What Is an Ecosystem? Living Things and Their Environment

Ecologists often divide ecosystem components into biotic and abiotic factors. Biotic factors are living or once-living parts of the system, including plants, animals, fungi, and microorganisms. Abiotic factors include light, water, temperature, soil, minerals, salinity, and other physical or chemical conditions that influence life.

The central idea in an ecosystem is interaction. A tree uses sunlight, water, carbon dioxide, and soil nutrients. Insects may eat its leaves, birds may eat those insects, fungi may decompose fallen material, and microorganisms can transform nutrients in the soil. None of these relationships makes full sense when studied completely in isolation.

Most ecosystems receive energy from sunlight. Producers such as plants capture some of that energy through photosynthesis and store it in chemical compounds. Consumers obtain energy by eating producers or other consumers, while decomposers gain energy from dead material and waste. At each transfer, some energy becomes less available for further biological work.

Matter behaves differently because atoms can be reused. Water, carbon, nitrogen, phosphorus, and other materials move between organisms and the physical environment. When revising ecosystems, this contrast is useful: energy generally flows through the system, while matter can cycle again and again through different reservoirs and organisms.

A pond can be treated as an ecosystem, but it is not sealed off from everything around it. Rain carries materials in, streams may carry water out, birds move between habitats, leaves fall from nearby trees, and sunlight enters from outside. Scientists choose a useful boundary so they can study particular flows and relationships.

That means an ecosystem does not have one fixed physical size. A rotting log can be studied as a small ecosystem, while an entire lake or forest can be treated as a larger one. The chosen scale changes which interactions become most important. Good ecology questions therefore identify both the organisms involved and the environmental conditions that connect them.

Ecosystems are not static. Some species matter far more than their numbers suggest: remove sea otters from a kelp forest and sea urchins, no longer eaten, graze the kelp to bare rock. Ecologists call these keystone species. Ecosystems also change over time through succession. Bare rock left by a retreating glacier is colonised first by lichens and mosses, then grasses, then shrubs and trees over decades, each stage altering the soil and light for the next.

Disturbances such as fire, flood or storm restart parts of that sequence, and many systems depend on them: some pine cones open only in the heat of a fire. This is why a healthy ecosystem is better described as resilient than as balanced. It absorbs shocks and recovers, and the question in conservation is usually how much disturbance a system can take before it shifts into a different state.

Nobody can count every organism in a forest or lake, so ecologists sample. For plants and slow moving animals, a common tool is the quadrat, a square frame placed at random points so the species inside can be counted and scaled up to estimate the whole area. Where conditions change across a landscape, such as from a shoreline up a beach, ecologists run a transect, a line along which they record organisms at regular intervals.

Mobile animals need other methods. In mark and recapture, researchers catch a sample, mark them harmlessly and release them. Later they catch a second sample and note what fraction is already marked. If one in ten is marked, the population is roughly ten times the number originally marked. Scientists also measure abiotic factors such as light, temperature, soil moisture and water chemistry alongside the counts, because patterns only make sense when living and nonliving data are compared.

One way to see how complicated ecosystems are is to try building one. In 1991 eight people sealed themselves inside Biosphere 2, a large glass-and-steel enclosure in the Arizona desert containing a small rainforest, desert, savanna, marsh, an ocean with a coral reef and a farm. The plan was for the crew to live for two years in a closed system, with air, water and nutrients recycled by the organisms inside. It became one of the most instructive experiments in ecology, largely because of what went wrong.

The oxygen level inside fell steadily, from about 21 percent towards 14 percent, low enough to leave the crew short of breath, and extra oxygen had to be pumped in. The cause lay in the soil, which was rich in organic matter that microbes broke down, consuming oxygen and releasing carbon dioxide. Much of that carbon dioxide was then absorbed by exposed concrete in the structure, so the missing oxygen was hard to trace at first. Many pollinating insects died out, while ants and cockroaches spread, showing how hard an ecosystem's behaviour is to predict.

A rough rule helps explain why food chains are short. On average, only about ten percent of the energy stored at one feeding level ends up stored at the next. Imagine grassland plants that build 10,000 kilojoules of energy into their tissues. Rabbits eating those plants might store about 1,000 kilojoules in their own bodies. Foxes eating the rabbits would store roughly 100, and a predator that ate the foxes would gain only about 10. Real efficiencies vary, often somewhere between about five and twenty percent, but a steep drop at each step appears in almost every ecosystem.

The missing energy has not vanished. Much of it is used for movement, growth and keeping warm, then lost as heat through respiration. Some sits in parts that are never eaten, such as roots, bones or fur, and some passes out in faeces, where decomposers use it instead. This pattern explains why a fifth or sixth link in a food chain is rare: too little energy is left to support a viable population of animals at that level. It also explains why land growing crops that people eat directly can feed far more people than the same land used to raise animals for meat.

In 1977 scientists diving in the submersible Alvin near the Galápagos Islands went to study hot springs on the deep sea floor, expecting little life at depths that sunlight never reaches. Instead they found dense communities of giant tube worms, clams, mussels and crabs clustered around the vents. The food chain rested on bacteria that gain energy by chemically oxidising hydrogen sulphide in the hot vent water, a process called chemosynthesis, and use it to make sugars from carbon dioxide. The discovery overturned the assumption that almost every ecosystem depends on photosynthesis.

The giant tube worm shows how close these partnerships can be. Adults have no mouth or gut. Instead they carry billions of the bacteria inside a special organ, and their red plumes, coloured by a form of haemoglobin, collect hydrogen sulphide, oxygen and carbon dioxide to supply them. Vent communities are not completely cut off from the Sun, though, because much of the oxygen that vent microbes use comes from photosynthesis near the surface. Individual vents can also shut down within years or decades, so their animals rely on drifting larvae to find and colonise new ones.

An ecosystem is a system formed by living communities and their nonliving environment interacting together. Look for biotic and abiotic factors, trace energy through feeding relationships, and follow matter as it cycles. Most importantly, remember that an ecosystem is about relationships rather than a simple list of organisms.