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Ecosystems and Energy Flow
Every living thing needs energy to survive, grow and reproduce. In an ecosystem, energy flows from the Sun through plants to animals and finally to decomposers. Understanding this flow explains why there are fewer top predators than plants, why some species are especially important, and why changes in one part of a food web can affect many others.
What is an ecosystem?
An ecosystem is a community of living organisms (biotic factors) interacting with their non-living environment (abiotic factors) in a particular area.
- Biotic factors: plants, animals, fungi, bacteria
- Abiotic factors: sunlight, temperature, water, soil, nutrients
Ecosystems range from a small pond to a vast rainforest. Large regions with similar climate and vegetation, such as deserts, grasslands, tundra and tropical rainforests, are called biomes.
Roles in an ecosystem
| Role | What it does | Examples |
|---|---|---|
| Producers (autotrophs) | make their own food, usually by photosynthesis | plants, algae, some bacteria |
| Primary consumers | eat producers | rabbits, caterpillars, zooplankton |
| Secondary consumers | eat primary consumers | frogs, small fish |
| Tertiary consumers | eat secondary consumers | hawks, sharks |
| Decomposers | break down dead matter and recycle nutrients | fungi, bacteria |
Photosynthesis
Producers capture sunlight and use it to convert carbon dioxide and water into glucose (sugar) and oxygen:
carbon dioxide + water + light energy → glucose + oxygen
This process is the foundation of almost every food chain on Earth. (Some deep-sea ecosystems are instead based on chemical energy from hydrothermal vents.)
Food chains, food webs and trophic levels
A food chain shows one path of energy: grass → grasshopper → frog → snake → hawk. A food web shows many interconnected food chains, which is far more realistic, because most animals eat, and are eaten by, several species.
Each feeding step is called a trophic level: producers are level 1, primary consumers level 2, and so on.
The roughly 10 percent rule
Only a small fraction of energy passes from one trophic level to the next, often summarised as about 10 percent, although the actual figure varies widely between ecosystems. The rest is used for life processes such as movement and keeping warm, or lost as heat, or is never eaten.
Example
Suppose producers in a grassland capture 100,000 units of energy. Using the approximate 10 percent rule:
| Trophic level | Organism | Energy available (approx.) |
|---|---|---|
| 1 | grass | 100,000 units |
| 2 | grasshoppers | 10,000 units |
| 3 | frogs | 1,000 units |
| 4 | snakes | 100 units |
| 5 | hawks | 10 units |
This explains why food chains rarely have more than four or five levels, and why ecosystems support far fewer top predators than plant-eaters. It also explains why, in general, plant-based foods require less land and energy to produce than the same number of calories from meat.
Keystone species and trophic cascades
A keystone species has a much larger effect on its ecosystem than its numbers would suggest. Removing it can change the entire community.
Example: sea otters
Sea otters eat sea urchins. Sea urchins graze on kelp. Where otters were hunted heavily, urchin populations often increased and ate away large areas of kelp forest, which shelter many fish. Where otters returned, kelp forests often recovered. This chain of effects through trophic levels is called a trophic cascade.
Example: wolves in Yellowstone
Grey wolves were reintroduced to Yellowstone National Park in the United States in 1995. Researchers have linked the wolves to changes in elk behaviour and numbers and to recovery of some streamside plants. Scientists still debate how large and widespread these effects are, which is a good reminder that real ecosystems are complex and that scientific understanding develops over time.
Ecosystems change over time
Ecosystems are not fixed. Ecological succession is the gradual change in the species living in an area. Primary succession starts on bare rock, for example after a volcanic eruption: lichens and mosses arrive first, slowly building soil for grasses, shrubs and eventually trees. Secondary succession happens where soil remains after a disturbance such as a fire or an abandoned field, so recovery is usually faster. Understanding succession helps scientists plan ecosystem restoration.
Key takeaways
- Ecosystems include living (biotic) and non-living (abiotic) parts.
- Energy flows from producers to consumers to decomposers.
- Only a small fraction of energy, often around 10 percent, passes to the next trophic level.
- Keystone species can trigger trophic cascades that reshape ecosystems.
Practice
1. Write a four-step food chain for an ocean ecosystem. 2. If producers capture 50,000 units of energy, about how much reaches the third trophic level using the 10 percent rule? 3. Explain why removing a keystone species can affect many other species.
Answers: 1 for example, phytoplankton → zooplankton → small fish → seal. 2 about 500 units (50,000 → 5,000 → 500). 3 because it strongly influences other populations through the food web, so its loss can cause a chain reaction, such as urchins increasing and destroying kelp forests.