Food Chains and Food Webs

A food chain is the simplest representation of feeding relationships. ARROWS point from the EATEN to the EATER — they show the direction of energy and nutrient flow.

Example: Grass → Rabbit → Fox → (Eagle).

Each step is a trophic level:

Trophic level	Name	Example
1	Producer	Grass, algae, oak tree
2	Primary consumer (herbivore)	Rabbit, cow, caterpillar
3	Secondary consumer	Fox, frog, small bird
4	Tertiary consumer	Eagle, lion

Producers are usually plants. They make their own food by photosynthesis — the ENERGY ENTRY POINT for the whole ecosystem.

Consumers eat other organisms. Herbivores eat plants; carnivores eat animals; omnivores eat both.

Decomposers (bacteria and fungi) feed on dead bodies and waste. They aren't usually shown in food-chain arrows but they're vital: they recycle nutrients back to the producers.

In reality a single species feeds on (and is fed on by) many others. A food web shows these overlapping relationships.

A simple woodland food web might include:

• Producers: oak tree, grass, bramble.
• Caterpillars eat oak leaves; rabbits eat grass; mice eat bramble berries.
• Foxes eat rabbits AND mice.
• Owls eat mice.
• Birds eat caterpillars.

A web like this shows that most consumers have several food sources — and several predators. This is more REALISTIC and more STABLE than a single chain.

What happens if you remove a species? Trace the arrows. If mice die out:

• Owls and foxes lose part of their diet → owl numbers fall first (they only eat mice).
• Plants get less eaten → may overgrow.
• Caterpillars and rabbits may increase (less competition + foxes shift to them).

This kind of analysis is what the MYP asks for.

Sea otters and kelp forests (Pacific coast). Sea otters eat sea urchins. Sea urchins eat kelp. Hunters nearly eliminated sea otters in the 1800s → sea urchin population exploded → kelp forests vanished → fish and seal populations crashed. Restoring otters has slowly restored the kelp.

Wolves of Yellowstone. Wolves had been removed from Yellowstone by 1926. Elk population grew unchecked → grazed willow saplings → no trees → beavers had no building material → wetland habitats collapsed. Reintroducing wolves in 1995 restored the cascade — even the river paths changed because vegetation regrew.

Bee decline. Bees pollinate ~30% of crops. Pesticides + habitat loss have reduced bee populations. Knock-on: fruit and vegetable yields drop.

These are examples of trophic cascades — a change at the top (or bottom) of a food web rippling through every other level.

MYP criterion D: 'Discuss the impact of human activity on a food web.' A strong answer picks ONE specific case (e.g. Yellowstone wolves), traces the cascade through several levels, and notes the wider lesson — top predators and key species are critical.