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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Food Chains and Webs
All living organisms need energy to survive. This energy ultimately comes from the sun and flows through ecosystems in a series of feeding relationships. Understanding how organisms feed on each other helps us make sense of how ecosystems function and how energy moves through the natural world.
Key Definitions:
A food chain shows a single, linear pathway of energy flow from one organism to another. Each step in a food chain is called a trophic level. Food chains always start with a producer (usually a green plant) and end with a top predator.
Example: Grass → Rabbit → Fox
The arrows in a food chain mean "is eaten by" or "energy flows to".
Food webs are more realistic models of feeding relationships in ecosystems. They show multiple interconnected food chains, reflecting the fact that most organisms eat more than one type of food and can be eaten by multiple predators.
Food webs help us understand how changes to one population can affect many others in an ecosystem.
Organisms in food chains and webs can be grouped according to their feeding position or trophic level:
First trophic level. Plants and algae that make their own food through photosynthesis using sunlight, water and carbon dioxide.
Examples: Grass, trees, phytoplankton
Second trophic level. Herbivores that eat plants.
Examples: Rabbits, caterpillars, cows
Third trophic level. Carnivores that eat herbivores.
Examples: Foxes, frogs, small birds
Fourth trophic level. Carnivores that eat other carnivores.
Examples: Eagles, sharks, lions
Break down dead organisms and waste materials, recycling nutrients back into the ecosystem.
Examples: Bacteria, fungi, worms
As energy moves up a food chain, a significant amount is lost at each trophic level. This happens because:
Only about 10% of the energy available at one trophic level is transferred to the next level. This is why food chains rarely have more than 4 or 5 trophic levels - there simply isn't enough energy left!
Scientists use pyramids to show the relationships between different trophic levels in an ecosystem:
Shows the number of individual organisms at each trophic level.
Usually pyramid-shaped but can be inverted (e.g., one tree supporting many insects).
Shows the total dry mass of organisms at each trophic level.
More reliable than numbers, as it accounts for organism size.
Shows the amount of energy stored at each trophic level.
Always pyramid-shaped due to energy loss between levels.
Food webs help us understand complex feeding relationships in ecosystems. When interpreting food webs, remember:
When wolves were reintroduced to Yellowstone National Park in 1995, they didn't just affect elk populations (their prey). The reduced elk numbers allowed trees and shrubs to recover, which provided habitat for birds and beavers. Beaver dams created wetlands, supporting fish and amphibians. This cascade of effects from adding one predator is called a trophic cascade and shows how interconnected food webs really are.
Human activities can disrupt food webs in various ways:
When habitats are destroyed, organisms lose their food sources and shelter. This can cause population declines or extinctions that ripple through the food web.
Example: Deforestation removes trees that many animals depend on for food and shelter.
Taking too many individuals from a population can affect both their predators (who lose a food source) and their prey (whose populations may explode without control).
Example: Overfishing cod in the North Atlantic led to increases in their prey (shrimp and crab) and decreases in seabird populations that relied on cod.
Non-native species can disrupt food webs by outcompeting native species, introducing new predator-prey relationships, or altering habitats.
Example: Cane toads introduced to Australia have no natural predators there and poison native animals that try to eat them.
Changing temperatures and weather patterns can affect when plants grow and animals reproduce, disrupting the timing of food availability in food webs.
Example: Earlier spring warming may cause plants to flower before their pollinators emerge.
To construct a food web:
In exams, you might be asked to predict the effect of changes to a food web, such as removing a species or introducing a new one. Think about direct effects (on predators and prey of the affected species) and indirect effects (on species further away in the web). Remember that changes often have unexpected consequences due to the complex nature of food webs!
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