Organisms and Populations - Notes | Class 12 | Part 1: Organisms and its Environment


Ecology is the study of interactions among organisms and between the organism and its physical (abiotic) environment. 

Ecology is concerned with 4 levels of biological organization: Organisms, Populations, Communities & Biomes.

Physiological ecology (Ecology at the organismic level) is the study of adaptation of an organism to environments in terms of survival and reproduction.

The rotation of earth and the tilt of its axis cause annual variations in temperature & seasons. Major biomes (desert, rain forest, tundra etc.) are formed due to these variations & precipitation (rain & snow).

Regional and local variations within a biome lead to the formation of different habitats.

Life exists even in extreme & harsh habitats. E.g. 
  • Rajasthan desert, rain-soaked Meghalaya forests, deep ocean trenches, torrential streams, permafrost (snow laden) polar regions, high mountain tops, thermal springs & compost pits.
  • Our intestine is a habitat for many microbes.
The physico-chemical (abiotic) components (water, light, temperature, soil etc.) & biotic components (pathogens, parasites, predators, competitors etc.) lead to variation of different habitats.

The distinct role and position of an organism in its environment is called its niche. By this, each organism tolerates various conditions, utilises various resources etc. 

Abiotic Factors 

a. Temperature 

The most ecologically relevant environmental factor.

Temperature on land varies seasonally. It gradually decreases from equator to the poles and from plains to mountain tops. It ranges from subzero levels (in polar areas & high altitudes) to >500° C (in tropical deserts). 

Average temperature in thermal springs & deep-sea hydrothermal vents is above 100° C.

Mango trees cannot grow in temperate countries (Canada, Germany etc.). There is no Snow leopard in Kerala forests. Tuna fishes are rare beyond tropical latitudes in the ocean.

Temperature affects kinetics of enzymes, basal metabolism and other physiological functions of the organism.

Based on range of thermal tolerance, organisms are 2 types:
  • Eurythermal: They can tolerate a wide range of temperatures.
  • Stenothermal: They can tolerate only a narrow range of temperatures.
b. Water 

It is the second most important factor.

Desert organisms have special adaptations to limited water.

Productivity & distribution of plants is dependent on water.

For aquatic organisms, water quality (pH, chemical composition) is important. The salt concentration (salinity in parts per thousand) is less than 5 in inland waters, 30-35 in the sea and > 100 in some hypersaline lagoons.

Based on the tolerance to salinity, organisms are 2 types:
  • Euryhaline: Tolerate a wide range of salinities.
  • Stenohaline: Tolerate only a narrow range of salinity.
Many freshwater animals cannot live for long in sea water and vice versa because of the osmotic problems.
c. Light 

Plants need sunlight for photosynthesis.

Small forest plants (herbs & shrubs) are adapted to photosynthesize optimally under very low light because they are overshadowed by tall, canopied trees.

Many plants depend on sunlight for photoperiodism (e.g. flowering).
Many animals use diurnal and seasonal variations in light intensity and photoperiod for timing their foraging, reproductive & migratory activities.

Sun is the ultimate source for light & temperature on land. Deep (> 500m) in the oceans, the environment is dark and there is no energy available from sun.

The spectral quality of solar radiation is also important for life. The UV spectrum is harmful to many organisms. Not all the colour components of the visible spectrum are available for marine plants.

d. Soil 

Nature & properties of soil is differed due to climate, weathering, sedimentation, method of soil development etc.

Soil composition, grain size & aggregation determine the percolation and water holding capacity of the soils.
These characteristics and parameters like pH, mineral composition & topography determine the vegetation and animals in an area.

In aquatic environment, the sediment-characteristics determine the type of benthic animals. 
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