ECOSYSTEMS

 

ENERGY FLOW IN ECOSYSTEM:

• Except for deep sea hydrothermal ecosystem, sun is the only source of energy for all ecosystems on earth.
• Less than 50% of incident solar radiation is photosynthetically active radiations. (PAR).
• Plants capture 2-10 % of PAR and used in photosynthesis.
• All organisms depend on the producers, either directly or indirectly.
• Energy flow in the ecosystem is unidirectional i.e. energy transferred from producer to consumers.
• Energy transfer is not absolute, and spontaneous, unless energy is degraded it can not be transfer. When energy transferred from one trophic level to another, lot of energy lost in the form of heat to the environment.
• Only 10% of energy transferred from one trophic level to other.

Food chain:

• Grazing food chain: it extends from producers through herbivore to carnivore.
• Detritus food chain: Begins with dead organic matter (detritus) and pass through detritus feeding organism in soil to organisms feeding on detritus-feeders.
• In aquatic ecosystem GFC is the major conduit for energy flow.
• In terrestrial ecosystems a much larger fraction of energy flows through the detritus food chain than through GFC
• Different food chains are naturally interconnected e.g. a specific herbivore of one food chain may serve as food of carnivores of other food chains. Such interconnected matrix of food chains is called food web.
• Trophic level: A group of organism irrespective of their size having same source of energy or similar food habit constitute a trophic level.
• Standing crop: each trophic level has a certain mass of living material at a particular time called as the standing crop.
• The standing crop is measured as the mass of living organisms (biomass) or the number in a unit area.
• The number of trophic levels in a food chain is restricted by 10 % flow of energy, less amount of energy available to the last trophic level.

ECOLOGICAL PYRAMID:

• The base of the pyramid is broad and it narrows down at the apex. The similar shape is obtained when food or energy relationship between organisms at different trophic level.
• The relationship can be expressed in terms of number, energy or biomass.
• The base of the pyramid represented by producer and apex is the top consumer; other trophic levels are in between.
• In most ecosystems, all the pyramids, of number, of energy and biomass are upright.
• The pyramid of number in a tree ecosystem is inverted.
• The pyramid of biomass in sea also inverted because the biomass of fishes is 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, some energy is always lost as heat at each step.

Limitations of ecological pyramids:

• It does not take into account the same species belonging to two or more trophic levels.
• It assumes a simple food chain, it never exits in nature.
• It dose not accommodate food web.
• Saprophytes are not given place in ecological pyramids.

ECOLOGICAL SUCCESSION:

• The gradual and fairly predictable change in the species composition of a given area is called ecological succession.
• Composition and structure of the community constantly change in response to changing environmental condition.
• This change is orderly and sequential, parallel with the changes in the physical environment.
• All the changes lead finally to a community that is in near equilibrium with the environment and that is called climax community.
• During succession some species colonize and area and their populations become more numerous, whereas populations of other species decline and even disappear.
• The entire sequences of communities that successively change in a given area are called sere.
• The individual transitional communities are termed as seral stages.
• In the successive seral stages there is a change in the diversity of species of organisms, in crease in number of species and total biomass.
• Primary succession: succession that starts where no living organisms are there- these could be areas where no living organism ever existed may be a bare rock or new water body.
• Secondary succession:  succession that starts in areas that somehow, lost all the living organisms that existed there.
• Primary succession occurs in:-
• newly cooled lava,
• bare rock,
• Newly created pond or reservoir.
• Secondary succession begins in areas where natural biotic communities have been destroyed such as
• In abandoned farm lands.
• Burned or cut forest,
• land that have been flooded
• Since some soil or sediment is present, secondary succession is faster than primary succession.

Succession in plants:

• Based on the nature of habitat – whether it is water or it is on very dry areas- succession of plants is called hydrarch or xerarch.
• Hydrarch succession takes place in water areas and the successional series progress from hydric to mesic condition.
• Xerarch succession takes place in dry areas and the series progress from xeric to mesic conditions.
• Both hydrarch and xerarch successions lead to medium water conditions (mesic) – neither too dry (xeric) nor too wet (hydric)

Xerarch succession: Succession in bare rock:

• The species that invades bare area are called pioneer species.
• In primary succession on bare rock the pioneer species is the lichen.
• Lichen secretes acid to dissolve rock, helping in weathering and soil formation.
• The little soil, leads to growth of bryophytes (mosses).
• The mosses speed up the process of soil accumulation by trapping wind-blown particles.
• Lichen moss carpet provides suitable substratum for the germination of seeds of herbaceous plants.
• Gradually more soil is accumulated and herbaceous species make way for the invasion of shrubs followed by trees.
• The climax community is generally dominated by trees.

 

CBSE Biology (Chapter Wise) Class XII ( By Mr. Hare Krushna Giri )
Email Id : [email protected]