Interaction of biotic and abiotic components result in a physical structure that is characteristic for each type of ecosystem.
Identification and enumeration of plant and animal species of an ecosystem gives its species composition.
Vertical distribution of different species occupying different levels is called stratification.
For example, trees occupy top vertical strata or layer of a forest, shrubs the second and herbs and grasses occupy the bottom layers.
The components of the ecosystem are seen to function as a unit when consider the following aspects:
(i) Productivity;
(ii) Decomposition;
(iii) Energy flow; and
(iv) Nutrient cycling.
A pond is a shallow water body in which all the above mentioned four basic components of an ecosystem are well exhibited.
The abiotic component is the water with all the dissolved inorganic and organic substances and the rich soil deposit at the bottom of the pond.
The solar input, the cycle of temperature, day-length and other climatic conditions regulate the rate of function of the entire pond.
The autotrophic components include the phytoplankton, some algae and the floating, submerged and marginal plants found at the edges.
The consumers are represented by the zooplankton, the free swimming and bottom dwelling forms.
The decomposers are the fungi, bacteria and flagellates especially abundant in the bottom of the pond.
This system performs all the functions of any ecosystem and of the biosphere as a whole, i.e., conversion of inorganic into organic material with the help of the radiant energy
of the sun by the autotrophs; consumption of the autotrophs by heterotrophs; decomposition and mineralisation of the dead matter to release them back
for reuse by the autotrophs, these event are repeated over and over again.
There is unidirectional movement of energy towards the higher trophic levels and its dissipation and loss as heat to the environment.
PRODUCTIVITY
A constant input of solar energy is the basic requirement for any ecosystem to function and sustain.
Primary production is defined as the amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis.
It is expressed in terms of weight (gm–2) or energy (kcal m–2).
The rate of biomass production is called productivity.
It is expressed in terms of gm–2 yr–1 or (kcal m–22) yr–1 to compare the productivity of different ecosystems.
It can be divided into gross primary productivity (GPP) and net primary productivity (NPP).
Gross primary productivity of an ecosystem is the rate of production of organic matter during photosynthesis.
Gross primary productivity minus respiration losses (R), is the net primary productivity (NPP).
i.e.e GPP – R = NPP
Net primary productivity is the available biomass for the consumption to heterotrophs (herbiviores and decomposers).
Secondary productivity is defined as the rate of formation of new organic matter by consumers.
Primary productivity depends on the plant species inhabiting a particular area.
It also depends on a variety of environmental factors, availability of nutrients and photosynthetic capacity of plants.
Therefore, it varies in different types of ecosystems.
DECOMPOSITION
Decomposers break down complex organic matter into inorganic substances like carbon dioxide, water and nutrients and the process is called decomposition.
Dead plant remains such as leaves, bark, flowers and dead remains of animals, including fecal matter, constitute detritus, which is the raw material for decomposition.
The steps in the process of decomposition are fragmentation, leaching, catabolism, humification and mineralisation.
Detritivores (e.g., earthworm) break down detritus into smaller particles.
This process is called fragmentation.
By the process of leaching, watersoluble inorganic nutrients go down into the soil horizon and get precipitated as unavailable salts.
Bacterial and fungal enzymes degrade detritus into simpler inorganic substances.
This process is called as catabolism.
Humification and mineralisation occur during decomposition in the soil.
Humification leads to accumulation of a dark coloured amorphous substance called humus that is highly resistant to microbial action and undergoes decomposition at an extremely slow rate.
The humus is further degraded by some microbes and release of inorganic nutrients occur by the process known as mineralisation.
Decomposition is largely an oxygen-requiring process.
The rate of decomposition is controlled by chemical composition of detritus and climatic factors.
In a particular climatic condition, decomposition rate is slower if detritus is rich in lignin and chitin, and quicker, if detritus is rich in nitrogen and
water-soluble substances like sugars.
Temperature and soil moisture are the most important climatic factors that regulate decomposition through their effects on the activities of soil microbes.
ENERGY FLOW
Except for the deep sea hydro-thermal ecosystem, sun is the only source of energy for all ecosystems on Earth.
Of the incident solar radiation less than 50 per cent of it is photosynthetically active radiation (PAR).
Plants capture only 2-10 per cent of the PAR and this small amount of energy sustains the entire living world.
All organisms are dependent for their food on producers, either directly or indirectly.
So there is unidirectional flow of energy from the sun to producers and then to consumers.
Ecosystems need a constant supply of energy to synthesise the molecules they require, to counteract the universal tendency toward increasing disorderliness.
The green plant in the ecosystem are called producers.
In a terrestrial ecosystem, major producers are herbaceous and woody plants.
Likewise, producers in an aquatic ecosystem are various species like phytoplankton, algae and higher plants.
Starting from the plants (or producers) food chains are formed such that an animal feeds on a plant or on another animal and in turn is food for another.
