GED Science: Trophic Levels and Food Pyramids
Above, the case was made that all food chains and food webs have things in common. For example, all food chains start with producers, or plants, that obtain energy from the sun to make their own food. All food chains have first-order consumers, or herbivores, that consume plants. In addition to having the same structure, all food chains and food webs are similar in terms of the amounts of food energy flowing through them. This similarity is not just interesting because it holds for all food chains/webs, but it also helps to explain population limits for each role in those food chains/webs. The following is a typical pyramid showing the amounts of energy available at each level in a food chain/web:
The general “rule” is that only 1/10th of the available energy in a trophic level of the food chain/web is available to the next higher trophic level in that chain/web. A tropic level is the space an organism occupies. For example, a grasshopper occupies the primary consumer/herbivore trophic level. Only 1/10th of the energy available from the producer (plant) level is available to the first-order consumers (herbivores). The reason for this proportional relationship is that some energy in each trophic level is lost due to waste production or inefficient digestion processes, etc. The following diagram shows the tropic levels in terms of our food chain example above:
This energy pyramid of trophic levels demonstrates two very important ecological ideas:
1) Not all of the energy at one trophic level (e.g., producers) is available to organisms at higher levels.
2) Fewer organisms can be supported at higher levels of the food/energy pyramid (relative to size).
It is difficult to compare the number of organisms from one level to the next, due to the variation in size of the organisms at each level. In other words, because hawks are so much larger than grasshoppers, it is not accurate or helpful to compare the numbers of hawks to the number of grasshoppers in a trophic level pyramid. Instead, ecologists often refer to the energy or the biomass available to each level. Biomass is the mass of living or recently living material in a trophic level. The diagram above shows that the grass at the producer level represents 10,000 kilocalories of biomass. However, only 1/10th of that…1,000 kilocalories of biomass is available to the primary consumer level, the grasshoppers.
Thus, in any ecosystem, due to loss from one trophic level to the next, fewer and fewer organisms can be supported (in terms of biomass). This situation is compounded and complicated by the fact that organisms increase in size at higher levels in the trophic pyramid. In other words, a hawk, at the quaternary level in the trophic pyramid has very little biomass left in terms of available food energy. Additionally, hawks are much larger than any organism at lower levels, so a hawk must consume large amounts of food at lower levels to support its nutritional needs. This trophic relationship explains why thousands of insects, etc. are found in ecosystems, and fewer quaternary consumers, or apex predators can be supported by most ecosystems.