The population size of a particular species that can be supported in any given ecosystem depends on the resource needs—ultimately, the energy needs—of that species.

Before the 1800s, the North American prairie was able to provide sufficient energy and other resources to support an incredible 30 million to 60 million bison. During the late 1800s all but a few hundred of these extraordinary animals were killed for their skins, meat, tongues, for sport, or to impact the Plains' Native American populations. An intensive breeding program was undertaken—this was among the first times that zoos took an active role to save a species from becoming extinct in the wild—and captive-bred bison were released back into the prairie. But most of the prairie has since been converted to farmland or extensively grazed by livestock. Therefore it would be impossible for bison herds to ever...