CBSE Class 12 Biology Notes Chapter 14 Ecosystem PDF Download

Ecological Pyramids

1. Pyramid of Number : This pyramid shows the number of individuals per unit area at different trophic levels, with producers at the base and consumers at successively higher levels. It is generally upright. However, in the case of a large tree, the pyramid of number can be inverted because the number of insects feeding on the tree exceeds the number of trees.
2. Pyramid of Biomass : This pyramid represents the biomass at various trophic levels. It is usually upright, except in aquatic food chains that involve short-lived organisms like plankton. In the sea the pyramid of biomass can be inverted because the biomass of fish can exceed that of phytoplankton.
3. Pyramid of Energy : This pyramid graphically represents the amount of energy captured by different trophic levels per unit area. The pyramid of energy is always upright and cannot be inverted. This is because energy is always lost as heat during each transfer from one trophic level to the next (e.g., during feeding, digestion, assimilation and respiration).

Ecological Succession

• The orderly and sequential changes that lead to a stable community are known as the climax community .
• The entire sequence of communities that successively change in a given area is referred to as a sere , while the individual transitional communities are called seral stages or seral communities .

Types of Ecological Succession :

1. Primary Succession : This type of succession begins in an area where no organisms exist, such as on bare rocks or cooled lava from a volcano.
2. Secondary Succession : This occurs in areas where living organisms have been removed or destroyed due to disturbances like forest fires or earthquakes.

Succession of Plants

• Hydrach Succession : This occurs in wetter areas, progressing from hydric (wet) to mesic (moderate) conditions.
• Xerarch Succession : This takes place in dry areas, moving from xeric (dry) to mesic (moderate) conditions.

Pioneer Species : The species that invade bare areas are called pioneer species. For example, in primary succession on rocks, lichens act as pioneer species by secreting acids that help weather the rock to form soil. In aquatic environments, small phytoplankton serve as pioneer species before being replaced by free-floating angiosperms.