Eat your dinner! Everybody, whether you're a sophomore in high school or a fruitfly or the leaf of a banana plant, needs energy in order to grow and stay alive. To get that energy, organisms must eat. The term "food chain" refers to the flow of food through different levels of life. A "food web" is a collection of food chains.

Life on Earth is as diverse as anyone could imagine. It ranges from a bacterium to an elephant, and a lot more and everything in between. But one fact is common to all the creatures on the planet: Everybody must eat in order to continue living and to produce succeeding generations, whether they are butterflies or Bostonians.

Science has coined the term “food chain” to describe the flow of food (another word for energy) as it passes through the life of an ecosystem (that is, as succeeding levels of life consume that food and are consumed themselves). At first glance, the notion of such a chain may seem simple, but in fact such chains are quite complex. They are so elaborate that they must be thought of as part of even more complicated arrangements, which are known as food webs.

Everybody's got to eat, including this corn earworm. (Fred Powledge)

To understand how food chains and food webs function, it is necessary to learn a few expressions, or to learn new meanings for old terms. Ecologists, those scientists who study the interactions of creatures with their environment, think in terms of ecosystems. These are groups of living organisms and their environments—the entire package of creature and environment. Ecosystems are distinguished by the pathways that exist and the actions that take place among them. Perhaps the most important of these actions involves collecting food.

Populations are groups of organisms that have the same form, origins, and functions, and that (in most cases) are able to reproduce with one another. A population can refer to animals — housecats or humans — or to creatures that are much smaller — mosquitoes or bacteria.

Within a particular ecosystem, populations are sorted out by the ways they get and use energy. These groups are called comunities. Ecosystems are the ecologists’ way of describing the units in which communities trade energy and other materials with each other and with the physical environment around them. That environment, or ecosystem, can be extremely large (all of Earth) or medium-sized (a river and its environs) or quite small (a few drops of rain water caught in the folds of an orchid’s leaf).

Nothing that takes place within an ecosystem is more important than the exchange of food — for, without it, none of the ecosystem’s communities, populations, or individual organisms would be able to survive and reproduce. For decades, ecologists have tried to study that exchange to see what goes on within it. They have had the most success in ecosystems that are relatively isolated from their surroundings, such as islands or lakes. Researchers have come up with these categories of exchange; when you put them all together, they make up a food chain:

• The sun produces radioactive energy which allows green plants to grow. In land-based plants, sunlight reacts with the pigments that form chlorophyll in leaves and carbon dioxide to made sugars through the process known as photosynthesis. In the sea and other waters, energy is created when organisms (such as plankton) fall to the bottom and their chemical constituents are consumed by the benthic (depth dwelling) marine life that lives there. The plants and plankton are called the primary producers of the food chain.
  
  
• The primary consumers are the creatures that consume the primary producers. Also known as herbivores, or plant-eating animals, these include insects, rabbits, or grasshoppers. They get their energy from organic matter in the producers, and they produce organic matter themselves — their feces and, later when they die, their own decomposing tissues.
  
  
• The secondary consumers are the ones who eat the primary consumers. They are known as carnivores, or meat-eating animals. The rabbit that eats the plant may in turn be consumed by a fox. If you eat a hamburger, you're a secondary consumer. The consumer at the top of the chain is called the top carnivore.
  
  
• Another component of the food chain is active at all levels. That is the reducer, or decomposer — Nature's undertaker. Bacteria, fungi, and some protozoa are constantly at work, decomposing dead plant matter and plant-eating animals and meat-eating animals and their feces. They reduce all these organisms to nutrients that are again used in the food chain. The decomposers are key players in nature’s great recycling center.

The scientist who thought up the term, “food chain,” British zoologist Charles Elton, also saw that chains were too complex to describe everything that was happening in the transfer of energy. So in 1927, Elton published a book in which he descibed “food webs,” as well. In each community of organisms, there may be many food chains. The relationships between and among all the chains in a community are known as a food web. A food web, then, is a collection of food chains.

Elton and others also came up with the idea of an energy pyramid. They reasoned that as food energy is transferred from a lower level to a higher one, as much as 90 percent of it is lost through respiration and heat — and that’s 90 percent lost at each level. Thus it’s easy to see who, usually, there are so many organisms at the bottom of a food chain and so few at the top.

Some researchers have described the pyramid in terms of weight. In one academic report, scientist G.E. MacGinitie wrote that 10,000 pounds of algae make 1,000 pounds of tiny crustaceans; 1,000 pounds of tiny crustaceans make 100 pounds of small fish; 100 pounds of small fish make 10 pounds of large fish, and 10 pounds of large fish make 1 pound of a human. That was written in 1935, and the 10 percent rule is still accepted today.