Food chains.

Step 1 of 5

Who eats whom

A grey heron stands at the edge of the lake and stabs its bill into a perch. The perch ate small fish, the small fish ate zooplankton, water fleas and copepods a few millimetres long, and those grazed algae, tiny cells floating near the surface. That line of eating is a food chain, and each step in it is a trophic level.

Every chain starts with producers, living things that make their own food from sunlight, by photosynthesis. Everything else is a consumer, and consumers are counted in the order they eat: primary, secondary, tertiary. Move close to the water in the Scene and look for every link.

Step 2 of 5

One way

Matter in the lake goes round in a cycle: the carbon in a dead fish’s body returns to the air and the water, and algae build sugar from it again. About energy you have to ask a different question. Answer, then press Energy pyramid under Build the energy pyramid and look at the orange rims of each tier.

Step 3 of 5

A tenth

The Scene counts energy in dots: each dot is a year of food for one heron, about 700 MJ. In the sunlight of a temperate climate, the lake’s algae, over about a square kilometre, capture $20\,000$⁠ dots a year. Most of the energy a level gets is not passed on: living things burn it to stay alive, in cellular respiration. Ecologists sum this up as the ten per cent rule. Here Energy passed on is fixed at 10%.

Step 4 of 5

Arrows

Biology books draw food chains and food webs with arrows from level to level. The arrow is part of the language, and it is worth knowing what it says before you read a whole web.

Step 5 of 5

Less sun

All the lake’s energy starts as the sunlight the algae capture, and they grow in proportion to the light. Energy passed on is fixed, and Sunlight is about to drop to the light of a shaded lake. The herons do not eat algae. Predict what happens to them.