Humans frequently disrupt the balance of nature but generally organisms in the ecosystem are adaptable to each other. The waste products produced by one species are used by another, and the resources required by some are replenished by others.
For example, the oxygen needed by humans to live is produced by plants, while the waste product of breathing, carbon dioxide, is used by plants as a raw material in photosynthesis; the process by which oxygen is made.
An old concept, balance of nature ruled ecological research for many decades and led to a popular belief among conservationists that nature was best left to its own devices, and that human intervention was unwelcome.
However, this has been largely discredited - chaotic changes in population levels are common, and there are examples in history showing that several modern day habitats originate from human interference.
For example, the abundance of grazing animals on Africa's Serengeti plain is thought by some ecologists to be partly due to human-made fires creating the fertile Savannah habitats found in Kenya.