# 115 Summary of Ecosystem
- Energy enters ecosystems in sunlight. Producers (photosynthetic plants) capture some of this energy and transfer it to organic substances such as carbohydrates. Consumers (animals and fungi) gets their energy by eating producers or other consumers.
- Food chains and food webs show how energy flows through an ecosystem. The level at which an organism feeds in a food chain is its trophic level.
- Energy is lost as it passed along food chains.
- The energy losses in food chains limit the length of the chain, so few food chains have more than five trophic levels.
- It would be more energy-efficient for humans to harvest and eat plant crops, rather than feeding the crops to animals and then eating those.
- Pyramids of numbers and pyramids of biomass are ways of showing the relative numbers or biomass at different trophic level in a food chain.
- The carbon cycle shows how CO2 from the air is used in photosynthesis to make organic compounds in plants, which are then eaten by animals. Decomposers obtain their carbon by feeding on dead plants or animals, or on their waist materials. Respiration by all organisms returns CO2 to the air.
- Nitrogen gas is very inert, and must be fixed (to produce ammonium ions or nitrate ions) before it can be used by living organisms. Some plants have nitrogen-fixing bacteria in their roots, and other nitrogen-fixing bacteria live freely in the soil. Plants absorb ammonium or nitrate ions and use them to make proteins, which can then be eaten by animals. Decomposers and nitrifying bacteria convert proteins to ammonia and nitrates, which can be re-used by plants. Denitrifying bacteria convert nitrates to nitrogen gas which is returned to the air.
- The size of a population of organisms is affected by environmental factors such as food supply, predation and disease.
- When a resource is in limited supply, the growth of a population often shows a lag phase, exponential phase, stationary phase and death phase.
- Age pyramids show the structure of a population at one moment in time, and can be used to predict how the population is likely to change in the future. The global human population is increasing, but there is hope that by the end of this century the growth will have slowed significantly.
