Ecosystems: Interactions, Energy, and Dynamics
Roles in an ecosystem
Producers, such as plants and algae, make their own food using sunlight. Consumers eat other organisms. Decomposers, such as fungi, bacteria, and earthworms, break down dead organisms and wastes, returning nutrients to the soil so producers can use them again.
Matter cycles, energy flows
Matter cycles: the same atoms move from soil to plant to rabbit and back to soil. Energy does not cycle; it flows one way from the Sun through producers to consumers and is gradually lost as heat. Arrows in a food web show the direction energy travels - from the organism that is eaten to the eater - so arrows point toward the consumer.
Traps to watch for
- “Decomposers are just rotting garbage.” They are essential recyclers; without them, nutrients stay locked in dead material.
- “Energy is recycled in a food web.” Matter cycles; energy does not.
- “Removing one species has no effect.” Removing a predator usually lets prey increase, which then overuses the plants, and the whole web shifts.
- “A food web shows who is strongest.” It shows feeding relationships, which do not always match size or strength.
Worked example
In a yard food web of grass, rabbits, and foxes, the foxes disappear. Predict the chain of effects: rabbits increase because fewer are eaten; grass decreases because more rabbits feed on it; then rabbit numbers fall as food runs short. A strong answer describes both directions of change, not just the first.
Rule of thumb
Before answering, label each organism as producer, consumer, or decomposer, then trace the arrows.