Every bite of food you have ever eaten was, at bottom, captured sunlight. Follow almost any meal far enough back and you reach a green cell turning light into chemical energy. Tracing these paths explains why some animals are common and others rare, and why the biggest animal on Earth eats some of the smallest.

Producers, consumers and decomposers

Ecologists sort organisms by how they get energy. Producers, also called autotrophs, make their own food. Through photosynthesis, producers capture light energy and store it in sugars. On land the main producers are plants; in the oceans they are mostly microscopic phytoplankton, which carry out roughly half of all the photosynthesis on the planet. Consumers must eat other organisms. Herbivores, or primary consumers, eat producers; carnivores that eat herbivores are secondary consumers, and those that eat other carnivores are tertiary consumers. Omnivores, including most humans, feed at several levels. Decomposers, such as fungi and bacteria, break down dead matter and return its nutrients to soil and water.

For a long time biologists assumed that all life ultimately depends on the sun. In 1977 scientists diving in the submersible Alvin near the Galápagos Islands found thriving communities of giant tube worms, clams and crabs around hot springs on the deep sea floor, far beyond the reach of light. There, bacteria make food from chemical energy, such as that in hydrogen sulfide, rather than from sunlight. This process is called chemosynthesis.

Trophic levels and the ten percent rule

Each feeding step is called a trophic level, from the Greek word for nourishment. In 1942 the young American ecologist Raymond Lindeman, drawing on his study of Cedar Bog Lake in Minnesota, argued that ecosystems could be understood by tracking energy as it moves between these levels. His work led to a famous rule of thumb: only about ten percent of the energy at one trophic level is stored at the next. The real figure varies, often between five and twenty percent, but it is always far below one hundred. The rest is spent on movement, growth and keeping warm, lost as heat, or never eaten or digested at all.

This steady loss has large consequences. A field of grass can support many rabbits, fewer foxes, and very few eagles. Food chains rarely have more than four or five levels, because too little energy remains at the top to support another. Drawn as a diagram, the energy at each level forms a pyramid, wide at the base and narrow at the peak. It also explains why large predators need huge territories.

From chains to webs

The British ecologist Charles Elton described food chains and the pyramid of numbers in his 1927 book Animal Ecology. Real feeding relationships, however, branch and cross. A fox eats rabbits, mice, berries and beetles; a hawk competes with it for mice. A food web maps all these links. For decades ecologists assumed that more complex webs were more stable, since a predator with many prey can switch when one declines. In 1972 the physicist turned ecologist Robert May challenged that idea with mathematics, showing that randomly assembled complex systems tend to be less stable, not more. Since real ecosystems persist, his result pushed researchers to ask what special structure makes natural webs hold together, a question still being worked out.

Energy flows, matter cycles

A key distinction runs through all of ecology: energy flows through an ecosystem in one direction and is lost as heat, while matter such as carbon and nitrogen is recycled again and again. Nitrogen is abundant in air but useless to most organisms until certain bacteria, including those living in the root nodules of beans and clover, convert it into usable forms.

Biomagnification

Some substances do not break down and are stored in fat rather than excreted. Biomagnification is the rise in concentration of a persistent toxin at each step up a food chain. In the 1950s, Clear Lake in California was treated with the pesticide DDD to kill gnats; the water held tiny amounts, but fish-eating grebes died with concentrations many thousands of times higher. The related pesticide DDT thinned the eggshells of bald eagles, ospreys and peregrine falcons. Mercury builds up the same way, which is why health agencies advise pregnant women to limit large predatory fish such as swordfish.

Worked example: the Southern Ocean

Around Antarctica, phytoplankton bloom in the summer light. Shrimplike krill graze on them, and penguins, seals, fish and great whales eat the krill. A blue whale feeds just two steps above the producers, which is why the largest animal ever known can be supported: little energy is lost along so short a chain. Whaling in the twentieth century removed most of the great whales, and many expected krill to boom without them. In some areas krill instead declined. One leading hypothesis is that whales were part of the nutrient cycle, releasing iron-rich waste near the surface that fertilized the phytoplankton. If so, removing a top consumer starved the base of its own food web, a reminder that matter cycles through every level, not just downward.