Energy flows through ecosystems. Plants collect the sun's energy with their leaves and use it to transform water and carbon dioxide into high-energy carbohydrate molecules. When an animal consumes these molecules (such as when a herbivore like a rabbit eats a carrot), the stored energy is released and helps to fuel the animal's cellular activities, including the production of new molecules, cells, and tissues. If a carnivore then consumes the herbivore (for example, if a wolf eats the rabbit), the carnivore will in turn be fueled by the energy stored in the herbivore's tissues. In this way the solar energy originally collected by plants is transferred from one organism to another.

The passage of energy through an ecosystem is anything but tidy, however. At every step of the food chain a tremendous amount of energy is used up or lost, which makes it unavailable to the next level of consumers. The most efficient plants, for example, convert less than 3 percent of the solar energy available to them into plant tissue, and much of this tissue is never eaten (although it does become available to decomposers, a very important component of the biota in an ecosystem). And of that which is eaten, only 10 percent on average is converted into herbivore tissue; the other 90 percent of the energy herbivores consume goes toward maintaining normal life functions or is passed on as waste. And so it goes until, at the top of the food chain, only a tiny fraction of the energy once absorbed during plant photosynthesis has been maintained and stored in the tissues of top-level carnivores.

The amount of energy that gets transferred from one level of the food chain to the next can be represented by a pyramid, with plants at the base and carnivores at the peak. In general, the shape of the energy pyramid reflects the relative numbers of individuals at each feeding level. But more accurately, it reflects the amount of tissue, called biomass, that each type of organism contributes to the ecosystem. For instance, if biologists weighed all of the organisms living on 100 acres of wilderness, they would find that the sum of all plants weighed the most, followed by all of the herbivores, and then the carnivores.

Because of the energy loss that occurs at each step of a food chain, there is a limit to the number of feeding levels, called trophic levels, a given ecosystem can have. In fact, most food chains are made up of only three or four feeding levels. An ocean ecosystem, for example, may have shrimp that feed on plankton, small fish that eat the shrimp, and larger fish and mammals that eat the fish. Each trophic level has in it fewer animals than the level below. In fact, there are simply too few animals at the top of the food chain to sustain a group of animals one level higher that feeds only on them.