You don’t forget the images: the olive ridley sea turtle, a plastic straw lodged in his nose. The Rubenesque sea lion, neck cinched tight by a thick plastic packing strap. The Cuvier’s beaked whale, stomach split open to reveal more than 80 pounds of plastic waste—snack bags, rope, rice sacks...

It all began with a deer on the cover of All Animals magazine. Terry Kline, administrator of the Botstiber Foundation outside Philadelphia, saw the photo in the spring of 2014 and turned to a feature about managing deer populations with fertility control (“Out of season,” May/June 2014). For years...