--- title: Balance in a food web — a lesson of Mova's school · trees and pragmatics description: «Balance in a food web», a lesson of the world course of Mova's primary school, sentence by sentence with dependency trees and pragmatics. It begins: When an animal eats, most of the energy in its food is used up for moving, keeping warm and living, and only a small part (about a tenth) is built int --- [Mova](https://md.mova.elamur.ai/free/en/) · [this page in HTML](https://mova.elamur.ai/free/en/corpora/school/world/30-balance-in-food-webs/) [home](https://md.mova.elamur.ai/free/en/) / [corpora](https://md.mova.elamur.ai/free/en/corpora/) / [school](https://md.mova.elamur.ai/free/en/corpora/school/) / [World](https://md.mova.elamur.ai/free/en/corpora/school/world/) / Balance in a food web # Balance in a food web Lesson 30 of the world course of Mova's primary school: 17 sentences, each with its syntax tree and pragmatics. The lessons are written for Mova, and their agreed trees are part of Mova corpus 0.4 (CC BY-SA 4.0). How Mova does on its tests: [the lesson at school](https://md.mova.elamur.ai/free/en/school-run/world/30-balance-in-food-webs/). - [0](https://md.mova.elamur.ai/free/en/corpora/school/world/30-balance-in-food-webs/0/) — When an animal eats, most of the energy in its food is used up for moving, keeping warm and living, and only a small part (about a tenth) is built into its body and can pass on to whoever eats it. - [1](https://md.mova.elamur.ai/free/en/corpora/school/world/30-balance-in-food-webs/1/) — So each step of a food chain has much less energy than the one below: there must be far more grass than zebras, and far more zebras than lions, which is why big predators are always rare. - [2](https://md.mova.elamur.ai/free/en/corpora/school/world/30-balance-in-food-webs/2/) — In a food web everything is connected, so a change to one living thing spreads: when wolves disappeared from Yellowstone Park, deer grew so many that they ate the young trees by the rivers; when wolves came back, the trees grew again. - [3](https://md.mova.elamur.ai/free/en/corpora/school/world/30-balance-in-food-webs/3/) — Sea otters eat sea urchins, and sea urchins eat kelp seaweed; where otters were hunted away, the urchins multiplied and ate whole kelp forests, and all the fish living in the kelp lost their home. - [4](https://md.mova.elamur.ai/free/en/corpora/school/world/30-balance-in-food-webs/4/) — To guess what happens when one living thing disappears, follow the arrows: what it ate will grow in number, what ate it will go hungry and must switch food, shrink or move away. - [5](https://md.mova.elamur.ai/free/en/corpora/school/world/30-balance-in-food-webs/5/) — Only about a tenth of the energy at one step of a food chain passes on to the next step. - [6](https://md.mova.elamur.ai/free/en/corpora/school/world/30-balance-in-food-webs/6/) — Most of the energy an animal eats is used for moving, growing, keeping warm and staying alive. - [7](https://md.mova.elamur.ai/free/en/corpora/school/world/30-balance-in-food-webs/7/) — Because energy is lost at each step, there are always far fewer predators than plant-eaters. - [8](https://md.mova.elamur.ai/free/en/corpora/school/world/30-balance-in-food-webs/8/) — Food chains rarely have more than four or five steps, because too little energy is left at the top. - [9](https://md.mova.elamur.ai/free/en/corpora/school/world/30-balance-in-food-webs/9/) — If a predator disappears, its prey usually increases in number. - [10](https://md.mova.elamur.ai/free/en/corpora/school/world/30-balance-in-food-webs/10/) — If prey animals increase too much, they can eat too many plants and harm the whole habitat. - [11](https://md.mova.elamur.ai/free/en/corpora/school/world/30-balance-in-food-webs/11/) — If a prey animal disappears, the predators that ate it must find other food, move away or decline. - [12](https://md.mova.elamur.ai/free/en/corpora/school/world/30-balance-in-food-webs/12/) — An animal that eats many kinds of food survives the loss of one food more easily than an animal that eats only one. - [13](https://md.mova.elamur.ai/free/en/corpora/school/world/30-balance-in-food-webs/13/) — Predators help keep a habitat healthy by stopping plant-eaters from growing too many. - [14](https://md.mova.elamur.ai/free/en/corpora/school/world/30-balance-in-food-webs/14/) — In Yellowstone Park, returning wolves reduced the number of deer and the riverside trees grew back. - [15](https://md.mova.elamur.ai/free/en/corpora/school/world/30-balance-in-food-webs/15/) — Sea otters protect kelp forests by eating the sea urchins that eat kelp. - [16](https://md.mova.elamur.ai/free/en/corpora/school/world/30-balance-in-food-webs/16/) — A field of crops can feed many more people directly than through feeding animals for meat. - ← previous: [Classifying unusual animals](https://md.mova.elamur.ai/free/en/corpora/school/world/29-classifying-unusual-animals/) - next →: [The skeleton and muscles](https://md.mova.elamur.ai/free/en/corpora/school/world/31-skeleton-and-muscles/) - ↑ back to [World](https://md.mova.elamur.ai/free/en/corpora/school/world/)