"Many people are under the illusion that before the meddling of humans, the populations of different types of plants and animals tended to be pretty much constant. This isn’t really the way things work, however, in a finite world. Instead, the populations of many species cycle up and down, depending on particular conditions such as the population of animals that prey on them, the availability of food, the prevalence of disease, and the weather conditions. Even forests exhibit surprising variability. Many undergo regular cycles of burning. In fact, some species of trees, such as the giant sequoias in Yosemite, require fire in order to reproduce. These cycles are simply part of the natural order of self-organizing ecosystems in a finite world."