Food Chains and Food Webs — Study Notes

A pond survey turns up thousands of algae cells, hundreds of water fleas, a few dozen minnows, and just one heron stalking the bank. That lopsided head-count isn't random — it's exactly what a food chain predicts once you understand what actually gets handed from one mouth to the next.

What a Food Chain Actually Shows

A food chain is a line-up of organisms showing who feeds on whom, linked by arrows. Every chain opens with a producer — a green plant or alga able to build its own food from sunlight, carbon dioxide and water — because nothing further along can manufacture food from scratch. Every organism after it is a consumer, labelled by how far removed it sits from that producer: feed on the producer directly and you're a primary consumer; feed on that primary consumer and you're a secondary consumer, and upward from there.

The trickiest habit to build is the arrow. It's tempting to picture the predator "doing" the eating and let the arrow start there — but the arrow actually tracks the energy, which only ever flows out of whatever gets consumed and into whatever consumes it. So the arrow begins at the organism lower down and finishes at whatever feeds on it next, never the reverse.

Worked Example — Ordering a Jumbled List Into a Chain

Four organisms are given out of order — heron, algae, minnow, water flea — plus two clues: the water flea feeds on the algae, and the minnow feeds on the water flea, and the heron feeds on the minnow.

1. Identify the producer first: algae is the only one here that photosynthesises, so it opens the chain.
2. Work outward one link at a time: the water flea feeds on algae, so it comes next, joined by an arrow running from algae toward it.
3. Continue the pattern: the minnow feeds on the water flea, so it follows, arrow again pointing from consumed to consumer.
4. Place the heron last, since it feeds on the minnow: algae → water flea → minnow → heron.

Labelling each link confirms the order: the water flea is primary consumer, the minnow is secondary, and the heron is tertiary — three steps removed from the producer, matching how rare herons are compared with algae.

That ordering trick is only the starting skill — how chains combine into a full web, why each step loses most of its usable energy, and what that does to a biomass pyramid all get built up properly with an audio walkthrough and worksheet in the full lesson below.