Plants can only use so much of the energy they absorb from the sun. It is difficult to measure, but certainly no more than an eighth of one per cent of the sunlight falling on a plant can be stored by being converted into sugar. The chemical energy in the sugar is used by the plant to build up the complicated organic compounds that go to form its structure. It is this sugar, and the energy contained in it, that a herbivorous animal obtains when it eats grass. However, it cannot convert all the plant's stored energy into its own requirements - the maximum efficiency of an animal is about ten per cent. This ten per cent factor is present at all stages of the food chain and means that in any environment a hundred herbivores can support only ten carnivores, and these ten carnivores can in turn support only one "second-stage" carnivore. These figures are oversimplified and refer to animals of the same size. The important factor is the weight of the animals rather than the numbers of individuals. The ten per cent factor holds true for every stage in the complicated pattern of food chains and is an important factor leading to the stable shape of the food pyramid.
The dependency of feeding efficiency on sunlight is the reason why different parts of the earth support quite different numbers of organisms. In the tropics, where the sunlight is intense, much more solar energy is available to be absorbed by plants. Hence, where other factors such as rainfall allow, there is more vegetation per unit area than in temperate or polar climates. This large amount of plant material is able to support a large number of herbivorous animals, which in turn support a large number of carnivores. In the Arctic, on the other hand, the low level of solar energy produces a much more sparse vegetation, and hence there are fewer herbivores and even fewer predators.
The variety of species at each level in the pyramid depends on the variety of plants at the base. On tropical grassland, for example, where there are short grasses, herbs, tall grasses, bushes and trees, each of the large number of indigenous animal species eats a different collection of plants. Therefore the animal that eats roots does not compete with the animal that eats the low herbs or the animal that eats the tall grasses. Even those that do have a broadly similar diet are sufficiently different in some way so as not to compete directly - for example, one may eat in the daytime and the other at night. In this way the ecological niches are multiplied and the processes of evolution ensure they are all filled.
The principle that �nature abhors a vacuum� is as true in biology as it is in physics. An ecological niche is never left vacant for long and something will evolve to occupy it as soon as one appears. Within each species, however, competition is strong and each particular niche will support only so many individuals. Struggles between members of the same species are usually formalized into stylized displays in which little real damage is done. Territory is preserved and mates are chosen without recourse to any actual combat. This appears to be the strategy that leads to greatest success in maintaining a creature's position in the ecosystem.
The predations of carnivores do little to upset the balance of the food pyramid. By preying only upon the weak, sick and elderly - a practice forced upon them by the fact that a healthy adult can usually outrun or fight off an attack - it ensures that only the fittest survive. If the fit, healthy adults of a species cannot outrun or fight off an assault their species will swiftly become extinct and the niche will be taken by another creature. In this respect predators can be thought of as no more than impatient scavengers.