Those conflicts often pit neighbor against neighbor. After a small dog in Wheaton, Ill., was mauled by a coyote and had to be euthanized, officials hired a nuisance wildlife mitigation company. Its operator killed four coyotes and got voice-mail death threats. A brick was tossed through a city official's window, city-council members were peppered with threatening emails and letters, and the FBI was called in. After Princeton began culling deer 12 years ago, someone splattered the mayor's car with deer innards.

Welcome to the nature wars, in which Americans fight each other over too much of a good thing—expanding wildlife populations produced by our conservation and environmental successes. We now routinely encounter wild birds and animals that our parents and grandparents rarely saw. As their numbers have grown, wild creatures have spread far beyond their historic ranges into new habitats, including ours. It is very likely that in the eastern United States today more people live in closer proximity to more wildlife than anywhere on Earth at any time in history.

In a world full of eco-woes like species extinctions, this should be wonderful news—unless, perhaps, you are one of more than 4,000 drivers who will hit a deer today, or your child's soccer field is carpeted with goose droppings, or feral cats have turned your bird feeder into a fast-food outlet, or wild turkeys have eaten your newly planted seed corn, or beavers have flooded your driveway, or bears are looting your trash cans. And that's just the beginning.

In just a few decades we have turned a wildlife comeback miracle into a mess that's getting messier, and costlier. How did this happen? The simple answer: Forests grew back over the past two centuries, wildlife came back over the past century and people sprawled across the landscape over the past half-century.