One important fact that is often forgotten is that ecosystems are, as it were, living
structures. They are constantly changing and subject to evolution, so that at various times
they will be composed of different organisms. According to pollen data and information
gathered from fossil bones, few modern ecosystems are the same as they were 10,000
years ago. They originate bit by bit as the environment changes, and as species become
extinct or shift their geographical range or their anatomy or behavior.