Sun is the primary source of energy for all ecosystems on Earth. Of the incident solar radiation less than 50 per cent of it is photosynthetically active radiation (PAR). Photosynthetically active radiation, often abbreviated PAR, designates the spectral range (wave band) of solar radiation from 400 to 700 nanometers that photosynthetic organisms are able to use in the process of photosynthesis.
Plants capture only 2-10 per cent of the PAR and this small amount of energy sustains the entire living world. The energy of sunlight fixed in food production by green plants is passed through the ecosystem by food chains and webs from one trophic level to the next. In this way, energy flows through the ecosystem.
The Trophic Structure of Ecosystems
- The organisation and pattern of feeding in an ecosystem is known as the trophic structure.
- The levels through which food energy passes from one group of organism to the other group are called trophic levels.
Food Chain
The chain of transformation and transfer of food energy in the ecosystem from one group of organism
to another group through a series of steps or levels is called food chain.
Two types of food-chains are recognised:
Tropic levels in an ecosystem
- Grazing food-chain: In a grazing food-chain, the first level starts with plants as producers and
A Food Chain
ends with carnivores as consumers at the last level, with the herbivores being at the intermediate level.
There is a loss of energy at each level which may be through respiration, excretion or decomposition. The levels involved in a food chain range from three to five and energy is lost at each level. The phytoplanktons
→zooplanktons →Fish sequence or the grasses →rabbit →Fox sequences are the examples, of grazing food chain.
- Detritus food chain: This type of food chain goes from dead organic matter into microorganisms and then to organisms feeding on detritus (detrivores) and their predators. Such ecosystems are thus less dependent on direct solar energy. These depend chiefly on the influx of organic matter produced in another system. For example, such type of food chain operates in the decomposing accumulated litter in a temperate forest.
Food Web
When the feeding relationship in a natural ecosystem become more complicated, the food chain does not remain simple and linear rather it is also complicated by several inter-connected overlapping food chains. This happens when greater number of species feed on many kinds of prey.
Such complicated food chain is called food web.
- Thus, Energy is passed through the system in food chains and webs. The flow of energy in ecosystems is
- The important point to note is that the amount of energy decreases at successive trophic levels. The number of trophic levels in the grazing food chain is restricted as the transfer of energy follows 10 per cent
law – only 10 per cent of the energy is transferred to each trophic level from the lower trophic level.
A Food Web
- Storage of energy in the system is shown by the amount of living material in both the plants and animals present. The amount of living material present is called the standing crop.
- This can be expressed in several ways but is usually shown as biomass (living material) per unit area, measured as dry weight, ash weight or calorific value.
- Usually the amount of standing crop in each trophic level decreases with each step on the food chain away from the plants. This can be shown diagrammatically by Ecological pyramids.
Ecological Pyramid
The pyramid shape of decrease of total number of species, total biomass and energy availability with successive higher trophic levels in the food chain in a natural ecosystem is called ecological pyramid.
- The base of each pyramid represents the producers or the first trophic level while the apex represents tertiary or top level consumer. The three ecological pyramids that are usually studied are:
- pyramid of number;
- pyramid of biomass and
- pyramid of energy.
- Number Pyramid is the pyramid formed by the number of species from one trophic level to higher trophic levels.
- Biomass pyramid includes the total weight of the organic matter (total biomass) of each trophic level. Thus, the pyramid formed by total biomass at each trophic level is called biomass pyramid.
- Energy Pyramid is the pyramid representing total amount of energy present at each trophic
Energy flow through different trophic levels
level of food chain in a natural ecosystem per unit area per unit time. The energy is expressed in kilocalories per square meter per day or per year. (Kcal/m2/day or year)
Any calculations of energy content, biomass, or numbers has to include all organisms at that trophic level. No generalisations we make will be true if we take only a few individuals at any trophic level into account.
- A given organism may occupy more than one trophic level simultaneously. One must remember that the trophic level represents a functional level, not a species as such. A given species may occupy more than one trophic level in the same ecosystem at the same time; for example, a sparrow is a
Figure 10: An ideal pyramid of energy. Observe that primary producers convert only 1% of the energy in the sunlight available to them into NPP
primary consumer when it eats seeds, fruits, peas, and a secondary consumer when it eats insects and worms.
- In most ecosystems, all the pyramids, of number, of energy and biomass are upright, i.e., producers are more in number and biomass than the herbivores, and herbivores are more in number and biomass than the carnivores. Also energy at a lower trophic level is always more than at a higher level.
- There are exceptions to this generalisation, for example the number of insects feeding on a big
- The pyramid of biomass in sea is also generally inverted because the biomass of fishes far exceeds that of phytoplankton.
- Pyramid of energy is always upright, can never be inverted, because when energy flows from a particular trophic level to the next trophic level, some energy is always lost as heat at each step
