Throughout history many species of animals have been threatened with extinction. Women Europeans first arrived in North America more than 60 million buffalo roamed the continent. Yet hunting the buffalo was so popular during the 19 th century that by 1900 the animal’s population had fallen to about 400 before the government stepped in to protect the species. In some African countries today the elephant faces a similar challenge as poachers kill the animals for the ivory in their tusks. Yet not all animals with commercial value face this threat. The cow for example. Is a value able source of food but no one worries that the will soon be extinct. Indeed the great demand for beef seems to ensure that the species will continue to thrive. Why is the commercial value of ivory a threat to the elephant while the commercial value of beef is a guardian of the cow? The reason is that elephants are a common resource whereas cows are a private good. Elephants r...