How organisms interact with each other and their environment, and why biodiversity matters at genetic, species and ecosystem levels.
No living thing lives alone. A frog needs insects to eat, plants for those insects, water to breed in and a temperature it can tolerate. Ecology is the study of these connections. This article explains how ecosystems work, how energy and materials move through them, why variety of life (biodiversity) matters, and what threatens and protects it.
Written for students up to O-Level (grades 8 to 10), and useful for parents and teachers. General education.
An ecosystem is a community of living organisms together with the non-living parts of their surroundings, interacting as a system. A pond, a forest, a desert, a coral reef and even a school garden can each be studied as an ecosystem. Ecosystems have no sealed walls: energy, water, nutrients and animals move in and out.
| Term | Meaning |
|---|---|
| Habitat | The place where an organism lives |
| Population | All the organisms of one species in an area |
| Community | All the populations living together in an area |
| Ecosystem | The community plus its non-living environment |
| Biosphere | All the parts of Earth where life exists |
Biotic factors are living influences: predators, food, competitors, disease and partners. Abiotic factors are non-living: temperature, light, water, pH, salinity, soil type and oxygen. Each species has a range of conditions it can tolerate. Outside that range it survives poorly or not at all, which is why different organisms are found in different places.
A student wants to estimate how many daisies grow on a 400 m² lawn. She places 10 random quadrats (each 1 m²) and counts 3, 5, 2, 4, 6, 3, 5, 4, 2, 6 daisies.
This is an estimate. More quadrats, placed randomly, make it more reliable. Random placement avoids choosing only the crowded or the sparse patches.
Nearly all ecosystems are powered by sunlight. Producers (plants, algae and some bacteria) capture light energy by photosynthesis and store it in chemical form. Consumers get energy by eating: primary consumers eat producers, secondary consumers eat primary consumers, and so on. Each feeding step is a trophic level.
Energy flows in one direction: it enters as light and leaves as heat. At each step most of the energy is not passed on. Some is used by the organism in respiration (movement, keeping warm, growth), some is lost as heat, and some parts, such as bones, are not eaten or digested. Usually only about 10% passes to the next level (the real figure varies, often between 5% and 20%). This is why food chains rarely have more than four or five links, and why there are far fewer lions than zebras.
A field of grass takes in 20 000 kJ of energy. Rabbits eat the grass and gain 2000 kJ. Foxes eat the rabbits and gain 180 kJ.
The numbers are practice values, but the pattern (about a tenth passed on) is typical.
A food chain shows who eats whom in a single line, for example grass → grasshopper → frog → hawk. Arrows point in the direction of energy flow, from food to eater. Real feeding relationships are tangled, so scientists draw food webs.
In a web, losing one species can affect many others. If the frog disappears, the grasshopper population may grow and eat more grass, while the hawk depends more on rabbits. A species that has an effect much larger than its numbers suggest is called a keystone species; for example, sea otters that eat sea urchins help keep kelp forests healthy in some coastal ecosystems.
For a closer look at feeding relationships and how energy moves along them, see our article on food chains, food webs and energy flow.
Energy flows through an ecosystem, but matter is recycled. Decomposers, mainly bacteria and fungi, break down dead organisms, waste and fallen leaves, releasing nutrients (such as nitrates and phosphates) back into the soil and carbon dioxide into the air. Without decomposers, dead material would pile up and nutrients would stay locked away.
Carbon moves between air, living things and the soil. Plants take carbon dioxide from the air by photosynthesis and build it into sugars, proteins and other molecules. Animals get carbon by eating. All living things release carbon dioxide by respiration, and decomposers do so when they break down dead matter. Burning fuels and wood also adds carbon dioxide to the air.
Fossil fuels are the remains of ancient organisms; burning them returns carbon that was locked away for millions of years. The extra carbon dioxide is increasing the greenhouse effect and contributing to climate change, a consequence studied in more detail in physical-science and geography courses.
In the water cycle, the Sun's heat evaporates water, plants release water vapour by transpiration, vapour condenses to form clouds, and rain or snow returns water to land and sea.
Nitrogen makes up about 78% of the air, but most organisms cannot use nitrogen gas. In the nitrogen cycle, some bacteria (nitrogen-fixing bacteria, for example in root nodules of peas and beans) convert nitrogen gas into forms plants can use. Decomposers release ammonium from dead matter; nitrifying bacteria convert ammonium to nitrates, which plants absorb and use to make proteins. Denitrifying bacteria return some nitrogen to the air. Farmers use crop rotation with legumes and fertilisers to supply nitrogen to soil.
| Interaction | What happens | Example |
|---|---|---|
| Predation | One organism kills and eats another | Hawk and rabbit |
| Competition | Organisms use the same limited resource | Two plants competing for light |
| Mutualism | Both species benefit | Nitrogen-fixing bacteria in legume roots; bees and flowers |
| Parasitism | One lives on or in another and harms it | Tapeworm in a gut; mosquito feeding on blood |
A population grows when births and immigration exceed deaths and emigration. When resources are plentiful a population can grow quickly, but eventually limits such as food, space, disease and predators slow growth. The largest population an environment can support over time is its carrying capacity.
Biodiversity is the variety of life. It is described at three levels: genetic diversity (variation within a species), species diversity (the number and abundance of different species) and ecosystem diversity (variety of habitats). Tropical rainforests and coral reefs have especially high species diversity.
Counting species gives species richness. A more informative measure also considers how evenly individuals are spread among species. Two fields may both have 3 species, but one with 98 individuals of one species and 1 each of the others is less diverse than one with about 33 of each.
| Threat | Explanation |
|---|---|
| Habitat loss and fragmentation | Clearing forest, draining wetlands and building roads destroy or split habitats; this is considered the greatest threat on land. |
| Over-exploitation | Hunting, fishing or harvesting faster than a population can recover. |
| Pollution | Fertiliser runoff, plastics, pesticides and oil harm organisms. Fertiliser runoff can cause eutrophication: algae bloom, then decomposers use up oxygen and fish die. |
| Invasive species | Non-native species can out-compete or prey on native species. |
| Climate change | Changing temperatures and rainfall alter where species can live. |
Local ecosystems matter to local people. Mangroves protect coastlines from storm waves and act as nurseries for young fish, so protecting them can support both biodiversity and local fishing communities.
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