The chain or web is formed because of this interdependency.
No energy that is trapped into an organism remains in it for ever.
The energy trapped by the producer, hence, is either passed on to a consumer or the organism dies.
Death of organism is the beginning of the detritus food chain/web.
All animals depend on plants (directly or indirectly) for their food needs. They are hence called consumers and also heterotrophs.
If they feed on the producers, the plants, they are called primary consumers, and if the animals eat other animals which in turn eat the plants (or their produce)
they are called secondary consumers.
The primary consumers are herbivores.
Some common herbivores are insects, birds and mammals in terrestrial ecosystem and molluscs in aquatic ecosystem.
The consumers that feed on these herbivores are carnivores, or primary carnivores (though secondary consumers).
Those animals that depend on the primary carnivores for food are labelled secondary carnivores.
A simple grazing food chain (GFC) is shown below:
Grass(Producer) → Goat(Primary Consumer) → Man(Secondary consumer)
The detritus food chain (DFC) begins with dead organic matter.
It is made up of decomposers which are heterotrophic organisms, mainly fungi and bacteria.
They meet their energy and nutrient requirements by degrading dead organic matter or detritus.
These are also known as saprotrophs (sapro: to decompose).
Decomposers secrete digestive enzymes that breakdown dead and waste materials into simple, inorganic materials, which are subsequently absorbed by them.
In an aquatic ecosystem, GFC is the major conduit for energy flow.
As against this, in a terrestrial ecosystem, a much larger fraction of energy flows through the detritus food chain than through the GFC.
Detritus food chain may be connected with the grazing food chain at some levels.
Some of the organisms of DFC are prey to the GFC animals, and in a natural ecosystem, some animals like cockroaches, crows, etc., are omnivores.
These natural interconnection of food chains make it a food web.
Based on the source of their nutrition or food, organisms occupy a specific place in the food chain that is known as their trophic level.
Producers belong to the first trophic level, herbivores (primary consumer) to the second and carnivores (secondary consumer) to the third tropic level.
When any organism dies it is converted to detritus or dead biomass that serves as an energy source for decomposers.
Organisms at each trophic level depend on those at the lower trophic level for their energy demands.
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.
In nature, it is possible to have so many levels – producer, herbivore, primary carnivore, secondary carnivore in the grazing food chain
ECOLOGICAL PYRAMIDS
The base of a pyramid is broad and it narrows towards the apex.
One gets a similar shape, whether you express the food or energy relationship between organisms at different trophic level.
This, relationship is expressed in terms of number, biomass or energy.
The base of each pyramid represents the producers or the first trophic level while the apex represents tertiary or top level consumer.
The three ecological pyramids that are generally studied are:
(a) pyramid of number;
(b) pyramid of biomass and
(c) pyramid of energy.
In most ecosystems, all the pyramids, of number, of energy and biomass are upright, i.e., producers are more in number and biomass than the herbivores,
and herbivores are more in number and biomass than the carnivores.
Also energy at a lower trophic level is always more than at a higher level.
The pyramid of biomass in sea is generally inverted because the biomass of fishes 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 trophic level,
some energy is always lost as heat at each step.
Each bar in the energy pyramid indicates the amount of energy present at each trophic level in a given time or annually per unit area.
However, there are certain limitations of ecological pyramids such as it does not take into account the same species belonging to two or more trophic levels.
It assumes a simple food chain, which never exists in nature; it does not accommodate a food web.
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ECOLOGICAL SUCCESSION
The gradual and fairly predictable change in the species composition of a given area is called ecological succession.
During succession some species colonise an area and whereas populations of other species decline and even disappear.
The entire sequence of communities that successively change in a given area are called sere(s).
The individual transitional communities are termed seral stages or seral communities.
In the successive seral stages there is a change in the diversity of species of organisms, increase in the number of species and organisms as well as an increase in the total biomass.
Succession is hence a process that starts in an area where no living organisms are there – these could be areas where no living organisms ever existed, like bare rock;
or in areas that somehow, lost all the living organisms that existed there. The former is called primary succession, while the latter is termed secondary succession.
Examples of areas where primary succession occurs are newly cooled lava, bare rock, newly created pond or reservoir.
The establishment of a new biotic community is generally slow.
Before a biotic community of diverse organisms can become established, there must be soil.
Depending mostly on the climate, it takes natural processes several hundred to several thousand years to produce fertile soil on bare rock.
Secondary succession begins in areas where natural biotic communities have been destroyed such as in abandoned farm lands, burned or cut forests, lands that have been flooded.
Since some soil or sediment is present, succession is faster than primary succession.
At any time during primary or secondary succession, natural or human induced disturbances (fire, deforestation, etc.), can convert a particular seral stage of succession to an
earlier stage.
