Reading a web is two skills: putting each organism in its role, and understanding the energy pyramid that explains the whole shape. Learn these and you can read any web you're handed, and predict who'll be common and who'll be rare.
The Four Roles
Producer — the green plants. They make their own food from sunlight, so they're the energy's way in; every web starts here. Herbivore (primary consumer) — eats producers, like the grasshopper munching grass. Carnivore (secondary / tertiary consumer) — eats other consumers: the frog that eats the grasshopper is a secondary consumer, the snake that eats the frog is tertiary, one rung higher again. Decomposer — the fungi, bacteria and worms that break down dead things and droppings and recycle the nutrients back into the soil for the producers to use again.
The level an organism feeds at — first, second, third — is its trophic level. Producers are level 1, herbivores level 2, the carnivores that eat them level 3, and so on up.
The Energy Pyramid
Here's the engine of the whole topic. Energy does not pass along the chain neatly — most of it leaks away at every step, mainly as heat (animals are warm, they move, they breathe, they live). Only about a tenth of the energy at one level makes it into the next. So each step up has far less energy to share around than the step below.
That's why a food web is shaped like a pyramid: heaps of grass at the bottom, fewer grasshoppers, fewer frogs again, and only a handful of hawks right at the top. There simply isn't enough energy left up high to feed many big predators. The pyramid isn't a coincidence — it's energy running out as it climbs.
A Worked One, Slowly
Question: in the chain grass → grasshopper → frog → snake → hawk, name each organism's role and trophic level, and say why hawks are rare.
The grass is the producer, trophic level 1 — it catches the Sun's energy. The grasshopper is a herbivore, the primary consumer, level 2. The frog is a carnivore, the secondary consumer, level 3. The snake eats the frog, so it's a tertiary consumer, level 4. The hawk is higher still, a top predator. Why are hawks rare? Because only about a tenth of the energy survives each step, so by the time you've climbed five levels there's almost nothing left — barely enough to keep a few hawks going. Name the role, give the level, then pin the rarity on energy lost up the pyramid. That structure earns full marks every time.