A food web shows who eats whom in an ecosystem and how energy moves through living things. It starts with the Sun, which supplies energy that producers use to make food by photosynthesis. Consumers get energy by eating producers or other consumers.
Understanding food webs helps explain population changes, ecosystem stability, and the effects of human impacts such as habitat loss or overfishing.
Trophic levels are the feeding positions in a food web, such as producers, primary consumers, secondary consumers, and tertiary consumers. Energy flows in one direction through these levels, while matter is recycled by decomposers back into the soil and water. Only a small fraction of energy is passed to the next level, so higher-level predators are usually less abundant.
A change in one species can ripple through the web because many feeding relationships are connected.
Understanding Environmental Science: Food Webs and Trophic Levels
The arrows in a food web have a specific meaning. They point from the organism being eaten toward the organism that eats it. A grass arrow pointing to a rabbit means the rabbit receives food energy from the grass.
This can feel backward at first because students may expect arrows to show the direction of hunting. Reading each arrow as energy moves from food to eater prevents many mistakes. Organisms can occupy more than one feeding position.
A bear that eats berries, insects, fish, and small mammals does not fit neatly into one single level. Its position changes with its food source and the season.
Most energy taken in by an organism does not become new body tissue. Plants use energy for life processes, and animals use much of their food energy for movement, body temperature, growth, and repair. A large amount leaves the organism as heat during respiration.
Some food is not digested and becomes waste. Only the energy stored in new tissue can be eaten by the next consumer. This explains why ecosystems can support many grasses, fewer rabbits, and very few foxes.
It also explains why large predators need wide territories or productive feeding grounds. A diagram called an energy pyramid shows this pattern through its narrowing upper levels.
Decomposers are easy to underestimate because they often work out of sight. Fungi, bacteria, worms, and many small animals break down dead leaves, fallen trees, waste, and dead organisms. They release nutrients into soil or water, where plants and algae can use them again.
Without this work, usable nutrients would become trapped in dead material. Scavengers such as vultures and crabs eat dead animals, but they do not complete all decomposition. They break material into smaller pieces.
Microorganisms carry out much of the final chemical breakdown. In a pond, lake, or ocean, decomposers can use up dissolved oxygen when there is too much dead material, which can harm fish.
Food webs help scientists predict indirect effects. If wolves disappear from an area, deer or elk numbers may rise. Heavy grazing can then reduce young trees and shrubs.
Fewer plants can affect insects, birds, stream banks, and soil. This kind of linked change is called a trophic cascade. Human actions can start cascades through pesticide use, hunting, introduced species, pollution, or removing habitat.
Some pollutants become more concentrated at higher feeding positions because predators consume many contaminated prey over time. When studying a web, identify the arrows carefully, note organisms with several food sources, and think about what would happen if one population increased or decreased. Real ecosystems rarely respond in one simple straight line.
Key Facts
- Producers make food using sunlight: 6CO2 + 6H2O + light energy = C6H12O6 + 6O2.
- Energy flows from the Sun to producers to consumers, but nutrients cycle through organisms, soil, water, and air.
- Primary consumers eat producers, secondary consumers eat primary consumers, and tertiary consumers eat secondary consumers.
- The 10 percent rule estimates that only about 10% of energy transfers to the next trophic level.
- Energy available at the next level can be estimated by E_next = 0.10 x E_current.
- Food webs are more realistic than food chains because most organisms eat and are eaten by more than one species.
Vocabulary
- Food web
- A food web is a network of connected food chains that shows feeding relationships in an ecosystem.
- Trophic level
- A trophic level is a feeding position in an ecosystem based on how an organism gets energy.
- Producer
- A producer is an organism, such as a plant or algae, that makes its own food using sunlight or chemical energy.
- Consumer
- A consumer is an organism that gets energy by eating other organisms.
- Decomposer
- A decomposer is an organism, such as a fungus or bacterium, that breaks down dead matter and returns nutrients to the environment.
Common Mistakes to Avoid
- Drawing arrows toward what gets eaten is wrong because food web arrows show the direction of energy flow from food to eater.
- Assuming all energy transfers to the next trophic level is wrong because most energy is lost as heat, movement, and life processes.
- Treating decomposers as separate from the food web is wrong because decomposers recycle nutrients from every trophic level.
- Calling every large animal a top predator is wrong because trophic level depends on what the organism eats, not its size.
Practice Questions
- 1 A grassland producer level stores 20,000 kJ of energy. Using the 10 percent rule, how much energy is available to primary consumers, secondary consumers, and tertiary consumers?
- 2 In a pond food chain, algae provide 8,000 kJ of energy. Estimate the energy available to zooplankton, small fish, and herons if each transfer passes on 10 percent of the energy.
- 3 In a food web, a pesticide greatly reduces the population of insects that eat plants. Explain how this could affect plants, insect-eating birds, and decomposers.