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Trophic levels are the position an organism has within a food chain. In a food chain, the bottom level is always represented by an autotrophic organism. This is also known as a producer because it can manufacture organic nutrients by the process of photosynthesis. This organism is an algae or a plant. Next, are the primary consumers or herbivores. These directly eat the producers and are usually small in size although not necessarily so. They can be grasshoppers, mice, rabbits, but they can also be something as large as a deer consuming plants. Next, are the secondary consumers or carnivores. These eat the animals in the previous level. They can be wolves that eat the rabbit or deer, they can even be frogs eating the grasshoppers. There are also omnivores that can feed on flesh or plants. There are also decomposers that feed on the dead organic matter in the environment--like bacteria and fungi. In your question, although it generally holds that smaller animals are at the bottom and largest at the top, you must remember that it takes a lot of food to maintain a large-bodied organism, therefore, they must eat a lot of smaller prey. Also, at each successive level of a food chain, some food energy is lost as heat due to maintaining the metabolism of the organisms at each level and when they hunt or gather food, they use some energy in their search for food. However, in a food chain, you can have a plant, a mouse, a fox and an eagle, with bacteria as the decomposers. Although the eagle occupies the fourth trophic level feeding on the fox, it is not necessarily larger than the fox!
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