An ecosystem is a community of living organisms interacting with the nonliving parts of their environment, such as sunlight, water, air, and soil. Energy enters most ecosystems as sunlight and is captured by producers like plants and algae. That energy moves through food webs as organisms eat and are eaten.
Matter is different because atoms are reused again and again within the ecosystem.
Understanding Ecosystems
Food webs are more realistic than simple food chains because most organisms have more than one food source. A fox may eat rabbits, insects, berries, and small birds. Each feeding link transfers stored energy, but much of that energy is used immediately.
Organisms need energy for movement, growth, repair, keeping warm, and reproduction. A large part leaves their bodies as heat.
This explains why ecosystems usually support many plants, fewer herbivores, and still fewer large predators. A small number of hawks needs a wide area with enough prey beneath them.
Feeding positions are called trophic levels. Producers form the first level. Herbivores occupy the next level, while carnivores may occupy higher levels.
Omnivores can fit into more than one level because their diet changes. Humans are a familiar example. Someone eating beans receives energy through fewer feeding steps than someone eating beef, since cattle eat plants before becoming food.
This does not make one food choice automatically right or wrong, but it shows why food production needs land, water, and plant growth. Energy loss at each step limits the size of predator populations.
Decomposers do work that is easy to miss. Fungi, bacteria, worms, and insects break down dead organisms and waste. In doing so, they release substances into soil, water, and air.
Plant roots can take up many of these materials again. Without decomposition, nutrients would remain trapped in dead leaves, fallen trees, and animal remains. Nitrogen is especially important because organisms need it to build proteins and genetic material.
Certain bacteria change nitrogen into forms that plants can use. Other bacteria return nitrogen to the air. These tiny organisms help keep entire ecosystems functioning.
Changes in one part of a system can spread through many feeding links. Removing a top predator can allow some prey populations to rise quickly. Those animals may then eat more plants, changing shelter and food for other species.
Pollution can move through food webs too. Some chemicals do not break down easily and collect in body tissues. Predators may receive larger amounts because they eat many contaminated prey.
Fertilizer washed from farms or lawns can cause rapid algae growth in ponds and lakes. When the algae die, decomposition can use up oxygen needed by fish.
When studying ecosystem diagrams, pay close attention to what each arrow means. In a food web, an arrow usually shows the direction that food energy moves, from the organism being eaten toward the eater. In a nutrient cycle, arrows may show matter moving between air, soil, water, and living things.
Do not assume every organism fits one neat category. A mushroom is a decomposer, but it can interact with plant roots.
A bear may act as a predator or a plant eater. Real ecosystems change with seasons, weather, migration, disease, and human activity.
Key Facts
- Producers convert light energy into chemical energy by photosynthesis: 6CO2 + 6H2O + light -> C6H12O6 + 6O2.
- Cellular respiration releases energy from food: C6H12O6 + 6O2 -> 6CO2 + 6H2O + energy.
- Energy flows one way through ecosystems and is eventually lost as heat.
- Matter cycles through organisms, soil, water, and air because atoms are conserved.
- Only about 10% of energy is usually transferred from one trophic level to the next.
- Biomass generally decreases at higher trophic levels because less energy is available.
Vocabulary
- Ecosystem
- An ecosystem is all the living organisms in an area plus the nonliving environment they interact with.
- Producer
- A producer is an organism, such as a plant or algae, that makes its own food using energy from sunlight or chemicals.
- Consumer
- A consumer is an organism that gets energy by eating other organisms or organic matter.
- Decomposer
- A decomposer is an organism, such as a fungus or bacterium, that breaks down dead matter and returns nutrients to the environment.
- Trophic level
- A trophic level is a feeding position in a food chain or food web, such as producer, primary consumer, or secondary consumer.
Common Mistakes to Avoid
- Saying energy cycles through an ecosystem is wrong because energy flows in one direction and is eventually released as heat.
- Forgetting decomposers is wrong because decomposers recycle nutrients from dead organisms and waste back into soil, water, and air.
- Assuming all energy from one trophic level moves to the next is wrong because most energy is used for life processes or lost as heat.
- Confusing food chains with food webs is wrong because a food chain shows one path of energy transfer, while a food web shows many connected feeding relationships.
Practice Questions
- 1 A meadow producer level contains 50,000 kJ of energy. Using the 10% rule, how much energy is available to primary consumers, secondary consumers, and tertiary consumers?
- 2 A pond contains 800 kg of producer biomass. If primary consumers have 12% of the producer biomass and secondary consumers have 15% of the primary consumer biomass, what is the biomass of each consumer level?
- 3 Explain why removing decomposers from a forest ecosystem would affect both plant growth and the flow of matter through the community.