Ecological Pyramids and Succession

Ecological Pyramids

Ø The energy relationship between the different trophic levels is represented by the ecological pyramids.

Ø Their base represents the producers or the first trophic level while the apex represents the tertiary or top level consumer.

Ecological pyramids are of 3 types:

· Pyramid of number

· Pyramid of biomass

· Pyramid of energy

Pyramid of numbers in a grassland ecosystem. Only three top-carnivores are supported in an ecosystem based on production of nearly 6 millions plants

Pyramid of biomass shows a sharp decrease in biomass at higher trophic levels

Inverted pyramid of biomass-small standing crop of phytoplankton supports large standing crop of zooplankton

An ideal pyramid of energy. Observe that primary producers convert only 1% of the energy in the sunlight available to them into NPP

In most ecosystems, the three pyramids are upright except in some cases:

o The pyramid of biomass is inverted in an ocean ecosystem since a small standing crop of phytoplankton supports a large number of zooplankton.

o The pyramid of number can be inverted when, say, a large tree is eaten by small insects.

o However, the pyramid of energy is always upright.

A trophic level represents a functional level and not a single species as such. Also, a single species may become a part of more than one trophic level in the same ecosystem at the same time depending upon the role it plays in the ecosystem.

Limitations of ecological pyramids:

Ø The ecological pyramids do not take into account the same species belonging to more than one trophic level.

Ø It assumes a simple food chain that almost never exists in nature. It does not explain food webs.

Ø Saprophytes are not given a place in ecological pyramids even though they play a vital role in ecosystem.

Ecological Succession

Ø The composition of all ecosystems keeps on changing with change in their environment. These changes finally lead to the climax community.

Ø Climax community − It is the community which is in equilibrium with its environment. Gradual and fairly predictable change in the species’ composition of a given area is called ecological succession.

Ø Sere(s) − It is the sequence of communities that successively change in a given environment. The transitional communities are called seral stages or seral communities.

Ø Succession happens in areas where no life forms ever existed as in bare rocks, cool lava, etc. (primary succession), or in areas which have lost all life forms due to destructions and floods (secondary succession).

Ø Primary succession takes hundreds to thousands of years as developing soil on bare rocks is a slow process. Secondary succession is faster than primary succession since the nature does not have to start from scratch.

Primary succession

Ø During succession, any disturbances (natural/man-made) can convert a particular seral stage to an earlier one.

Ø Hydrarch succession − It takes place in wet areas and converts hydric conditions to mesic.

Ø Xerarch succession − It takes place in dry areas and converts xeric conditions to mesic.

Ø Pioneer species − These are the species that first invade a bare area. On land, these could be lichens that secrete enzymes to dissolve the rock surfaces for soil formation while in water, pioneer species could be phytoplanktons.

Ø The ultimate result of all successions is a climax community, a mesic.