Images below by Tom Dempsey stimulate thoughts on how people have impacted world ecosystems over human history. People have long modified vast ecosystems by burning ancestral forests into grasslands, industrializing agriculture, and paving and building to support today's worldwide population of 7 billion. Human destruction or transformation of nature is often irrevocable, as when species are forced into extinction. Plant and animal pioneers can quickly reclaim areas left alone by people, but the invasive weed species usually overwhelm previously-diverse natural environments.