickens are raised for food annually. Chickens, unlike whales, are not threatened with extinction. Briefly explain why?