Ecological Pyramids

Ecological Pyramids

Ecological food webs

Ecological pyramids

Sustaining top carnivores

Resources

Ecological pyramids are graphical representations used to represent the energy in various tophic levels of ecosystems. Ecological pyramids may depict the number of individuals, the biomass, or the amount of energy in each trophic level. They are organized plants on the bottom, 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.

As energy is passed along a food chain through trophic interactions, substantial energy losses occur during each transfer. These energy losses are a consequence of the second law of thermodynamics, which states that whenever energy is transformed from one state to another, entropy increases. In biological energy transfer, increases in entropy generally result in the prodction of heat. Energy is converted from a highly ordered state in biomass to a much less-ordered condition as heat. Transfers of energy between organisms along food chains are inefficient resulting in a

pyramid-shaped representation of productivity in ecological food webs.

Ecological food webs

Ecological food webs are based on the productivity of autotrophs, which are organisms capable of utilizing inorganic forms of energy to synthesize organic compounds. The major autotrophs in ecosystems are plants, which perform photosynthesis for growth and reproduction. On average, plantphotosynthesis 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, the energetic basis of the productivity of heterotrophic organisms, such as animals and microorganisms. The biomass of plants is consumed by herbivores, animals in the next trophic level. However, herbivores cannot convert all of the energy that they consume into biomass. The efficiency of this process is about 1-20%. The rest of the energy of the plants is either not assimilated 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.

Ecological pyramids

Because of the second law of thermodynamics, the trophic structure of energy, or productivity, is always pyramid shaped. In many ecosystems this shape is reflected in the number of individuals and the biomass at different trophic levels. However, these latter variables are not pyramid shaped for all ecosystems. In the open ocean planktonic ecosystem, the phytoplankton (or single-celled algae) typically have a similar biomass as the small animals (called zooplankton) that feed on them. However, the phytoplankton cells are relatively short-lived, and their biomass is regenerated quickly. The herbivorous zooplankton has longer life cycles, and their turnover rates are slower 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. Inverted ecological pyramids may occur in grasslands, where the dominant plants are relatively small but quite productive. In this case, the plants will not have much biomass at any time, but the rates of energy fixation are high. Indeed, the annual productivity of the plants in grasslands is much larger than that of the herbivores. In these regions, 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 of long-lived herbivores such as deer, bison, antelopes, gazelles, hippopotamuses, rhinos, elephants, and other large animals.

When building ecological pyramids from numbers of individuals, inversions can also occur. For example, insects are the most important herbivores in most forests, where they can be found in great numbers. In contrast, the numbers of trees are much smaller, because each individual is large and occupies a great deal of space. However, building an ecological pyramid using energy 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 an ecosystem, there must be a suitably large biomass of prey that these animals can exploit. Their prey must in turn be sustained by a suitably large biomass of autotrophs. Because of these energetic constraints, only very productive ecosystems can sustain top predators.

African savannas and grasslands sustain more species of higher-order carnivores than any other terrestrial ecosystem. The most prominent of these top predators are lion, leopard, cheetah, hyena, and wild dog. Although these species may kill each other during aggressive interactions (lions and hyenas are well known for their mutual enmity), they do not regularly prey upon each other, and are all top predators. In this unusual case, a large number of top predators can be sustained because the grasslands ecosystem is very productive at its base.

See also Autotroph; Carnivore; Food chain/web; Herbivore; Heterotroph; Trophic levels.

Resources

BOOKS

Odum, Eugene and Gary W. Barrett. Fundamentals of Ecology. Stamford, CT: Brooks Cole, 2004.

Smith, Robert Leo and Thomas M. Smith. Elements of Ecology. 6th ed. San Francisco, CA: Benjamin Cummings, 2005.

Bill Freedman