According to modern ecological theory, high diversity at any trophic level of a community is possible only under the influence of cropping. Until herbivores evolved, single-celled algae of the Precambrain were resource-limited, and a small number of species saturated aquatic environments. In the near-absence of vacant niches, life diversified slowly. Because the changes required to produce the first algae-eating heterotrophs were therefore delayed, the entire system was self-limiting. When the "heterotroph barrier" was finally crossed in the late Precambrian, herbivorous and carnivorous protists arose almost simultaneously, for no major biological differences separate the two groups. These events automatically triggered the formation of a series of self-propagating feedback systems of diversification between adjacent trophic levels. Comparable systems arose among multi-cellular groups, which radiated rapidly from the newly diversifying protist taxa. The sudden proliferation of complex food webs formed by taxa invading previously vacant adaptive zones produced an explosive diversification of life over a period of a few tens of millions of years. The rapid appearance of skeletons in various groups, though of special geological importance, was no more dramatic than other aspects of the radiation. The overall rate of diversification was comparable to rates for less-extensive adaptive radiations of the Phanerozoic.
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