Organisms and Populations - Notes | Class 12 | Part 2: Responses to Abiotic Factors



Responses to Abiotic Factors 
Organisms maintain a stable internal environment (homeostasis) despite varying external environmental conditions. This is possible by following processes.

a. Regulate 

It is the maintenance of homeostasis by physiological & behavioural means. It ensures constant body temperature (thermoregulation), constant osmotic concentration (osmoregulation) etc. E.g. All birds & mammals, very few lower vertebrates and invertebrates.

Thermoregulation in mammals: The success of mammals is mainly due to their ability to maintain a constant body temperature.
In summer, when outside temperature is more than body temperature (37° C), sweating occurs. This results in evaporative cooling and brings down body temperature.
In winter, when the temperature is below 37° C, shivering occurs. It produces heat and raises the body temperature.

Most of the organisms are not regulators or are partial regulators because thermoregulation is energetically expensive especially for small animals (shrews, humming birds etc.). They have a larger surface area relative to their volume. So they lose body heat very fast when it is cold outside. Then they have to expend much energy to generate body heat. So, very small animals are rare in Polar Regions.

b. Conform 

99% of animals and nearly all plants cannot maintain a constant internal environment. Their body temperature or osmotic concentration change with the surrounding conditions. They are called conformers. 

In aquatic animals, osmotic concentration of body fluids changes with that of the ambient osmotic concentration.

c. Migrate 

Many animals like birds move away temporarily from stressful habitat to a more hospitable area and return when stressful period is over.

E.g. During winter, Keolado National Park (Bhartpur, Rajasthan) hosts migratory birds coming from Siberia and other extremely cold northern regions.

d. Suspend 

In bacteria, fungi & lower plants, thick walled spores help to survive unfavourable conditions. Under suitable conditions, they germinate.

In higher plants, seeds and some vegetative reproductive structures serve to tide over periods of stress by reducing their metabolic activity. They germinate under favourable moisture and temperature.

In animals: Examples are
  • Hibernation of bears during winter.
  • Aestivation of some snails and fishes during summer.
  • Diapause (a stage of suspended development) of many zooplanktons in lakes & ponds.

Adaptation is the morphological, physiological & behavioural attribute that enables an organism to survive and reproduce in its habitat.

Many adaptations have evolved over a long evolutionary time and are genetically fixed.

Adaptations of kangaroo rat in North American deserts:
  • Internal fat oxidation gives water as byproduct if there is no external source of water.
  • Ability to concentrate urine so that minimal volume of water is used to remove excretory products.
Adaptations of desert plants:
  • Presence of thick cuticle on leaf surfaces.
  • Sunken stomata minimise water loss due to transpiration.
  • CAM photosynthetic pathway enables their stomata to remain closed during day time.
  • Desert plants like Opuntia have no leaves (they are reduced to spines). Photosynthesis is done by stems.
Adaptations of mammals:
  • Mammals from colder climates have shorter ears and limbs to reduce heat loss. This is called Allen’s Rule.
  • Aquatic mammals like seals have a thick layer of fat (blubber) below their skin that acts as an insulator and reduces loss of body heat.
Physiological and biochemical adaptations:
  • Archaebacteria are found in hot springs & deep-sea hydrothermal vents where temperature is >100° C. Many fish thrive in Antarctic waters (temperature is below 0° C).
  • Many marine invertebrates & fishes live at great depths in the ocean where the pressure is >100 times the normal atmospheric pressure.
  • At a high-altitude place (>3,500 m) we feel altitude sickness. Its symptoms are nausea, heart palpitations & fatigue. This is due to low atmospheric pressure. So the body does not get enough O2. Gradually, we acclimatize the situation and the body compensates low O2 availability by increasing RBC & breathing rate and decreasing the binding capacity of hemoglobin.
Behavioural adaptations:
  • Desert lizards bask in the sun and absorb heat when their body temperature is low, but move into shade when the ambient temperature starts increasing.
  • Some species burrow into the soil to hide and escape from the above-ground heat.
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