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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Producers and Consumers
Ecosystems are full of living things that depend on each other for survival. One of the most important ways organisms interact is through feeding relationships - who eats what! These relationships form the backbone of how energy flows through ecosystems and helps us understand how all living things are connected.
Key Definitions:
Producers are organisms that make their own food using energy from the environment. They form the first trophic level in any ecosystem.
Key characteristics:
Examples: Green plants, algae, some bacteria
Consumers cannot make their own food and must eat other organisms to get energy. They form the second and higher trophic levels.
Key characteristics:
Examples: Humans, lions, birds, insects
Consumers are classified based on what they eat and their position in the food chain. Understanding these classifications helps us map out how energy flows through ecosystems.
What they are: Herbivores that eat only plants
Position: 2nd trophic level
Examples: Rabbits, caterpillars, cows, deer
What they are: Carnivores that eat herbivores
Position: 3rd trophic level
Examples: Foxes, frogs, small birds, spiders
What they are: Carnivores that eat other carnivores
Position: 4th trophic level
Examples: Eagles, sharks, lions, wolves
Omnivores eat both plants and animals. They can occupy multiple trophic levels depending on what they're eating at the time.
Examples: Humans, bears, pigs, crows
Decomposers break down dead organic matter and return nutrients to the soil. They're crucial for recycling nutrients in ecosystems.
Examples: Fungi, bacteria, earthworms
Food chains and food webs are ways to visualise feeding relationships in ecosystems. They show how energy flows from one organism to another.
A food chain is a simple linear sequence showing who eats whom. It always starts with a producer and ends with a top predator.
Example: Grass → Rabbit → Fox → Eagle
Each arrow means "is eaten by" or "energy flows to".
Real ecosystems are more complex than simple food chains. Food webs show the interconnected feeding relationships in an ecosystem, with many food chains linked together.
Food webs are more realistic because:
The African savanna is home to a diverse range of species with complex feeding relationships:
If one species disappears (like elephants), it affects many others. Elephants create clearings by knocking down trees, which allows grass to grow, which feeds zebras and gazelles, which feed lions and hyenas.
As energy moves up the food chain, a lot of it is lost at each step. This is why food chains rarely have more than 4 or 5 links.
Only about 10% of the energy available at one trophic level is transferred to the next level. This means that for every 1000 joules of energy captured by producers, only about:
Energy is lost at each trophic level through:
Biomass is the total mass of living material at each trophic level. Like energy, biomass decreases as you move up the food chain.
A pyramid of biomass shows the total dry mass of organisms at each trophic level. It's usually pyramid-shaped because:
This pattern exists because of the energy loss at each trophic level - fewer organisms can be supported at higher levels.
Understanding feeding relationships helps us make smarter choices about food production:
This is why some scientists suggest we should eat lower on the food chain to feed more people with less environmental impact.
Human activities can disrupt natural feeding relationships in several ways:
When habitats are destroyed, feeding relationships are disrupted as species lose their food sources or places to live.
Example: Deforestation removes trees that many animals depend on for food and shelter.
Removing too many fish from the ocean disrupts marine food webs, affecting both predators and prey.
Example: Overfishing of cod in the North Atlantic led to increases in their prey (shrimp and crab) and decreases in their predators (seals).
Understanding feeding relationships helps us predict how changes to one species might affect others and how we can better protect entire ecosystems rather than just individual species.
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