The complex web of species interaction is full of odd associations. Stocking a lake with fish cuts down on dragonflies, which helps pollinators, which helps the flowers bloom. Or it can cut down on amphibians such as newts, which is bad for garter snakes. Invasive flowering purple loosestrife is good for insects and birds that feed on them, but hard on plankton, which is at the bottom of the food chain for everything. Martha Foley and Curt Stager look an unintended consequences of human actions in nature.