Functioning of an Ecosystem

Functioning of an Ecosystem

An ecosystem is driven by the flow of energy and circulation of matter by biogeochemical cycle between the biosphere, lithosphere, hydrosphere and atmosphere. The solar radiation is the basic input of energy entering the ecosystem. In the living organisms or biosphere, the energy (chemical energy) and matter (food), collectively called as food energy, are transferred via food chain and food web through various trophic levels. Thus all the elements of an ecosystem are interdependent and integrated. An ecosystem itself is integrated with other ecosystems and thus they all become interdependent. In fact every ecosystem is part of the larger ecosystem, until the larger ecosystem of the earth-the biosphere – is formed. That is why we see climatic changes happening in one ecosystem affecting the other ecosystems.

Productivity of Ecosystem

Only a small fraction of sunlight striking the earth is concerted to chemical energy by primary producers. The rate of conversion of solar energy into chemical energy (organic matter) by autotrophs is called productivity of ecosystem. The productivity of ecosystem depends to two things:-

(i)                  The availability of solar radiation to autotrophic primary producers. The productivity of ecosystems goes in decreasing from equator to poles as the amount of solar energy received goes on decreasing from equator to poles. Therefore, plants in tropical areas will have higher productivity than plants in temperate or tundra region.

(ii)                The efficiency of plants to concert solar energy into food or chemical energy which is also called as primary production. It is measured in two ways:

(a)    Gross primary productivity which is total amount of chemical energy assimilated by the autotrophs from solar energy.

(b)   Net Primary Productivity is equal to Gross Primary Productivity minus the energy lost through respiration.

The Respiration means energy required to keep the metabolism of an organism going on. So highly mobile organisms as well as higher life forms such as mammals have higher respiratory rate. Complex plants such as those in tropical rainforests have higher respiratory rate than simple plants like algae or grasslands. Net primary productivity represents the usable amount of energy at the first trophic level, which is made available to higher trophic levels.

(iii)               Other than sunlight, the efficiency of the plants is also limited by nutrient availability. Generally the limiting nutrients are nitrogen or phosphorous but most often water and temperature, together limit the productivity of an ecosystem. For example, the Polar Regions having less sunlight and desert regions having less water have lower productivity. Besides these there are biological factors like mutualism, parasitism, predation, etc which increase the productivity of an ecosystem.

Secondary Productivity: The rate at which ecosystem’s consumers convert the chemical energy of the food they eat into their own new biomass in called the secondary productivity.

Energy Efficiency: The concept of primary and secondary productivity a gives the idea of energy efficiency. Not all the energy produced at one trophic level is available for transfer to next trophic level (as stored food energy) but majority is lost in respiration.

Energy efficiencies or trophic efficiency refers to the energy transferred to the next level. Tropic efficiencies generally range from 5 to 20% e. g. plants transfer around 7 – 10% of total energy produced by them to the herbivores. Herbivores being mobile have higher metabolic respiration. As a result relative loss of energy increases at successively higher trophic levels.

A common way of illustrating ecological efficiency is via pyramids of productivityThere are a few other terms. Biomass refers to the quantity or weight of living matter per unit area per unit time. It is represented in terms of dry weight. Biomass is comprised of plants and animals and therefore it is referred to as plant biomass or animal biomass.

Total plant biomass including above ground and subsurface plants is called standing crop.

On the basis, the Productivity of ecosystem refers to the rate of increase of biomass whereas Production is the amount of biomass of a given unit area at a given time. Mean net primary productivity for the whole earth is 320 dry gram/m2/ year. Also the productivity of continental ecosystems is higher than that of marine ecosystems.

Food Chain and Food web

The formation and transfer of food energy in the ecosystem takes place from one level to the next in a hierarchical manner. The levels through which food energy passes from one group of organisms to the other group are called tropic levels.

The chain of transformation and transfer, of food energy in the ecosystem from one group of organism to the other group through a series of steps or levels (trophic levels) is called food chain.

In other words, the chain of transfer of food energy form one group of organisms to the other group in the biosphere (ecosystem) is called food chain and the point where food energy is transferred from one group of organisms to the other group is called trophic level.

On an average there are four trophic levels but they can vary from 2,3 to even 5, 6 etc depending on the length of food chain or size and scale of ecosystem we take into account.

Trophic Level- 1: This is the base of food chain where autotrophic primary producers like green plants produce food from nutrients, CO₂ and water by the process of photosynthesis. Sunlight is the primary source of energy entering the ecosystem.

Thus green plants transform solar energy into food energy entering the ecosystem.

Thus green plants transform solar energy into food energy or chemical energy, which is stored by green plants to develop their tissues. Some portion of this energy is lost through respiration, while some other portion is transferred form trophic level one to next trophic levels when herbivores or omnivores eat the plants.

Trophic Level – 2: It includes herbivores. These constitute primary consumers. They do not produce food on their own but depend on autotrophy for food e.g. grazers like goat, rabbit, etc.

Trophic Level- 3: The animals which depend on herbivore animals for food are included in trophic level 3. They are called carnivores as well as secondary consumers.

Trophic Level – 4: The animals which derive their food from all the three lower trophic levels are included in trophic level 4 e.g. man. These animals are called as omnivores. Decomposers too drive their energy from all the trophic levels.

Thus food chain is the sequence of energy transfer from one trophic level to the next one. A food chain may be linear e.g. crops- food grains – mouse – snake- hawk.

A food chain may also be complex involving many interconnected and overlapping food chains. This happens when greater number of species feed on many kinds of prey. Such complicated food chain is called a food web. For example, plants produce grains which are eaten by mouse. Mouse is eaten by snakes, cats, and mongoose. Mongoose itself eats snakes. Similarly insects feeding on animals ‘bloods are eaten by frogs while frogs themselves are eaten by snakes and mongoose. The prey birds like hawk, eagles and vultures too are dependent on population of these small animals. On the other hand grass and plants are eaten by goats which are eaten by wolves, leopards and tigers and man. Plants in turn are dependent on birds, butterflies, squirrel, man etc for seed dispersal. Thus in a food web large number of organisms are dependent on a multitude of organisms for their survival. If population of any one of the species is increased or decreased with respect to carrying capacity of the region, then survival of the whole food chain is risked. When an ecosystem has large number of species and prey base, it becomes resilient to changes in population of a particular species, as alternative food is also available .A  complex food chain is an indicator of large and well developed biodiversity.