Ecology: The Biology of Interactions. 3.13. Ecological Pyramids

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3.12. Ecological efficiencies

D. Shabanov, M. Kravchenko. Ecology: Biology of Interactions Section 3. Biogeocenology and Ecology of Communities

3.14. (addendum) Flora, Fauna, Consortia

3.13. Ecological Pyramids Charles Elton proposed a way to graphically represent the relationships between trophic levels, which has become almost a symbol of ecology as a science. This refers to ecological pyramids. When constructing ecological pyramids, measures of the abundance of representatives of different trophic levels are shown as rectangles stacked on top of each other. This method is usually used to describe grazing food chains. Pyramids of numbers, biomass, and productivity are distinguished. Let's construct a few ecological pyramids ourselves. Our examples will be quite conditional: we will assume that the food chains we will describe do not have "branches". For example, when modeling the clover — sheep — wolves chain, we will assume that sheep feed only on clover, and wolves feed only on sheep, and we will be interested in the relationships between these trophic levels within a certain ecosystem where the ratio of the numbers of the considered levels has reached equilibrium. Naturally, ecological pyramids can also be used to describe natural ecosystems, and the simplifications we have adopted are only for greater clarity of our reasoning. Let's return to the clover — sheep — wolves chain. Estimating the population sizes in this chain, we will see that there are much more individual clover plants than sheep, and more sheep than wolves. Such a pyramid (with a base wider than the apex) is called a correct or upright pyramid. However, not all grazing chains will have upright pyramids of numbers. An example could be the oak — oak silkworm chain. A large number of caterpillars can inhabit a small number of large trees. The pyramid of numbers for this chain will be inverted (Fig. 3.13.1). Fig. 3.13.1. Pyramids of numbers. A. upright. B. inverted. The inverted nature of the second pyramid is related to the differences in individual sizes. It is not difficult to understand that the inverted nature of the second pyramid is related to the differences in the sizes of producers and consumers. Based on data on the weight of an average oak and an average silkworm, as well as their pyramid of numbers, we can construct a pyramid of biomass. Naturally, it will be upright (Fig. 3.13.2). Fig. 3.13.2. Pyramids of biomass. A. upright. B. inverted. The inverted nature of the second pyramid is related to the differences in individuals' "life speed" - the different intensity of energy flow through their biomass. Can biomass pyramids be inverted? Quite rarely, but they can. Consider the pelagic food chains in the ocean. Surprisingly, the biomass of producers (planktonic algae) in such chains is often less than the biomass of consumers. Does this mean that consumers in such chains do not exist at the expense of producers? No. A simple analogy will help to understand this. A large pond can exist thanks to a small stream flowing into it, although at any given moment the mass of water in the pond is much greater than in the stream. It is clear that this is possible because the water in the stream changes much faster than in the pond. Similarly, in water column communities, energy flows through different trophic levels at different rates. The turnover time of phytoplankton biomass is measured in hours, zooplankton in days, fish and whales in weeks and months. To account for this difference, we need to reflect the intensity of energy flow through each level in ecological pyramids. Based on data on the biomass of food chain links and the speed of its change, we can construct a pyramid of productivity (or energy flow; Fig. 3.13.3). Fig. 3.13.3. Pyramids of productivity are always upright. This pyramid will always be upright. The first law of thermodynamics (law of conservation of energy) "forbids" such a pyramid from being inverted, and the second law forbids "floors" of equal width, as with each energy transformation, part of it must be dissipated as heat. By the way, this is why real food chains are not very long, and ecological pyramids are not very high. In any real ecosystem, so little energy would reach the consumer of level X (after ten successive transformations!) that it would be impossible to collect the necessary amount of energy for it from the territory available to one individual. Now, having become acquainted with the logic by which ecological pyramids are constructed, let's consider two more specific examples. Eugene Odum calculated the parameters of a hypothetical food chain in which a twelve-year-old boy ate exclusively veal (note: eating only meat is unnatural!), and calves ate only alfalfa (this is more physiological, not counting that both the boy and the calves need to start life by drinking milk from their mothers). The characteristics of such a pyramid are given in Table 3.13.1. Table 3.13.1. Example of ecological pyramids for a hypothetical food chain

Numbers

Biomass

Productivity

Boy

48 kg

8,3×103

Calves

4,5

1,035 kg

1,2×106

Alfalfa

2×107

8,211 kg

1,5×107

2×10^7

—

6,3×109

And the next example (Fig. 3.13.4) concerns real data on the biomass of several mammal species in a North American deciduous forest. As you can see, herbivorous mammals have the highest biomass and carnivores the lowest, as expected from the considerations above. [IMG_4] Fig. 3.13.4. Biomass of several mammal species in a North American deciduous forest As you can see, ecological pyramids cannot be very tall because part of the energy is lost when moving from one level to another. However, different organisms lose different amounts of energy. In different communities, the average level of ecological efficiency differs and is closely related to the number of trophic levels, as shown in Table 3.13.2. Table 3.13.2. Mean number of trophic levels in different biomes (R. Ricklefs, 1977)

Biome

Fig. 3.13.4. Biomass of several mammal species in a North American deciduous forest

As you can see, ecological pyramids cannot be very tall because part of the energy is lost when moving from one level to another. However, different organisms lose different amounts of energy. In different communities, the average level of ecological efficiency differs and is closely related to the number of trophic levels, as shown in Table 3.13.2.

Table 3.13.2. Mean number of trophic levels in different biomes (R. Ricklefs, 1977)

25 %

7,1

Marine coast

20 %

5,1

Steppe

10 %

4,3

5.1

5 %

3,2

Ecological efficiency varies greatly at different trophic levels, and it is particularly low at the base of ecological pyramids. Feeding on plant matter is a more complex biochemical and physiological 'task' than feeding on animal matter. In most terrestrial ecosystems, there is an excess of plant food. However, the population of herbivorous animals (level I consumers) is usually well controlled by carnivorous animals. An excess of organisms at this level will be effectively 'eaten up' by organisms at the next level. According to the natural balance hypothesis, the main regulator of the ratio of trophic levels is the apex predator – the highest-level consumer. Therefore, in a system with an even number of trophic levels, odd levels (1st, 3rd, 5th...) are more effectively controlled by consumers, and in a system with an odd number of levels, even levels are controlled. Since the first level, the producer level, is the most difficult to control in terrestrial ecosystems, one can expect that terrestrial systems are more likely to have an odd number of trophic levels. Observations confirm this assumption.

3.12. Ecological efficiencies

D. Shabanov, M. Kravchenko. Ecology: Biology of Interactions Section 3. Biogeocenology and Ecology of Communities

3.14. (addendum) Flora, Fauna, Consortia