Class 12 Biology

Ecological Pyramids

The three ecological pyramids that are usually studied are

Pyramid of number

It shows a sharp decrease in number as we move up the trophic levels. This pyramid is generally upright. But there are exceptions to this rule. For example; number of insects feeding on a large banyan tree is numerous.

Pyramid of biomass

It shows a sharp decrease in biomass as we move up the trophic levels. This pyramid is upright in terrestrial ecosystems. But this pyramid is inverted in oceans because the biomass of fishes far exceeds the biomass of phytoplankton.

Pyramid of energy

This pyramid is always upright. The primary producers convert only 1% of the energy in the sunlight available to them into NPP. Out of this, only 10% is available for the organisms at the next trophic level.

Limitations of Ecological Pyramids

It does not take into account the same species belonging to two or more trophic levels. It is based on the assumption of a simple food chain; which never exists in nature. Saprophytes are not given any place in these pyramids.

Ecological Succession

The gradual and fairly predictable change in the species composition of a given area is called ecological succession. These changes finally lead to a community that is in near equilibrium with the environment and such a community is called a climax community.

Sere: The entire sequence of communities that successively change in a given area is called sere (s). The individual transitional communities are called seral stages or seral communities. In the successive seral stages there is a change in diversity, increase in number of species, and increase in total biomass.

Primary SuccessionSecondary Succession
Succession starts at a place where there was no organism.Succession starts at a place where living organisms were lost due to natural or man-made reasons.
This is a slow process.This is a fast process.
Examples: newly cooled lava, bare rock, new pond, etc.Examples: farm after harvest, flooded plains, etc.

Succession of Plants

Hydrarch Succession: When succession of plants takes place in wet areas, it is called hydrarch succession.

Xerarch Succesion: When succession of plants takes place in dry areas, it is called xerarch succession. In both cases, succession proceeds towards meric conditions, i.e. medium water conditions.

Pioneer Species: The species that invade a bare area are called pioneer species. Lichens are usually the pioneer species in primary succession on rocks. Lichens secrete acids to dissolve rocks, and thus help in weathering and soil formation. Lichens are followed by bryophytes. Higher plants come at the end of the process.

Small phytoplanktons are the pioneers in primary succession in water. they are followed by free-floating angiosperms, then by rooted hydrophytes, sedges, grasses and finally the trees. In due course of time, the water body is converted into land, and thus the climax is forest.

Nutrient Cycling

Standing State: The amount of nutrients present in the soil at any given time is called standing state. It varies in different types of ecosystem and also in different seasons.

Nutrient Cycling: The movement of nutrient elements through various components of an ecosystem is called nutrient cycling or biogeochemical cycle. There are two types of nutrient cycling, viz. gaseous and sedimentary. Atmosphere serves as the reservoir for gaseous type of nutrient cycle. The earth’s crust serves as the reservoir for sedimentary type nutrient cycle. The function of the reservoir is to meet with the deficit which occurs due to imbalance in the rate of influx and efflux.

Carbon Cycle

As per an estimate, 4 x 1013 kg of carbon is fixed in the biosphere through photosynthesis annually.

A considerable amount of carbon returns to the atmosphere as CO2 through respiration by organisms. Substantial amount of CO2 is contributed by decomposers during processing of waste materials. Burning of fossil fuels, wood, forest fire, combustion of organic matter and volcanic activities are other sources of CO2 release into the atmosphere.

Phosphorus Cycle

Rock is the natural reservoir of phosphorus. Phosphorus is present in rocks in the form of phosphates. Minute quantities of phosphates dissolve in soil solution during weathering of rocks. Phosphate is absorbed by plants, and consumers get it from plants. Decomposition by phosphate-solubilising bacteria facilitates release of phosphorus back to the atmosphere.

Ecosystem Services

The products of ecosystem processes are named as ecosystem services, for example, healthy forest ecosystems purify air and water, mitigate droughts and floods, cycle nutrients, generate fertile soils, provide wildlife habitat, maintain biodiversity, pollinate crops, provide storage site for carbon and also provide aesthetic, cultural and spiritual values.

Robert Constanza and his colleagues have put an average price tag of US $ 33 trillion a year on these fundamental ecosystems services.