Ecological pyramids are graphical representations of the trophic structure of ecosystems. Ecological pyramids are organized with the productivity of plants on the bottom, that of herbivores above the plants, and carnivores above the herbivores. If the ecosystem sustains top carnivores, they are represented at the apex of the ecological pyramid of productivity.
A fact of ecological energetics is that whenever the fixed energy of biomass is passed along a food chain, substantial energy losses occur during each transfer. These energy losses are a necessary consequence of the so-called second law of thermodynamics . This universal principle states that whenever energy is transformed from one state to another, the entropy of the universe must increase (entropy refers to the randomness of distributions of matter and energy). In the context of transfers of fixed biological energy along the trophic chains of ecosystems, increases in entropy are represented by losses of energy as heat (because energy is converted from a highly ordered state in biomass, to a much less-ordered condition as heat). The end result is that transfers of energy between organisms along food chains are inefficient, and this causes the structure of productivity in ecological food webs to always be pyramid shaped.
Ecological food webs
Ecological food webs are based on the productivity of green plants (or photoautotrophs), which are the only organisms capable of utilizing diffuse solar radiation to synthesize simple organic compounds from carbon dioxide and water . The fixed energy of the simple organic compounds, plus inorganic nutrients , are then used by plants in more complex metabolic reactions to synthesize a vast diversity of biochemicals. Plants utilize the fixed energy of their biochemicals to achieve growth and reproduction. On average, plant photosynthesis utilizes less than 1% of the solar radiation that is received at the surface of the earth . Higher efficiencies are impossible for a number of reasons, including the second law of thermodynamics, but also other constraining factors such as the availability of nutrients and moisture, appropriate temperatures for growth, and other environmental limitations. However, even relatively fertile plant communities can only achieve conversion efficiencies of 10% or so, and only for relatively short periods of time .
The solar energy fixed by green plants in photosynthesis is, of course, the energetic basis of the productivity of all heterotrophic organisms that can only feed upon living or dead biomass, such as animals and microorganisms . Some of the biomass of plants is consumed as food by animals in the next trophic level, that of herbivores. However, herbivores cannot convert all of the energy of the vegetation that they eat into their own biomass. Depending on the digestibility of the food being consumed, the efficiency of this process is about 1-20%. The rest of the fixed energy of the plant foods is not assimilated by herbivores, or is converted into heat. Similarly, when carnivores eat other animals, only some of the fixed energy of the prey is converted into biomass of the predator . The rest is ultimately excreted, or is converted into heat, in accordance with the requirement for entropy to increase during any energy transformation.
It is important to recognize that the second law of thermodynamics only applies to ecological productivity (and to the closely related variable of energy flow). Consequently, only the trophic structure of productivity is always pyramid shaped. In some ecosystems other variables may also have a trophic structure that is pyramid shaped, for example, the quantities of biomass (also known as standing crop) present at a particular time, or the sizes or densities of populations. However, these latter variables are not pyramid shaped for all ecosystems.
One example of plants having a similar, or even smaller total biomass as the herbivores that feed upon them occurs in the open ocean . In that planktonic ecosystem the phytoplankton (or single-celled algae ) typically maintain a similar biomass as the small animals (called zooplankton ) that feed upon these microscopic plants. However, the phytoplankton cells are relatively short-lived, and their biomass is regenerated quickly because of the high productivity of these microorganisms. In contrast, the herbivorous zooplankton are longer lived, and they are much less productive than the phytoplankton. Consequently, the productivity of the phytoplankton is much larger than that of the zooplankton, even though at any particular time their biomasses may be similar.
In some ecosystems, the pyramid of biomass may be inverted, that is, characterized by a larger biomass of herbivores than of plants. This can sometimes occur in grasslands , where the dominant plants are relatively small, herbaceous species that may be quite productive, but do not maintain much biomass at any time. In contrast, the herbivores that feed on the plants may be relatively large, long-lived animals, and they may maintain a larger total biomass than the vegetation. Inverted biomass pyramids of this sort occur in some temperate and tropical grasslands, especially during the dry seasons when there can be large populations, and biomasses, of long-lived herbivores such as deer , bison , antelopes, gazelles , hippopotamuses , rhinos, elephants, and other big animals. Still, the annual productivity of the plants in grasslands is much larger than that of the herbivores.
Similarly, the densities of animals are not necessarily less than those of the plants that they eat. For example, insects are the most important herbivores in most forests , where they can maintain very large population densities. In contrast, the densities of tree populations are much smaller, because each individual organism is large and occupies a great deal of space. In such a forest, there are many more small insects than large trees or other plants, so the pyramid of numbers is inverted in shape. However, the pyramid of productivity in the forest is still governed by the second law of thermodynamics, and it is much wider at the bottom than at the top.
Sustaining top carnivores
Because of the serial inefficiencies of energy transfer along food chains, there are intrinsic, energetic limits to the numbers of top carnivores that ecosystems can sustain. If top predators such as lions or killer whales are to be sustained in some minimal viable productivity and population size, there must be a suitably large productivity of animal prey that these animals can exploit. Their prey must in turn be sustained by a suitably large productivity of appropriate plant foods. Because of these ecological constraints, only very productive or extensive ecosystems can sustain top predators.
African savannas and grasslands sustain more species of higher-order carnivores than any other existing terrestrial ecosystems. The most prominent of these top predators are lion, leopard, cheetah, hyena , and wild dog. Although these various species may kill each other during some aggressive interactions (lions and hyenas are well known for their mutual enmity), they do not eat each other, and each can therefore be considered a top predator. In this unusual case, a large number of top predators can be sustained because the ecosystem is very extensive, and also rather productive of vegetation in most years. Other, very extensive but unproductive ecosystems may only support a single species of top predator, as is the case of the wolf in the arctic tundra .
Ricklefs, R.E. Ecology. New York: W. H. Freeman, 1990.