From sunlight to fox

About this video

A sparrow pecks at seeds. The seeds grew on a plant that stood in the sun all summer. Follow any meal backwards and it starts with sunlight. This video follows the Sun's energy from a plant to a fox.

Plants, algae and some bacteria make their own food from light, carbon dioxide and water. They are the producers, the base of almost every ecosystem. Everything else is a consumer: it gets its energy by eating other living things. A rabbit eats grass, a fox eats the rabbit. A food chain is one line of who eats whom, and its arrows point the way the energy goes, from the eaten to the eater.

At every step most of the energy is lost: a rabbit uses most of what it eats to move, keep warm and stay alive. On average only about 10% of the energy at one level passes on to the next. That is why there is far more grass than rabbits and far more rabbits than foxes, and why food chains rarely go beyond four to six steps.

Real animals rarely eat just one thing. A fox eats rabbits, mice, beetles and fruit; a hawk eats mice and small birds. Draw every link and the chains cross into a food web, where one change spreads along many paths. In the 1960s the ecologist Robert Paine kept pulling starfish off one stretch of rocky shore. Barnacles and then mussels spread over the rocks, and within a year the cleared rocks had gone from 15 species to 8. This video goes with the lesson Food Chains and Webs in Knaph's Biology Level 1 course.

Transcript

1. 0:00A sparrow pecks at seeds. The seeds grew on a plant in the summer sun. Follow any meal back, and it starts with sunlight.
2. 0:09Plants make their own food from light. They are the producers. Everything else is a consumer. A rabbit eats grass, a fox eats the rabbit. The arrows show where the energy goes, from the eaten to the eater.
3. 0:26At each step, most of the energy is used up, to move, to keep warm, to stay alive. Only about ten percent passes on.
4. 0:37That's why there is far more grass than rabbits, and far more rabbits than foxes. And why food chains are short.
5. 0:46Real animals rarely eat just one thing. A fox eats rabbits, mice, beetles and fruit. A hawk eats mice and small birds. The chains cross, and make a food web.
6. 1:02In the 1960s, the ecologist Robert Paine kept pulling starfish off one stretch of rocky shore.
7. 1:10Without them, barnacles and then mussels spread over the rocks and crowded out the rest.
8. 1:17Within a year, the cleared rocks went from fifteen species to eight.
9. 1:24Energy flows from the Sun to producers and on to consumers, losing most at each step. And in a web, one change travels along many paths.