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environment. For almost 3 billion years, life was limited to the oceans and consisted
of bacteria, blue-green algae, and other relatively simple one-celled forms. Then
complex cells evolved, incorporating organelles such as nuclear membranes, chloroplasts,
and cilia. Soon afterward, these cells evolved into still more complex multicellular
animals and plants. About 600 million years ago, the concentration of
oxygen in the atmosphere climbed rather quickly (by geological standards) to near
its current level, destroying most of the anaerobic life in the oceans and on land
surfaces. A shield of ozone accumulated in the stratosphere, protecting life from
harmful ultraviolet irradiation. For the first time, substantial numbers of larger
animals filled the seas, and the global variety of life climbed sharply. Plants invaded
the land, then animals, represented first by small arthropods and other invertebrates,
then jawless fishes. The diversity of life continued to rise. Biodiversity stalled on a
plateau during most of the Mesozoic Era, then climbed gradually to its current
high level. It is a supreme irony that mankind, the great destroyer of life, began as
one of the products of the living world’s maximum proliferation.
A second major principle of biodiversity is that smaller organisms are generally
more diverse than larger ones. The reason appears to be simply that they fit into
smaller spaces, consume less food individually, complete their life cycles more quickly,
and hence are able to divide the habitats in which they live into smaller and more
numerous niches. And the more numerous the niches, the more species that can be
packed into the same location. Take a typical epiphyte-laden tree in the rainforest
of Peru. It may be the home of several hundred species of beetles, 40 species of ants,
and as many as 50 species of orchids and other epiphytes. But it can only be the
partial home for a flock of parrots, which must range over portions of the forest that
contain many thousands of such trees in order to obtain enough food for survival.
Among smaller animals, insects dominate diversity. About 750,000 of the 1 million
animal species described to date are insects, and some estimates have placed the
actual number as high as 80 million. The reason for this amazing disproportion is
uncertain. It seems likely due to the metamorphosis experienced by the majority of
kinds of insects during the individual life cycle: egg to larva to pupa to adult, with
the egg and pupa as passive transitional stages and the larva and adult as the active
stage. Larvae and adults are radically different in appearance (recall the caterpillar
and butterfly), typically feed on different foods, and even live in different sites. As
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