Ecological pyramids of biomass and abundance
Ecological pyramids are graphical models showing the number of living organisms (pyramid of numbers), their biomass (pyramid of biomass) or the energy they contain (energy pyramid) at each trophic level.
There are three types of ecological pyramids: energy, biomass and abundance.
The ratio of living matter at different levels generally obeys the same rule as the ratio of incoming energy: the higher the level, the lower the total biomass and the number of its constituent organisms.
The principle of building ecological pyramids
The base of the pyramid is formed by producers (plants).
Above them are the first order consumers (herbivores).
The next level is represented by consumers of the second order (predators).
And so on up to the top of the pyramid, which is occupied by the largest predators. The height of the pyramid usually corresponds to the length of the food chain.
The biomass pyramid (1) shows the ratio of biomasses of organisms of different trophic levels, depicted graphically in such a way that the length or area of a rectangle corresponding to a certain trophic level is proportional to its biomass.
In any trophic chain, not all food is used for the growth of an individual, that is, for the formation of biomass (part of it is spent on meeting the energy costs of organisms: respiration, movement, reproduction, maintaining body temperature, etc.). Consequently, in each subsequent link in the food chain, a decrease in biomass occurs.
The rule of the ecological pyramid of biomass reflects the regularity according to which in any ecosystem the biomass of each next link is 10 times less than the previous one.
The pyramid of numbers
The pyramid of abundance, or numbers (2) – displays the number of individuals at each of the trophic levels of a given ecosystem.
The pyramids of numbers reflect only the population density of organisms at each trophic level, but not the rate of self-renewal (turnover) of organisms.
If the reproduction rate of the prey population is high, then even with a low biomass, such a population can be a sufficient food source for predators with a higher biomass, but a low reproduction rate.
For this reason, the number pyramids can be inverted, i.e. the density of organisms at a given moment at a low trophic level may be lower than the density of organisms at a high level.
For example, many insects can live and feed on one tree (inverted population pyramid).
An inverted pyramid of biomass is characteristic of marine ecosystems, where the primary producers (phytoplankton algae) divide very quickly (they have a high reproductive potential and a rapid change of generations). In the ocean, up to 50 generations of phytoplankton can change per year. Phytoplankton consumers are much larger, but reproduce much more slowly. During the time until predatory fish (and even more so, walruses and whales) accumulate their biomass, many generations of phytoplankton will change, the total biomass of which is much larger.
The biomass pyramids do not take into account the duration of the existence of generations of individuals at different trophic levels and the rate of formation and consumption of biomass.
That is why the pyramids of the rates of formation of living matter, i.e. productivity, are a universal way of expressing the trophic structure of ecosystems. They are commonly referred to as pyramids of energies, referring to the energetic expression of products.
Of the three types of ecological pyramids, the energy pyramid gives the most complete picture of the functional organization of communities, since it reflects the picture of the rates of passage of a mass of food through the food chain.