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Abstract

The ecosystems and biota we see today represent the evolution of life
over a period of almost four billion years. Over the past several hundred
million years the diversity of life has changed markedly as a result of
evolutionary and other forces. Environmental fluctuations have led to
changes in ecosystems and biota. Over the past 15 million years or so
many regions of the planet have become more arid and this has led to
changes in their constituent biota. As a result of changes in climate over
the past two million years, glaciers have expanded and retreated a number
of times. This has resulted in major expansions and contractions in the
range of biota and ecosystems such as estuaries, forests and savannas.
While these fluctuations have affected biological diversity, the rate of
change has often been sufficiently gradual to allow organisms to adapt.
They have, for example, been able to evolve, migrate and persist in
refugia as a response to unfavourable conditions. While natural environmental changes and calamities have at times destroyed ecosystems and
many organisms, populations of many species have survived and evolutionary
processes have continued.

Item Type:	Book Section
Authors/Creators:	Norton, TW
Publisher:	Prentice Hall/Pearson Education Ltd
Additional Information:	© 2001 Pearson Education Ltd
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