Who eats whom
An ECOSYSTEM is a group of living things and the place they live, all depending on each other. Every one starts with a PRODUCER, a plant or tiny plant-like plankton that makes its own food from sunlight. A CONSUMER eats the producer, and is eaten in turn by a PREDATOR, while its PREY tries not to be. Drawn as a line, that is a FOOD CHAIN; drawn with all the crossing links in a real HABITAT, it becomes a tangled FOOD WEB.
Only about a tenth of the ENERGY at each step is passed on to the next; the rest is used up living, moving and keeping warm. That is why there are so many more plants than plant-eaters, many more plant-eaters than meat-eaters, and so few top predators.
Nothing goes to waste
A DECOMPOSER, such as a fungus, a bacterium or an earthworm, breaks down dead plants and animals and returns their NUTRIENTS to the soil for plants to use again. Without them, dead leaves and bodies would simply pile up. Around 300 million years ago, vast swampy forests grew in what is now Britain and much of the world. Their fallen trees were buried in such huge amounts that they became coal, and some scientists think part of the reason is that the fungi able to rot wood had not yet evolved to deal with them.
Otters, urchins and kelp
Some species matter far more than their numbers suggest. On the Pacific coast of North America, sea otters eat sea urchins, and sea urchins eat kelp, the giant seaweed that forms underwater forests. Where otters were hunted almost to extinction in the 1800s and 1900s, the urchins multiplied and stripped the kelp bare, leaving a barren seabed. Where otters returned, the urchins were kept in check and the kelp forests grew back, along with the fish and other creatures that shelter in them. A species like that is called a keystone species.
Wolves back in Yellowstone
When wolves were brought back to Yellowstone National Park in the United States in 1995, they began hunting elk again. Scientists still argue about how much the wolves changed the park's plants and rivers, but they agree that one returning predator can ripple through a whole COMMUNITY.
Biomes from the tundra to the reef
Large areas with a similar climate and similar kinds of life are called a BIOME. The TUNDRA of the far north is cold and treeless, with frozen ground just below the surface. The SAVANNA of East Africa is grassland scattered with trees, home to huge herds of grazing animals. A tropical FOREST and a CORAL REEF hold an astonishing number of SPECIES in a small space. A WETLAND, such as a peat bog, looks unremarkable but stores huge amounts of carbon; peatlands cover only about three per cent of the land but hold more carbon than all the world's forests.
Every species has its own NICHE, its particular job and way of life, and every ADAPTATION, from a cactus's spines to a polar bear's fur, helps it fit. A POPULATION is all the members of one species in a place. Our coral reef page looks closely at one of the richest ecosystems of all.