Succession of Plants
Based on the nature of the habitat, whether it is water (or very wet areas) or it is on very dry areas – succession of plants is called hydrarch or xerarch, respectively.
Hydrarch succession takes place in wet areas and the successional series progress from hydric to the mesic conditions.
Whereas, xerarch succession takes place in dry areas and the series progress from xeric to mesic conditions.
Hence, both hydrarch and xerarch successions lead to medium water conditions (mesic) – neither too dry (xeric) nor too wet (hydric).
The species that invade a bare area are called pioneer species.
In primary succession on rocks these are usually lichens which are able to secrete acids to dissolve rock, helping in weathering and soil formation.
These later pave way to some very small plants like bryophytes, which are able to take hold in the small amount of soil.
They are, with time, succeeded by higher plants, and after several more stages, ultimately a stable climax forest community is formed.
The climax community remains stable as long as the environment remains unchanged.
With time the xerophytic habitat gets converted into a mesophytic one.
In primary succession in water, the pioneers are the small phytoplanktons, which are replaced with time by rooted-submerged plants, rooted-floating angiosperms
followed by free-floating plants, then reedswamp, marsh-meadow, scrub and finally the trees.
The climax again would be a forest. With time the water body is converted into land.
In secondary succession the species that invade depend on the condition of the soil, availability of water, the environment as also the seeds or other propagules present.
Since soil is already there, the rate of succession is much faster and hence, climax is also reached more quickly.
Important fact is that all succession whether taking place in water or on land, proceeds to a similar climax community – the mesic.
NUTRIENT CYCLING
The amount of nutrients, such as carbon, nitrogen, phosphorus, calcium, etc., present in the soil at any given time, is referred to as the standing state.
It varies in different kinds of ecosystems and also on a seasonal basis.
The movement of nutrient elements through the various components of an ecosystem is called nutrient cycling.
It is also called as biogeochemical cycles (bio: living organism, geo: rocks, air, water).
Nutrient cycles are of two types:
(a) gaseous and
(b) sedimentary.
The reservoir for gaseous type of nutrient cycle (e.g., nitrogen, carbon cycle) exists in the atmosphere and for the sedimentary cycle (e.g., sulphur and
phosphorus cycle), the reservoir is located in Earth’s crust.
Environmental factors, e.g., soil, moisture, pH, temperature, etc., regulate the rate of release of nutrients into the atmosphere.
Ecosystem – Carbon Cycle
Carbon constitutes 49 per cent of dry weight of organisms and is next only to water.
71% carbon is found dissolved in oceans.
This oceanic reservoir regulates the amount of carbon dioxide in the atmosphere.
Fossil fuel also represent a reservoir of carbon.
Carbon cycling occurs through atmosphere, ocean and through living and dead organisms.
A considerable amount of carbon returns to the atmosphere as CO2 through respiratory activities of the producers and consumers.
Decomposers also contribute substantially to CO2 pool by their processing of waste materials and dead organic matter of land or oceans.
Some amount of the fixed carbon is lost to sediments and removed from circulation.
Burning of wood, forest fire and combustion of organic matter, fossil fuel, volcanic activity are additional sources for releasing CO2 in the atmosphere.
Human activities have significantly influenced the carbon cycle. Rapid deforestation and massive burning of fossil fuel for energy and transport have significantly increased the rate of release of carbon dioxide into the
atmosphere.
Ecosystem – Phosphorus Cycle
Phosphorus is a major constituent of biological membranes, nucleic acids and cellular energy transfer systems.
Many animals also need large quantities of this element to make shells, bones and teeth.
The natural reservoir of phosphorus is rock, which contains phosphorus in the form of phosphates.
When rocks are weathered, minute amounts of these phosphates dissolve in soil solution and are absorbed by the roots of the plants.
Herbivores and other animals obtain this element from plants.
The waste products and the dead organisms are decomposed by phosphate-solubilising bacteria releasing phosphorus.
Unlike carbon cycle, there is no respiratory release of phosphorus into atmosphere.
The other two major and important differences between carbon and phosphorus cycle are firstly, atmospheric inputs of phosphorus through
rainfall are much smaller than carbon inputs, and, secondly, gaseous exchanges of phosphorus between organism and environment are negligible.
ECOSYSTEM SERVICES
Healthy ecosystems are the base for a wide range of economic,environmental and aesthetic goods and 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.
Though value of such services of biodiversity is difficult to determine, it seems reasonable to think that biodiversity should carry a hefty price tag.
Researchers have put an average price tag of US $ 33 trillion a year on these fundamental ecosystems services, which are largely taken for granted because they are free.
This is nearly twice the value of the global gross national product GNP which is (US $ 18 trillion).
Out of the total cost of various ecosystem services, the soil formation accounts for about 50 per cent, and contributions of other
services like recreation and nutrient cycling, are less than 10% each.
The cost of climate regulation and habitat for wildlife are about 6% each.
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