ecological pyramids

Ecological pyramids are used to represent trophic levels

Ecological pyramids are useful in representing the trophic levels of an ecosystem, and decomposers are not included in the pyramids. In them, each trophic level is represented by a rectangle, in which the length is proportional to the number of individuals in the pyramid of numbers; to biomass, in the biomass pyramid; and to energy, in the energy pyramid.

The pyramid of numbers indicates the number of organisms in each trophic level. Depending on the ecosystem, the pyramid of numbers may have its apex facing up (direct pyramid) or facing downwards (inverted pyramid).

When in an ecosystem many producers are needed to feed a few locusts, which will serve as food for an even smaller number of birds, we use the pyramid with the apex upwards.

The pyramid with the apex downwards will be used when, in a forest, a single tree supports a large number of aphids, which are eaten by a smaller number of birds.

The biomass pyramid is built from the evaluation of biomass at trophic levels of a chain. It is usually expressed in dry weight (since the water present in this material is not organic matter, therefore it is not used as energy), per unit area (g/m 2 or kg/m 2 ). The shape of the biomass pyramid may vary depending on the ecosystem. We can see in the figure an example of a direct biomass pyramid. In this figure we can see that eight tons of alfalfa support a ton of calves in a year and these calves feed a teenager weighing 47 kg in that period.

As with the number pyramid, the biomass pyramid can also be inverted. We can exemplify with aquatic ecosystems. In these ecosystems, phytoplankton biomass may be less than zooplankton. The inversion of the pyramid occurs because the biomass is only relative to that moment, not considering the reproduction speed of phytoplankton, which is greater than that of zooplankton, which allows its rapid renewal. If the average of a whole year is measured, we can see that the average amount of phytoplankton was greater than that of zooplankton.

The energy pyramid indicates the productivity of the ecosystem, always considering the time factor. For this reason, the energy pyramid is never inverted. In this pyramid there is a representation, at each trophic level, of the amount of energy accumulated in a given area (or volume) per unit of time.

The first trophic level of the pyramid represents the amount of food produced by the producers of the ecosystem in a given area (biomass), during a certain time interval. We call this gross primary production (GDP). A part of the PPB is used by the producer himself in his metabolism, another part is released in the form of heat. Organic matter not used by producers is incorporated into their tissues, becoming available for the following trophic levels. This organic matter is called net primary production (NPP).

Thus, NPP is the energy available for the second trophic level, represented by herbivores. Of the food that herbivores ingest, part is used in their metabolism, and the other part is eliminated in feces and urine and in the form of heat. What remains is incorporated into the animal’s tissues, and this incorporated organic matter is what will become available for the next trophic level. With carnivores the same process occurs.

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