NCERT Book Class 12 Biology Ecosystem

ECOSYSTEM

Introduction

An ecosystem can be visualised as a functional unit of nature, where living organisms interact among themselves and also with the surrounding physical environment. Ecosystem varies greatly in size from a small pond to a large forest or a sea. Many ecologists regard the entire biosphere as a global ecosystem, as a composite of all local ecosystems on Earth. Since this system is too much big and complex to be studied at one time, it is convenient to divide it into two basic categories, namely the terrestrial and the aquatic.

Forest, grassland and desert are some examples of terrestrial ecosystems; pond, lake, wetland, river and estuary are some examples of aquatic ecosystems. Crop fields and an aquarium may also be considered as man-made ecosystems. We will first look at the structure of the ecosystem, in order to appreciate the input (productivity), transfer of energy (food chain/web, nutrient cycling) and the output (degradation and energy loss). We will also look at the relationships – cycles, chains, webs – that are created as a result of these energy flows within the system and their inter- relationship. 

In chapter 13, you have looked at the various components of the  environment- abiotic and biotic. You studied how the individual bioticand abiotic factors affected each other and their surrounding. Let us look at these components in a more integrated manner and see how the flow of energy takes place within these components of the ecosystem. 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 you consider the following aspects:

 (i) Productivity;

 (ii) Decomposition;

 (iii) Energy flow; and

 (iv) Nutrient cycling.

To understand the ethos of an aquatic ecosystem let us take a small pond as an example. This is fairly a self-sustainable unit and rather simple example that explain even the complex interactions that exist in an aquatic ecosystem. 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 torelease 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.

14.2. 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 (g –2) or energy (kcal m–2). The rate of biomass production is called productivity.It is expressed in terms of g–2 yr –1 or (kcal m–2) yr–1 to compare the  productivity of different ecosystems. It can be divided into gross primaryproductivity (GPP) and net primary productivity (NPP). Gross primary productivity of an ecosystem is the rate of production of organic matter during photosynthesis. A considerable amount of GPP is utilised by plants in respiration. 

Gross primary productivity minus respiration losses (R), is the net primary productivity (NPP).

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.

EXERCISES

1. Fill in the blanks.

 (a) Plants are called as_________because they fix carbon dioxide.

 (b) In an ecosystem dominated by trees, the pyramid (of numbers) is_________type.

 (c) In aquatic ecosystems, the limiting factor for the productivity is_________.

 (d) Common detritivores in our ecosystem are_________.

 (e) The major reservoir of carbon on earth is_________.

2. Which one of the following has the largest population in a food chain?

 (a) Producers

 (b) Primary consumers

 (c) Secondary consumers

 (d) Decomposers

3. The second trophic level in a lake is-

 (a) Phytoplankton

 (b) Zooplankton

 (c) Benthos

 (d) Fishes

4. Secondary producers are

 (a) Herbivores

 (b) Producers

 (c) Carnivores

 (d) None of the above

5. What is the percentage of photosynthetically active radiation (PAR), in the incident solar radiation.

 (a) 100%

 (b) 50 %

 (c) 1-5%

 (d) 2-10%

6. Distinguish between

 (a) Grazing food chain and detritus food chain

 (b) Production and decomposition

 (c) Upright and inverted pyramid

 (d) Food chain and Food web

 (e) Litter and detritus

 (f) Primary and secondary productivity

7. Describe the components of an ecosystem.

8. Define ecological pyramids and describe with examples, pyramids of number and biomass.

9. What is primary productivity? Give brief description of factors that affect primary productivity.

10. Define decomposition and describe the processes and products of decomposition.

11. Give an account of energy flow in an ecosystem.

12. Write important features of a sedimentary cycle in an ecosystem.

13. Outline salient features of carbon cycling in an ecosystem.

Please refer to attached file for NCERT Class 12 Biology Ecosystem