If an ecosystem has been severely damaged, can it recover? Almost never, according to a new study—even when active restoration takes place. Researchers from Northern Illinois University looked at 400 studies of ecosystems worldwide that had been destroyed or damaged by logging, mining, oil spills, agriculture, or nutrient pollution. Human-driven restoration didn’t speed up their recovery, or restore biodiversity any better than simply leaving areas to recover on their own. The study’s authors say their research reinforces the need to conserve intact ecosystems, because disturbed ones take such a long time to recover, and never return to their previous state.