Ecosystem Interactions and Cycles of Matter

Living together

On the Food Webs and Ecosystems page you followed energy as it flows from producers to consumers, and saw why there is less of it at each level. This page looks at two other big ideas. The first is how populations of different species interact: they can help each other, harm each other, or fight over the same things. The second is how matter moves. Energy flows through an ecosystem and is lost as heat, but atoms are not used up. They are passed around again and again in cycles.

All the populations living in one place at the same time make up a community. Ecologists sort the interactions in a community by asking a simple question: does each partner gain, lose, or stay about the same?

Symbiosis: three ways to live together

Symbiosis means two different species living in a close relationship, often for a long time. There are three main kinds.

• Mutualism (+/+). Both species gain. Termites cannot digest wood on their own; microbes living in their gut break it down, and the microbes get food and shelter. A clownfish lives among the stinging tentacles of a sea anemone, which protect it from predators, and the clownfish chases away butterflyfish that would eat the anemone.
• Commensalism (+/0). One species gains and the other is neither helped nor harmed. A bird nesting in a tree is a common example. Cattle egrets follow grazing cattle and snap up the insects the cattle stir out of the grass.
• Parasitism (+/−). A parasite lives on or in another living thing, its host, and takes food from it, harming it but usually not killing it quickly. A tapeworm can live in a person's intestine for years, feeding on the food the person eats. A cuckoo is a different kind of parasite: the female lays her egg in another bird's nest, and the host parents raise the cuckoo chick instead of their own young.

Real relationships are not always this neat. The balance can shift with conditions, and scientists still argue about some famous examples. That is why the examples here are ones that have been studied closely.

Competition and predation

Competition (−/−)

When two populations need the same limited resource, such as food, water, light, or space, they compete, and each makes life harder for the other. In a classic laboratory experiment, two species of Paramecium each did well when grown alone. Grown together in the same tube on the same food, one species outcompeted the other until it died out. Ecologists call this the competitive exclusion principle: two species cannot share exactly the same needs in the same place forever.

In the wild, competition often decides where a species can live. In 1961 the ecologist Joseph Connell reported his experiments with two species of barnacles on the rocky shore at Millport, on the Isle of Cumbrae in Scotland. The young of one kind, Chthamalus, settled over a wide band of rock, but lower down a bigger barnacle, Balanus balanoides (now called Semibalanus balanoides), crowded them out. When Connell cleared away the Balanus, the Chthamalus survived lower on the rocks.

Predation (+/−)

In predation, one animal, the predator, kills and eats another, the prey. Predators and prey can affect each other's numbers. The most famous example comes from Hudson's Bay Company fur-trapping records that go back to the 1800s. The numbers of snowshoe hares and of the lynx that hunt them rise and fall in a cycle that lasts about 10 years, with lynx numbers lagging a year or two behind the hares. More hares mean more food for lynx; more lynx eat more hares, so hares decline, and then lynx decline too. Food supply for the hares matters as well, so predators are only part of the story.

Who eats whom, and how energy moves along those links, is mapped on the Food Webs page. Here the focus is on how predators change the numbers and behavior of other species.

Keystone species

A keystone species has a much bigger effect on its community than its numbers would suggest. You met the idea on the Food Webs page. Here is the evidence behind two famous cases, and the reasons scientists are careful about them.

Sea otters and kelp

Sea otters eat sea urchins, and sea urchins graze on kelp. In a 1974 study, James Estes and John Palmisano compared islands in the Aleutian chain of Alaska. Where sea otters lived, urchins were small and few, and kelp forests were thick. Where otters were missing, urchins were common and kelp was scarce. By eating urchins, otters indirectly protect a whole forest of seaweed and the animals that live in it. A chain of effects like this, passing down through a food web, is called a trophic cascade. The sea otter case study tells the full story.

Wolves in Yellowstone

Wolves were wiped out in Yellowstone National Park by people; the last pack there was killed in 1926. In 1995 and 1996, 31 gray wolves from western Canada were released in the park, and 10 more from Montana followed in 1997. Elk make up about 90 percent of the wolves' winter prey. Some studies reported that, with wolves back, young aspen trees, which elk browse, began to grow taller in some places.

This is often told as a simple story: wolves came back, so the trees came back. The truth is more complicated, and scientists still disagree about how strong the effect is.

• Wolves are not the only thing that affects elk. Other predators, elk hunting outside the park, and weather such as drought and harsh winters all play a part, according to the National Park Service.
• One 2022 study found that some earlier surveys measured mainly the tallest young aspen. Its authors estimated that this overstated how well aspen were recovering by roughly four to seven times. In their view the effect is real but weaker and patchier than many reports said.

So were wolves a keystone species in Yellowstone? They clearly matter, but the size of their effect is still being studied. Scientists are still testing the claim, and the answer has become more careful.

How populations change

A population is all the members of one species living in one area. It grows when births and newcomers outnumber deaths and leavers, and shrinks when the reverse is true. With plenty of food and space, a population can grow faster and faster. But no place has unlimited resources.

