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The planet has an estimated 7 million animal and plant species occupying every ecological niche, with predators being the smallest niche. All major ecological roles have been occupied since the Cambrian explosion, with predators evolving from herbivores. Ecological niches function in a pyramid format, with producers at the bottom and apex predators at the top.
There are a number of ecological roles. These include microscopic, small, medium and large herbivores, producers, predators, scavengers and parasites. There are an estimated 7 million animal and plant species on the planet today, most of them insects, occupying every imaginable niche. The most popular niches appear to be herbivorous and parasitic, with predators being the smallest niche. However, it also seems that, of all ecological roles, that of predator most captures the popular imagination.
All major ecological roles have been occupied almost continuously since at least the Cambrian explosion, a major episode of evolutionary diversification about 542 million years ago. Some paleontologists have speculated that the evolution of predation is partly responsible for the evolutionary diversification that occurred during the Cambrian. In any case, by the end of the Cambrian, all major ecological roles were populated, except that the largest organisms were only about a meter long. During the following period, in the Ordovician, some animals (such as nautiloids) that were 3.5 m (11.6 ft) long evolved, and in the Carboniferous the size range of organisms was similar to today.
Although animals are generally highly specialized in their ecological roles, some animals share roles and some evolutionary lineages can evolve from one ecological role to another. For example, the ancestors of whales evolved from medium-sized land predators to whale-sized aquatic filter feeders (literally). Predators evolved from herbivores, and many predators are also scavengers. Many predators grow large and hardy not only to take down prey, but to compete with other large predators for a fetched kill. The top size of predators is determined by the amount of available prey. Under excellent evolutionary circumstances, such as those experienced time and time again by crocodile predators in swamps, predators can grow to enormous sizes, up to seven times larger than their recent ancestors.
Ecological niches function in a pyramid format, with producers such as plants at the bottom, herbivores in the middle, and predators at the top, with apex predators such as tigers at the top. The higher the animal is on the pyramid, the less biomass is generally dedicated to that species and the rarer it is. The animals at the top of the pyramid are generally also more susceptible to extinction, but not always. In some historical extinctions where most plant life was wiped out, the surviving animals were actually scavengers rather than dedicated herbivores or carnivores.
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