In Elton's account of the natural community as a simplified economy he suggested the following four principles that could describe the economy of nature as it operates everywhere on earth.

Elton called first of his principles the "food chain."

In every community, plants, through the photosynthetic conversion of sunlight to food, form the first link in a chain of nutrition. Food, one might say, is the essential capital in the natural economic order. The remaining links- usually no more than two or three, and almost never more than four- include the herbivorous animals and their predators.

A typical food chain in North American oak woods might link acorns, quail, and foxes, or acorns, mice, and weasels; with some 200 species of birds and mammals alone feeding on the oaks. The potential number of food chains is extraordinarily large. They could be found in nature by the thousands, all showing a common pattern but no two alike in every respect. And the bottom of the chain, rather than the top, is the most important link: The plants make the whole system possible.

Elton referred to the sum total of chains in any community as the "food web", an exceedingly complex design of crisscrossed lines of economic activity. Such webs are easiest to analyze in the relatively unpopulated arctic zones and almost impossible to untangle in the warm, humid tropics, where life forms abound. In every food chain certain roles must be performed. The plants, for example, are all "producers." Animals can be described as either first- or second-order "consumers," depending on whether they eat plants or other animals. Those animals that feed on the most numerous plants in a habitat, like the bison on prairie grasses, or copepods on diatoms in the sea, are the "key industries" in those economies.

In 1926, August Thienemann had introduced the terms "producer," "consumer," and "reducer," or "decomposer," which describe economic roles of organisms in a specific ecological setting; Elton now generalised them for every food chain in nature. These labels emphasized the nutritional interdependence that binds species together, and they became the boxes of resources with which ecologists would increasingly build models of materials and energy flows through ecosystems.

In general, an animal as large as an elephant cannot survive on food as small and lively as insects. It would exhaust itself in the chase and need more food than it could catch. A more substantial, stationary, and unresisting kind of food, the leaves, twigs and outer branches of trees and shrubs, is called for.

The giant whale is able to live on tiny crustaceans only because they are so numerous and easy to harvest. The law of nature decrees that each species has an optimum food size, and this law determines the structure of the food chain. "The very existence of food-chains," Elton noted, "is due mainly to the fact that any one animal can only live on food of a certain size. Each stage in an ordinary food chain has the effect of making a smaller food into a larger one, and so making it available to a larger animal." The chain thus becomes an ascending scale of larger and larger mouths and stomachs, except where a predator has the advantage of such special weapons as poison, like the spider, or group tactics, like the wolf pack. Even then, of course, the spider cannot kill and consume an elephant.

The only species that ignores these rules is civilised man, with his artificial techniques for more efficient food gathering. He can kill the largest animals on earth, or he can gather the smallest grain and seeds, and so eat lower on the food chain. But for Elton, modern man is distinctly an outsider, not to be confused with the mainstream economy of nature and its workings.

To serve as food for organisms higher up on the food chain, plants and animals near the bottom must be more numerous and reproduce more rapidly. Their fertility is a function of their position in the economic order: The smaller a creature, the more common it is and the faster it reproduces. In contrast predators at the top of the chain must reproduce more slowly than their prey, or they will end up with nothing to eat. They must also distribute themselves more sparsely across the land; hence every tiger looks for his own hill and defends it against other tigers, or he perishes. The demarcation and defence of private territories for food or breeding insures that each successful individual will have an adequate base for survival. Such territorialism goes on where it must among the birds and mammals, which are constantly required to adjust their density to their food supply.

The system of interacting populations resulting from these different behavioural requirements Elton called the "pyramid of numbers." A single plot of ground may provide a home for millions of microscopic soil bacteria, thousands of grass plants and insects, hundreds of trees, a few dozen rabbits, sparrows, and squirrels and at the apex only a single hawk. As one goes up the chain, the total weight of protoplasm at each level (the "biomass") declines along with the numbers. One lion, for instance, may eat fifty zebras in a year, so that the total physical mass of the predator is but a small fraction of the combined weight of its prey. The plants at the food chain's base constitute by far the greatest part of the mass of living substance on earth.