Plants and animals are living things: they come to life, grow and change, and all the while they use energy to work and to repair themselves. Every living thing, plant and animal alike, dies at some time. When it does, its body stops working, and a body that has died does not start working again. Dead is forever. That is the plain biology, and it is only the beginning of the story, because the nutrients in a dead body go back into the soil, where plants can use them again.

When a person dies, they no longer breathe, eat or sleep. Beyond that, people and families believe different things about what death means, and it is a good question to put to the grown-ups in yours. Death can be hard to think about, and wanting to understand it is natural. What happens to the bodies of plants and animals afterward can be observed, and it turns out to involve a surprising amount of life.

Dead plants and animals are food. The eaters are called decomposers, a group that includes bacteria, fungi and earthworms, and they are often described as nature's recyclers, since without them waste would pile up. Bacteria are tiny living things, and the ones that live on dead material break it down into nutrients that go back to the soil. A fungus cannot make its own food, and because fungi have no chlorophyll, they cannot make food from sunlight the way plants do. Instead they release enzymes that break down dead plants and animals, then take in some of the simpler substances to grow, while other organisms, including plants, use the rest. Fungi can even break down tough parts of wood and plants, such as lignin, chitin and cellulose.

Earthworms eat dead plants and animals and loosen the soil as they burrow. Their droppings, called casts, are rich in nitrogen, phosphorus and potassium, which help plants grow. Humus is dark, nitrogen-rich soil matter that forms when plant and animal matter decay, and it holds nutrients for plants. Soil organic matter also acts like a sponge, able to hold up to 90 percent of its weight in water and then release most of it. In grasslands, forests, lakes and farm land alike, decomposition keeps soil fertile. The soil under your feet is, in a real sense, made of what once lived.

People copy this process on purpose. Composting lets tiny microorganisms break down materials like leaves and food scraps into compost, which feeds soil. The microorganisms in a compost pile use carbon and nitrogen to grow and reproduce, water to digest materials, and oxygen to breathe. Their busy lives show up on a thermometer: a well-kept backyard compost pile can reach 130 to 160 degrees Fahrenheit. A forest does something similar with a fallen dead tree, which can become a nurse log. New baby trees grow on it, getting shade, nutrients and water. As the log decays it fills with soil, moss, mushrooms and small plants.

Some of the first visitors to a carcass arrive on wings. Scavengers such as vultures and sea eagles find dead animals quickly and eat them, and insects are among the first to arrive too. As the insects eat, they help move nutrients into the soil. Blowflies are thought to double the rate at which a dead animal breaks down, and burying beetles coat a carcass in chemicals that suppress bacteria and repel ants. Many germs on a dead animal cannot survive passage through a vulture's highly acidic gut, so scavenging can reduce the spread of disease. A 1979 study estimated that vultures in Serengeti National Park, Tanzania ate about 14 million kilograms of meat a year, more than all the mammal meat-eaters there combined.

The ocean's depths get their food from nutrients falling down from the surface waters, so food there is often hard to find. When a whale dies and sinks to the sea floor, its body feeds deep-sea animals, an event called a whale fall. Scavengers such as sharks, hagfish, rattails and crabs eat the flesh off the bones, and they finish the soft parts in a matter of months, after which different animals arrive. Microbes live off chemical energy from sulfides released as whale bones decay, and they form the base of ecosystems for as long as the food lasts. In deep water a whale skeleton can feed animals for years, even decades, but in shallow oceans a whale fall may vanish within a year or two.

No two whale falls unfold alike. Whale carcasses in Monterey Canyon are expected to decompose in under 10 years, while whale carcasses studied off Southern California may last 50 to 100 years, and each of the studied whale falls followed a different sequence of community development, with different key species. Osedax worms are a foundation species for whale falls, not just another weird deep-sea animal. A whale fall found in 2009 off Canada was watched for over a decade, and tube worms were still living on its jaw bone. About four months after a whale died on Davidson Seamount, its carcass was crawling with octopuses, eelpouts, worms, grenadiers and crabs.

Pacific salmon show the same exchange in a river. After they lay their eggs, they die, and their bodies feed scavengers or rot, adding nutrients to the stream. Most Pacific salmon stop eating when they return to fresh water, so they lack the energy to swim back to the ocean, while Atlantic salmon do not die after spawning and can spawn again over several years. Riverside plants fed by dead salmon shade and cool the water and attract insects that young salmon eat. Fallen trees make pools where young salmon hide, and as those trunks rot they return nutrients to the river. The river and its banks are tied together by what dies in it.

Slime molds help decompose dead plants, though they are no longer classed as fungi, which shows how many kinds of life join in the work. Decomposers recycle dead plant and animal tissue, which helps life continue in all of Earth's ecosystems, and good ecosystem care includes keeping decomposers of all sizes active so the system stays productive. A dead body is an ending for one living thing and the start of work for many others. If thinking about death brings up questions or sad feelings, a parent, a teacher, a school counselor or another grown-up you trust is the right person to talk with.

Words to know

• decomposers: nature's recyclers
• enzymes: breakdown chemicals
• humus: dark soil matter
• scavengers: dead-food eaters
• sulfides: sulfur chemicals