A velvety butterfly called the coastal green hairstreak once peppered the fog-capped hills of San Francisco. But then the city began to build out, and new structures fractured the jewel-toned insect’s native range. The butterflies were scattered into isolated populations, some only a few blocks apart—an astronomical distance for a creature that, over its lifetime, can venture only a few hundred feet or so from the native shrubs it depends on. In 2007, when the species was nearly gone from San Francisco, a local nonprofit got to work on a simple solution: Create a butterfly highway between some of the scattered populations by adding more of the insect’s host plants. “We planted microhabitats wherever we could get—a swath of a triangle or a long stairway edge,” says Amber Mahlberg, executive director of Nature in the City, the organization leading the project. It worked. Butterflies from previously isolated populations followed the breadcrumb trail and interbred, boosting their genetic diversity and odds of long-term survival. Microhabitats like these are good for wildlife like butterflies, and they benefit humans, too: Mounting research shows that small green spaces could improve the health of people who live nearby. “Americans tend to think about [the] nature...