12 Biology


Learning Goals:

  • Organism and Its Environment
  • Populations

Ecology: The subject which studies the interactions among organisms and between the organism and its physical environment is called ecology. Ecologically is concerned with four levels of biological organization, viz. organisms, populations, communities and biomes.


Major Abiotic Factors

Temperature: Temperature has significant effect on metabolism of living beings. The level of thermal tolerance of species determines the geographical distribution of various species.

Eurythermal: Organisms which can tolerate and thrive in a wide range of temperatures are called eurythermal. Very few organisms come in this category.

Stentothermal: Organisms which can thrive in a narrow range of temperatures are called stentothermal. Most of the organisms belong to this category.

Water: Life cannot be sustained without water. Productivity and distribution of plants heavily depends on water. For aquatic organisms, quality of water is very important. Chemical composition and pH are the criteria for determining the quality of water.

Euryhaline: Organisms which can thrive in a wide range of salinities are called euryhaline organisms. Very few organisms come under this category.

Stentohaline: Organisms which can thrive within a narrow range of salinities are called stentohaline organisms. Most of the organisms come under this category.

Light: You have read the significance of light for photosynthesis. So, importance of light for autotrophs is easy to understand. Plants are also dependent on sunlight to meet their photoperiodic requirements for flowering. Many animals use the diurnal and seasonal variations in light intensity and duration as cues for timing their foraging, reproductive and migratory activities. For organisms living in deep sea, the source of energy is not light, rather they depend on chemosynthesis to obtain energy. The spectral quality of solar radiation is also important for living beings. You know that UV radiations are harmful for most of the living beings. All the colour components of the visible spectrum are not available for marine plants living at different depths in the ocean.

Soil: Important characteristics of soil are; soil composition, grain size and aggregation. These characteristics determine the percolation and water holding capacity of soil. Along with these characteristics, other parameters like pH, mineral composition and topography determine the vegetation in any area. The vegetation, in turn determines the type of animals which can be sustained in that area. The sediment characteristics in aquatic environment determine the type of benthic animals which can be sustained there.

Responses to Abiotic Factors

Regulate: Some organisms are able to maintain homeostasis by physiological (sometimes behavioral also) means to maintain a constant body temperature, constant osmotic concentration, etc. All birds and mammals, and a very few lower vertebrate and invertebrate species are capable of such regulation. Due to this, mammals and birds are found in almost all parts of the earth.

Conform: Almost 99% of animals and nearly all plants cannot maintain a constant internal environment. Osmotic concentration of the body fluids of aquatic animals changes with that of the ambient water osmotic concentration. These organisms are called conformers because they conform to the ambient environmental conditions. These organisms have followed the classic principle of ‘cost to benefit ratio’. Thermoregulation is energetically expensive for many organisms. Heat loss or heat gain is a function of surface area. Small animals have a larger surface area relative to their volume. Hence, these animals tend to lose body heat very fast when it is cold outside. In such a condition, they need to expend too much energy to generate body heat through metabolism. This is the reason, very small animals are rarely found in polar regions.

Some species have evolved the ability to regulate. But they can do so only over a limited range of environmental conditions. Beyond this range, these species become conformers. For escaping environmental stress for a short duration, these organisms follow either of the two alternatives which are as follows:

Migrate: The organism can move away temporarily from the stressful habitat to a more hospitable area and return when stressful period is over. Many species of birds migrate from Siberia to India during the winter in the northern hemisphere. These birds migrate to escape the harsh climate during these months. They return to their original habitat once the winter season is over.

Suspend: Many organisms suspend metabolic activities to tide over unfavorable conditions. Bacteria, fungi and lower plants form thick-walled spores to tide over unfavorable conditions. The spores germinate when the conditions become favorable. In higher plants, seed dormancy is a means to tide over unfavorable conditions. Some vegetative structures of higher plants also help in serving the same purpose. Bears go into hibernation during winter months. Frogs go into summer aestivation to avoid harsh summers. Many zooplankton species in lakes and ponds enter diapause, i.e. a stage of suspended development.

Adaptations: Any attribute of the organism that enables the organism to survive and reproduce in its habitat is called adaptation. Adaptations can be morphological, physiological and behavioral. For example; the kangaroo rat in North American deserts is capable of meeting all its water requirements through internal fat oxidation in which water is a byproduct.

Allen's Rule: This rule was formulated by Joel Asaph Allen in 1877. This rule states that animals adapted to cold climates have shorter limbs and body appendages than animals, adapted to warm climates. This rule states that the body volume-to-surface area ratio for homeothermic animals varies inversely with the average temperature of the habitat. The ratio is high in cold climates and low in hot climates. For example; mammals from colder climates generally have shorter ears and limbs to minimize heat loss.

Physiological adaptations allow and organism to respond quickly to a stressful situation. Altitude sickness is often seen in a person who lives in the plains and goes to a hilly area. Within a few days, the person gets acclimatized and symptoms of altitude sickness vanish. This happens because of increased production of red blood cells. This is an example of physiological adaptation.

Desert lizards show good example of behavioral adaptation. When their body temperature drops below a certain level, these lizards bask in the sun to absorb heat. When the ambient temperature increases, these lizards go into shade.



NCERT Solution