Food Web and Energy Flow Lab

A food chain is a simplification. Real ecosystems are food webs — tangled networks where most species eat and are eaten by several others. That tangle is what makes ecosystems resilient, and also what makes them surprisingly fragile in specific places.

Where the energy goes

Energy enters through producers — plants and algae capturing sunlight by photosynthesis. It then passes to herbivores, then to carnivores. But at every step most of it is lost:

• Much of what is eaten is not digested and leaves as waste.
• A large share is burned in respiration just to keep the organism alive.
• Only what is built into new tissue is available to the next level.

The rough rule is the ten percent rule: only about 10% of the energy at one trophic level reaches the next. That is why food chains rarely exceed four or five links — by the fifth level there is simply not enough energy left to support another population.

Keystone species and cascades

Some species matter far more than their numbers suggest. Remove a keystone species — often a top predator — and the effects ripple down through every level. This is a trophic cascade.

The classic case: sea otters were hunted out along parts of the Pacific coast. Sea urchins, their prey, exploded in number and stripped the kelp forests bare, destroying habitat for hundreds of other species. Restore the otters and the kelp returns.

Try this

• Remove the top predator and watch herbivore numbers surge while plant biomass collapses.
• Remove a producer instead — the effect is immediate and severe, since everything rests on that base.
• Remove a mid-level generalist with many alternatives. The web often absorbs it with barely a wobble.

Why it matters

Food web thinking guides fisheries quotas, rewilding programmes (the wolves of Yellowstone changed river courses by altering grazing patterns), pest control, and predictions of how ecosystems respond to climate change.