A trophic level is a level on the food chain: producers (plants, aglae, etc.), primary consumers, secondary consumers, etc. An example for a food chain would be:

Grass-->Insect-->Mouse-->Snake-->Hawk

Often, the real world isn't quite a simple linear path for the food chain, so a food web is a more realistic diagram, showing various routes through which energy (in the form of food) travels to various animals (there can be several organisms per trophic level, animals can have multiple trophic levels, etc.)

 There are three main components to an ecosystem, into which all trophic levels fit:

  1. producers: such as plants and aglae
  2. consumers:(whether herbivores, carnivores, or omnivores) such as cows, fox, humans.
  3. decomposers: such as bacteria

The producers are autotrophic, while the consumers are heteratrophic, relying on lower levels of the food chain for preproduced energy.

Generally, a pyramidal structure arises when the biomass, or bulk weight of a particular species per kilmoter, is compared, with the higher trophic levels (the high-number consumers, for example), making the smaller, upper part of the pyramid.

Generally, the number of organism follows this trend (less individual consumers, more producers).

The energy, of course, starts from the producers, which use photosynthesis to create the energy, sucrose, etc. used by all the other levels. Without a supply of producers, any system would fail. The energy then flows up, as consumers, well, consume. Of course, there is energy lost in this process (not all the grass's energy is passed up to the insects, for example). Only about 10% of the energy from one level passes to the next. The other 90% is used for growth, lost as heat, animal movement, anabolic processes, etc.

Several factors limit the number of trophic levels (you aren't going to find a 100th level consumer, for example). One is the lack of energy, due to the 10% rule. Much higher then 4th or 5th level and the animal uses to much energy catching food to be able to simply survive. The other is the pyramidal structure: as the biomass decreases with each level, pretty soon you don't have much biomass to make a species with. Similarly, the declining number of individuals per level narrows down to a dangerously low number or zero altogether. Only a limited number of producers can survive in a given area, which also limites the size of a pyramid, with affects from top to bottom. And again, high level organisms are innefficient: spending all your time hunting for little energy is not a good survival strategy.

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