Trap One: the Arrow Points at the Eater, Not the Food
This is the big one, and it catches almost everybody. The arrow in a food web shows where the energy goes — so it always points to the eater, the thing receiving the meal. Grass → grasshopper, because the energy travels from the grass into the grasshopper. People want to draw it the other way, as if the hawk is “reaching down” to grab its food, but the arrow isn't about who's hunting — it's about which way the energy flows. Follow the food, point at the eater.
Trap Two: Pull Out One Species and the Whole Web Wobbles
It's tempting to think each animal only matters to the one directly above it. Not so. Remove a single species and the ripple runs right through the web. Take out the frogs, and the grasshoppers they ate are suddenly free to multiply — so the grass gets hammered — while the snakes that ate the frogs go hungry, so they drop too. One change, felt in every direction: up, down and sideways. That's what makes a web a web and not a stack of separate chains.
Trap Three: “top” Means Highest Level, Not Biggest Body
A “top predator” is the one at the highest trophic level — the end of the chain, the one nothing else eats. It is not automatically the biggest animal. A wedge-tailed eagle sits at the top of its web while weighing a few kilos; the kangaroo it might scavenge is far heavier but feeds much lower down, on plants. Body size is a red herring. Ask only one thing: what does it eat, and what eats it?
And the Quiet One: Decomposers Aren't Optional Extras
Decomposers — the fungi, bacteria and worms — get left off diagrams because they're unglamorous and out of sight. But they're doing the job that keeps the whole web running: breaking down dead bodies and droppings and recycling the nutrients back into the soil so the producers can grow again. Skip them and nutrients would lock up in corpses forever and the producers would starve. They close the loop — leave a web without them and, sooner or later, nothing else can live there either.