Last fall, the U.K.'s Department for Environment, Food and Rural Affairs targeted badgers for culling—the selective killing of a species as a population control measure.

Proposed culls have made headlines in the United States as well. Wildlife managers have targeted bison in Montana and swans, geese, and deer in New York.

What's driving these high-profile culling programs? Are they necessary? Can they be done ethically? And what's at the heart of the debate between their proponents and their detractors?

We explored the controversy over culling with Mary Pearl, a conservationist with the City University of New York who formerly served as president of the Wildlife Trust, a nonprofit organization now called EcoHealth Alliance.

I would say nature has been a culler, in