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The basic trophic levels in any food chain are the producers, the consumers and the decomposers. Most of the websites I've searched say they are the decomposers. But shouldn't it be the producers? In any given food chain, the number of individuals progressively decreases towards the higher trophic levels.

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The concept of a trophic "level" really only applies within the category of consumers (and even there it's only a rough approximation).

This is because some consumers eat producers (e.g., herbivores eating plants), while others eat other consumers (e.g., carnivores). So the notion of a "level" is basically counting how far something is from producers, which in turn creates the pyramid structure of bigger and badder predators eating smaller predators, and so on. This is really very specific to predatory animals because it is only in the context of predation that size goes up and numbers go down as photosynthesis becomes more remote.

With other types of interactions like parasitic worms infesting your gut or mosquitos sucking your blood, you could categorize the organism as a consumer and count it as another level removed from producers, but since it's only tapping a small amount of resources, rather than the more total interaction of predation, the ecological consequences are very different, and you can have a lot more mosquitos than people in a way that you can't have more wolves than deer.

So it is with decomposers as well, for yet another different reason. Decomposers don't need to be big (though they can be), since they aren't trying to hunt and kill something, they're extracting value from something that is already either dead or cast off. Thus, once again the concept of "level" doesn't really apply, and a vast number of flies can be born from the corpse of a single dead lion.

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