The largest population an environment can support over time is its carrying capacity. As a population nears it, food, water, space, and shelter run short, disease spreads more easily, and predators may find prey more often. Growth slows, and the population levels off. Things that hold a population in check are called limiting factors. Carrying capacity is not fixed: a drought can lower it, and a good year can raise it.

Sometimes a population shoots past its carrying capacity and then crashes. In 1944, 29 reindeer were released on St. Matthew Island in the Bering Sea, where there were no wolves or other large predators. By the summer of 1963 there were about 6,000. In the late winter of 1963–64, during extremely deep snow, almost all of them starved, and by 1966 only 42 were left. The biologist David Klein reported that the reindeer had already eaten away the lichens they relied on in winter, and he concluded that the food supply, together with the harsh weather, controlled the herd. A 2005 study disagreed: its authors argued that the severe winter weather was the main cause, because the herd was still growing fast right up to the crash. Scientists still debate how much each factor mattered.

The carbon cycle

Carbon is in every living thing: in sugars, fats, proteins, and DNA. The same carbon atoms move again and again between the air, living things, the ocean, and the ground.

• Photosynthesis: plants, algae, and some bacteria take carbon dioxide (CO₂) from the air or water and build it into sugar. (See Plants and Photosynthesis.)
• Eating: animals get carbon by eating plants or other animals.
• Respiration: almost all living things break down sugar for energy and breathe out carbon dioxide.
• Decomposition: decomposers, such as fungi and bacteria, break down dead matter and waste, releasing carbon dioxide too.
• Long-term storage: some carbon gets buried. Over millions of years, the remains of ancient living things became coal, oil, and natural gas, the fossil fuels.
• Burning: when people burn fossil fuels or forests, that stored carbon goes back into the air as carbon dioxide, much faster than it was buried.

Because of that extra burning, the amount of carbon dioxide in the air keeps rising. At the Mauna Loa Observatory in Hawaii, the monthly average in September 2026 was 426.07 parts per million (ppm), up from 424.37 ppm a year earlier.

The nitrogen cycle

Living things need nitrogen to make proteins and DNA. About 78 percent of the air is nitrogen gas (N₂), but plants and animals cannot use it in that form. Its two atoms are held together so tightly that only a few living things can pull them apart.

• Nitrogen fixation: certain bacteria, including cyanobacteria and the Rhizobium bacteria that live in lumps called root nodules on beans, peas, peanuts, and clover, turn nitrogen gas into ammonium. This partnership is a mutualism: the plant gets nitrogen and the bacteria get sugars. Read more about helpful bacteria on the Microorganisms page.
• Nitrification: other soil bacteria turn ammonium into nitrites and then nitrates.
• Absorption: plant roots take up ammonium and nitrates and use the nitrogen to build proteins. Animals get their nitrogen by eating plants or other animals.
• Decomposition: when plants and animals die, or animals produce waste, decomposers return the nitrogen to the soil as ammonium.
• Denitrification: still other bacteria turn nitrates back into nitrogen gas, which returns to the air.

People have changed this cycle too. Factories now make huge amounts of fertilizer from nitrogen in the air. Fertilizer helps feed billions of people, but when extra nitrogen washes into rivers and lakes it can feed blooms of algae that harm the water.

Where water fits in

The water cycle links the other cycles together. Water evaporates from oceans and lakes, rises from plant leaves, falls as rain or snow, soaks into the ground, and runs off into rivers. Moving water carries dissolved nitrates from fields into streams. The ocean takes up carbon dioxide from the air and stores huge amounts of carbon. And plants cannot photosynthesize without water. A change in one cycle, such as a drought, ripples into the others.

Who figured this out?

Key words

• Communityall the populations of different species living in one place at the same time
• Symbiosisa close, long-term relationship between two different species
• Mutualisma relationship in which both species gain
• Commensalisma relationship in which one species gains and the other is not affected
• Parasitisma relationship in which one species, the parasite, gains by harming its host
• Competitionwhen two populations need the same limited resource
• Predationwhen one animal kills and eats another
• Keystone speciesa species with a much bigger effect on its community than its numbers suggest
• Trophic cascadea chain of effects that passes down a food web
• Carrying capacitythe largest population an environment can support over time
• Limiting factoranything, such as food or space, that keeps a population from growing
• Nitrogen fixationturning nitrogen gas from the air into a form living things can use
• Decomposera living thing, such as a fungus or bacterium, that breaks down dead matter and waste
• Fossil fuelcoal, oil, or natural gas, formed from the remains of ancient living things

Game: Name That Relationship

You will see twelve pairs of living things, one at a time. Choose the relationship between them: mutualism, commensalism, parasitism, predation, or competition. A pair you name right on your first try earns 1 point, for a total of 12. Your best score is saved on this device.

Quick quiz

Six questions. Pick one answer for each, then press Check my answers.