Ecosystem

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● Ecosystem is the interaction of living things among themselves and

with their surrounding environment.

● There are two basic ecosystems − terrestrial and aquatic.

Structure of Ecosystem

● The interactions between the various biotic and abiotic factors of an

ecosystem lead to the maintenance of the ecosystem.

● Stratification is the vertical distribution of the different species

occupying the different levels. For example, trees occur at a higher

level then shrubs.

● The various aspects taken into consideration to study the functioning

of ecosystem are:

○ Productivity

○ Decomposition

○ Energy flow

○ Nutrient cycling

Productivity

● A constant supply of sunlight is required for the proper functioning of

any ecosystem.

● The amount of biomass produced per unit area over a time period by

plants during photosynthesis is defined as the primary productivity.

● It is expressed as weight (g−2) or energy (Kcal m−2).

● Productivity can be mainly divided into gross primary productivity

(GPP) and net primary productivity (NPP). GPP is the rate of

production of organic matter during photosynthesis.

NPP = GPP − Respiratory losses (R)

● Secondary productivity is defined as the rate of formation of new

organic matter by consumers.

● Primary productivity depends upon

○ type of plant species inhabiting a particular area

○ photosynthetic capacity of plants

○ nutrient availability

● Annual net productivity for whole biosphere is about 170 b tons of

organic matter.

Decomposition

● It is the process of breakdown of complex organic matter into

inorganic substances such as carbon dioxide, water, nutrients, etc.

● Fragmentation − Breaking down of detritus (dead plant and animal remains,

faecal matter) into smaller particles by detritivores (decomposers)

● Leaching - Process by which these inorganic matters enter the soil

● Catabolism − Process by which detritus is degraded into simpler inorganic

substances by bacterial and fungal enzymes

● Humification − Accumulation of humus in the soil.

Humus is resistant to microbial action and decomposes at an extremely slow

rate. It acts as a reservoir of nutrients.

● Mineralization − Process by which humus further degrades to release

minerals into the soil

● It is an oxygen consuming process and is controlled by the chemical

composition of detritus and climatic conditions.

Energy Flow

● Sun is the sole source of energy for all ecosystems on the earth.

● Plants and other photosynthetic organisms utilize less than 50% of the

solar radiation known as the photosynthetically active radiation

(PAR).

● In an ecosystem, plants are called producers and all animals depend

upon the plants directly or indirectly for their food. Hence, they are

known as consumers or heterotrophs.

● The consumers can be further divided into primary consumers

(herbivores), secondary consumers (primary carnivores), and tertiary

consumers (secondary carnivores).

● Food chain − The energy flow among the various constituent animals is known

as the food chain.

● Food web − The interconnection of the various food chains is called the food

web.

● Trophic level − Every organism occupies a specific level in their food chain

known as the trophic level.

● Standing crop − Each trophic level contains a certain amount of living

material at a certain time known as the standing crop.

● The number of trophic levels in a food chain is restricted since the

energy transfer follows the 10 percent law i.e., only 10% of the energy

is transferred from a lower trophic level to a higher one.

Ecological Pyramids

● The energy relationship between the different trophic levels is

represented by the ecological pyramids.

● 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

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

cases:

○ The pyramid of biomass is inverted in an ocean ecosystem since

a small standing crop of phytoplankton supports a large number

of zooplankton.

○ The pyramid of number can be inverted when, say, a large tree

is eaten by small insects.

○ 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:

○ The ecological pyramids do not take into account the same

species belonging to more than one trophic level.

○ It assumes a simple food chain that almost never exists in

nature. It does not explain food webs.

○ Saprophytes are not given a place in ecological pyramids even

though they play a vital role in ecosystem.

Ecological Succession

● The composition of all ecosystems keeps on changing with change in

their environment. These changes finally lead to the climax

community.

● 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.

● 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.

● 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).

● 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.

● During succession, any disturbances (natural/man-made) can convert

a particular seral stage to an earlier one.

● Hydrarch succession − It takes place in wet areas and converts hydric

conditions to mesic.

● Xerarch succession − It takes place in dry areas and converts xeric

conditions to mesic.

● 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.

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

Nutrient Cycling

● The amount of nutrients present in the soil at a given time is known as

the standing state.

● Nutrients are never lost from the ecosystem. They are only recycled

from one state to another.

● The movement of nutrients through the various components of the

ecosystem is called nutrient cycling or biogeochemical cycles. They are

of two types:

○ Gaseous − Reservoir for these types of cycles exist in the atmosphere.

○ Sedimentary − Reservoir for these types of cycles exist in the earth’s

crust.

Carbon Cycle

● About 49% of the dry weight of living organisms is made up of carbon.

● The ocean reserves and fossil fuels regulate the amount of CO2 in the

atmosphere.

● Plants absorb CO2 from the atmosphere for photosynthesis, of which a

certain amount is released back through respiratory activities.

● A major amount of CO2 is contributed by the decomposers who

contribute to the CO2 pool by processing dead and decaying matter.

● The amount of CO2 in the atmosphere has been increased considerably

by human activities such as burning of fossil fuels, deforestation.

Phosphorus Cycle

● Phosphorus is an important constituent of cell membranes, nucleic

acids, and cellular energy transfer systems.

● Rocks contain phosphorus in the form of phosphate.

● When rocks are weathered, some of the phosphate gets dissolved in

the soil solution and is absorbed by plants.

● The consumers get their phosphorus from the plants.

● Phosphorus returns back to the soil by the action of phosphatesolubilising

bacteria on dead organisms.