Английский язык Контрольная

Food Chains and Food Webs

A feeding relationship in an ecosystem is called a food chain, while the totality of the chains constitutes a food web. Often the food webs are quite complex with many different feeding relationships. In addition to primary producers, herbivores and carnivores, omnivores are common in some ecosystems. Many bird species, such as finches and sparrows are examples of omnivores. In autumn they feed on seeds and are herbivores, but in summer they feed their offsprings on insects and are then predators. However, it is often possible to differentiate between the levels described. They are called trophic levels. In an ecosystem there are normally four trophic levels but there might be more or fewer.

The biomass in an ecosystem is very unevenly distributed between the trophic levels. This is because not all of the biomass consumed is converted to the body mass on the next level; rather it is used as energy source for running, flying, swimming, for keeping the body temperature, etc. Thus, no more than 1- 10% of the mass of a lower level becomes biomass on the next level. This explains why an ecosystem normally does not support more than three or four trophic levels. So in the transfer between trophic levels, most energy is lost as heat (much like the engine in an automobile - much petrol is wasted as the 

From this description we can see the distinct  difference between the transfer of energy and nutrients in the ecosystem. Energy enters the ecosystem through photosynthesis and is gradually lost as heat through the trophic levels.