Ecology and Ecosystems

Ecologists work at several scales, each one containing the one before:

Producers (autotrophs) make their own food. Most use photosynthesis: plants on land, and algae and cyanobacteria in water. A few bacteria near deep-sea vents use chemical energy instead.

Consumers (heterotrophs) get energy by eating other organisms. Primary consumers eat producers, secondary consumers eat primary consumers, and so on. Each feeding step is a trophic level. Many animals feed at more than one level: a bear that eats berries and salmon is both a primary and a higher-level consumer.

Decomposers, mainly bacteria and fungi, break down dead matter and waste. Without them, nutrients would stay locked in dead bodies and the soil would run out.

A food chain is a single path of who eats whom: seaweed → limpet → sea star. Real communities are tangled, because most species eat, and are eaten by, several others. A food web shows all of those links at once.

In 1963 the ecologist Robert Paine began a simple experiment on the rocky shore at Mukkaw Bay, Washington. He kept prying ochre sea stars off one stretch of rock and throwing them back into the sea, and left a nearby stretch alone. The plot without sea stars was soon covered by California mussels, which crowded out barnacles, seaweeds, chitons and limpets. Within a few years the number of species there had fallen from 15 to 8. Paine later called a species like this, whose effect is far larger than its numbers suggest, a keystone species.

Trophic cascades. Removing a top predator can ripple down through several levels. Gray wolves were wiped out of Yellowstone National Park by the 1920s and reintroduced in 1995 and 1996. Elk numbers fell in the years that followed, and some studies report that willows and aspens began to recover where elk browsing eased. Other researchers argue the effect is weaker or patchier than the popular story suggests, because drought, hunting, bears and cougars also affect elk, and aspen recovery varies from place to place. Claims that wolves changed the course of Yellowstone's rivers are especially debated. Wolves are back in the park's food web, but ecologists are still sorting out which of the changes they caused.

Energy enters most ecosystems as sunlight captured by producers and leaves as heat. Only a fraction of the energy at one trophic level ends up in the bodies of the next. The rest is used for the organisms' own respiration and lost as heat, or it is in parts that are never eaten or digested.

On average about 10% passes from one level to the next, which is why the hero sketch at the top of this page drops from 10,000 to 1,000 to 100 to 10. This is a rough average, not a law: measured transfer efficiencies vary, roughly between 5% and 20%, depending on the ecosystem and the animals involved.

The rule explains why food chains rarely have more than four or five levels and why top predators are scarce. Pyramids of numbers or biomass can sometimes be upside down (one oak tree can feed thousands of caterpillars), but a pyramid of energy never is.

Energy flows through an ecosystem in one direction, but matter is recycled. Click a pink step in either diagram, or use the buttons, to read about it.

Nitrogen is often the nutrient that limits plant growth, which is why farmers add fertilizer. Fertilizer that washes off fields into rivers can feed huge blooms of algae. When the algae die and decay, the bacteria breaking them down use up the oxygen in the water, creating low-oxygen "dead zones" such as the one that forms in the Gulf of Mexico most summers.

With plenty of food and space and nothing holding it back, a population grows exponentially: the more individuals there are, the faster it grows, giving a J-shaped curve. Bacteria in fresh broth grow this way for a while.

No population can keep that up. As it grows, food, space and other resources run short and disease and predators take more. Growth slows and the population levels off near the carrying capacity (K), the largest population the environment can support over time. This S-shaped curve is called logistic growth.

Real populations can overshoot. In 1944, 29 reindeer were released on St. Matthew Island in the Bering Sea, which had no predators. By 1963 there were about 6,000. They ate the island's lichens faster than they could regrow, and after a hard winter only 42 were left.

Limiting factors that get stronger as a population gets denser, like competition for food and disease, are density-dependent. Others, like a hard freeze or a wildfire, hit regardless of density and are density-independent.

Mutualism, commensalism and parasitism are the three kinds of symbiosis, a close, long-term relationship between two species. The lines between them can blur: a relationship can shift as conditions change.

In the 1930s the Russian biologist Georgii Gause grew two species of Paramecium separately, and each thrived. Grown together on the same food, P. aurelia and P. caudatum could not both last: P. caudatum always died out in his cultures. Yet when he paired P. caudatum with P. bursaria, which fed in a different part of the tube, the two coexisted. This idea, that two species competing for exactly the same resources cannot coexist indefinitely, later became known as the competitive exclusion principle. In nature, competitors often survive by splitting resources, for example by feeding at different times or in different places.

Ecological succession is the gradual change in which species live in a place after a disturbance.

Primary succession starts with no soil at all: on new volcanic rock, or ground left bare by a retreating glacier. Pioneer species such as lichens and mosses arrive first. As they grow, die and help break down rock, a thin soil forms, and grasses, shrubs and eventually trees can follow. This can take centuries. The volcanic island of Surtsey, which rose out of the sea off Iceland in 1963, has been watched since its birth as a natural laboratory of primary succession.

Secondary succession follows a disturbance that removes the community but leaves the soil, such as a fire, a flood or an abandoned farm field. Seeds and roots already in the soil give it a head start, so it is much faster.

Older textbooks describe succession ending in a fixed "climax community." Ecologists now see most communities as always changing, as disturbances of different sizes keep resetting parts of the landscape.

Biodiversity is the variety of life, counted at three levels: genetic diversity within a species, the number of species, and the variety of ecosystems. Diverse ecosystems tend to be more stable and provide services people rely on, such as pollination, clean water and fertile soil.

In 2019 the IPBES global assessment, written by hundreds of experts, estimated that around one million species of animals and plants are threatened with extinction. It ranked the main direct causes as:

1. changes in how land and sea are used, such as clearing forest for farms
2. direct exploitation: hunting, fishing and logging
3. climate change
4. pollution, including fertilizer runoff and plastics
5. invasive species

Not every trend points down. A 2020 analysis of insect surveys found that land insects declined by roughly 9% per decade on average, while freshwater insects increased in many places, partly thanks to cleaner rivers. Trends differ between places and groups, which is why long-term counts matter. Protected areas, cleaner water and controls on hunting have brought many species back from the brink.