Of those that lived 20,000 years ago, many are gone: There are no more saber-toothed tigers or dire wolves or woolly mammoths. Many others, like rhinoceros and South China tigers, are so few in number that from an ecological perspective they're already extinct. Other big animals have retreated into fragments of their former ranges.

Despite the ubiquity of these changes, however, the ecological consequences are hard to perceive. Most people grew up in places where so-called apex predators and megaherbivores were already gone. Absence is a tricky thing to quantify. Ecologists have also been challenged to study processes that can involve subtle interactions over large regions, and take decades if not centuries to become apparent.

In recent years, however, the science has matured. Researchers have a better understanding of just how important large animals are to their environments. They affect what lives and grows, how nutrients cycle and even how disease spreads. Take them away — or reintroduce them — and you change the very nature of nature.

This research is described in a review published July 14 in Science. On the following pages, Wired.com looks at comparisons of environments without and with large animals.

At left, the coral reef ecosystem around Kirimati Island in the South Pacific, where fishing pressures have eliminated large fish. At right, a reef ecosystem around nearby Jarvis Island, which is unfished.

Image: Science