Outside every front door, life reigns. The air teems with pollen and spores. Bacteria the weight of a cow multiplies underneath a half-acre of soil. Studies in North Carolina and Pennsylvania have turned up hundreds of millions of insects per acre on samples five inches deep. The larger fauna might be more visible, but they still stay out of sight to the homeowners that host them: bobcats in Dallas, bears in Aspen, and coyotes as far north as Alaska. This biodiversity thrives with little effort from caretakers, as long as they don’t actively work against it—as they have been doing since the invention of the lawn.