All of these ecosystem services are valued and are reasons why many people in our societies wish to ensure that forests are protected or managed sustainably. But sustainability is a tricky thing to define. It reflects a suite of societal values such as those listed earlier, and it necessarily implies a time frame. For how long will we be able to sustain these values? Species go extinct. Climate changes. Forests burn and regrow. And throughout these disturbances and other changes that occur in forests, habitat for species is destroyed and regrows, or it may be eliminated or appear in a place where it never occurred before. The one thing that is constant about our environment is change, and the way that species, including humans, have persisted in the face of those changes is through maintaining the genetic diversity and behavioral flexibility to allow some members of each species to persist.