In 1866 the German biologist Ernst Haeckel coined the word ecology, from the Greek oikos, meaning house or household. Ecology studies how living things interact with one another and with their surroundings: the household of nature.
Levels of ecology
A population is all the members of one species in an area, such as the deer in a forest. A community is all the populations living together there. An ecosystem adds the nonliving parts: soil, water, air and climate. The biosphere is every ecosystem on Earth together.
Who eats whom
Every ecosystem has three basic roles.
- Producers, mostly plants and algae, make their own food by photosynthesis.
- Consumers eat other organisms. Herbivores eat plants, carnivores eat animals, and omnivores, like most humans, eat both.
- Decomposers, such as fungi and bacteria, break down dead matter and return its nutrients to the soil, where producers can use them again. Without them, the world would be buried in dead leaves and bodies.
A food chain traces one path: grass → rabbit → fox. Real ecosystems are tangled food webs, since most animals eat several things and are eaten by several others.
Energy flows, matter cycles
As the carbon atom in Level 3 showed, matter cycles, but energy flows through and is lost as heat. At each step up a food chain, much of the energy is used for living or lost as heat. As a rule of thumb, only about 10 percent of the energy at one feeding level passes to the next. That is why there are far fewer wolves than deer, and far fewer deer than plants. It is also why feeding people on plants directly takes less land than feeding them on animals raised on plants.
Matter, by contrast, is recycled. In the carbon cycle, photosynthesis pulls carbon dioxide from the air and respiration returns it. Burning coal, oil and gas releases carbon that plants stored millions of years ago, which is the main reason carbon dioxide levels in the air are rising.
In the nitrogen cycle, the air is about 78 percent nitrogen, yet plants and animals cannot use nitrogen gas directly. Certain bacteria, called nitrogen-fixing bacteria, convert nitrogen gas into forms living things can use. Some live in lumps on the roots of beans, peas and clover. In the early twentieth century the German chemists Fritz Haber and Carl Bosch developed an industrial way to do the same thing, making cheap fertilizer that now helps feed billions of people.
Living together
Species interact in many ways. They compete for food and space. Predators eat prey. In symbiosis, species live in close association: in mutualism both species benefit, as bees and flowers do; in commensalism one benefits and the other is unaffected; in parasitism one benefits at the other's expense.
Keystone species
In the 1960s the ecologist Robert Paine began removing a predatory sea star from a stretch of rocky shore on the coast of Washington State. Without the sea stars, mussels spread unchecked and crowded out other creatures, and the number of species on the shore fell sharply. In 1969 Paine named such species keystone species, after the central stone that holds an arch together: species whose effect on their community is far larger than their numbers suggest. Wolves returned to Yellowstone National Park in 1995 are often cited as another example, though ecologists still debate how strongly they changed the park's plants.
Biodiversity and its threats
Ecosystems provide services people rely on: pollination of crops, clean water, fertile soil and protection from floods. Scientists identify several main threats to biodiversity: habitat loss, usually ranked as the largest, along with invasive species, pollution, overharvesting and climate change.
How to respond is debated. In 2022 nearly 200 countries adopted a goal of protecting 30 percent of the world's land and seas by 2030. Supporters see protected areas as essential. Critics worry about the costs to farmers and local communities, and some argue that land managed by Indigenous peoples and local communities often conserves biodiversity as well as parks do, and more fairly.
Worked example: why the lake turned green
A small lake surrounded by farms turns bright green each summer, and one August, dead fish wash up on shore. Trace the chain of causes.
- Nutrients. Rain washes nitrogen and phosphorus fertilizer from fields into the lake.
- Producers boom. Algae, the lake's producers, grow explosively on the extra nutrients. This is an algal bloom.
- Decomposers boom. When the algae die, decomposers break them down. Their respiration uses up the oxygen dissolved in the water.
- Consumers die. Fish suffocate. The same process creates a large “dead zone” in the Gulf of Mexico each summer, fed by fertilizer carried down the Mississippi.
Each step uses an idea from this level: nutrient cycles, producers, decomposers and the links in a food web. Solutions involve trade-offs. Strips of grass and trees along streams can catch runoff; farmers can apply fertilizer more precisely. But these cost money and land, and farmers need to make a living. Good ecology identifies the problem; good civic deliberation decides how to share the cost of solving it.