A food web is a representation of how the energy and mass are transported between different organisms in an ecosystem. A food web consists of producers, consumers and decomposers. If we remove any one level from the food web, all the other levels are automatically affected. For example, if we remove the consumers, producers will have no predator and hence will grow unchecked, while any higher level consumer dependent upon these consumers will die out. For illustration, if we were to remove say, deer, from a food web, grass will grow unchecked and cover large areas. On the other hand, tigers (who predate on deer) and other big carnivores will die out due to lack of food. Thus, changes in one level of a food web will change all other levels.
Note that, removal of all consumers will cause such drastic effect. If we just remove one type of consumer, there will be observable change in the food web, but not such drastic changes. Food web is very complex and has a number of organisms at each level, such that removal of any one, may shift the food web, but not destroy it.
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