Organisms are equipped in a number of ways to cope with their environment. The adjustments made by the individuals in response to the specific environmental conditions are known as adaptations. Adaptations are structural or physiological characteristics that allow an organism to exist under the conditions imposed by its habitat.
All organisms have adaptations that help them survive and thrive. Some adaptations are structural, while some are behavioural. Structural adaptations are physical features of organisms that help them to cope with the environment like the modifications of the leaves in a cactus, aerenchyma tissue in hydrophytes, webbed toes in a duck or the fur of a bear. Behavioural adaptations are the things organisms do to survive. For example, bird calls and migration are behavioural adaptations.
Categories of Adaptations:
Adaptations may be classified into three main categories:
a. Inherited Adaptations:
Inherited adaptations are those inherited from one generation to another.
b. Acquired Adaptations:
These are acquired by a species under the influence of the environment. For example, the skin colour changes after exposure to sunlight. Similarly the body produces antibodies to protect against toxins.
c. Ecological Adaptations:
Ecological adaptations are those developed by plants and animals in different habitats. Accordingly adaptations may be further classified into desert, fresh water, marine, pelagic, etc. For example, plants and animals living in aquatic habitat are adapted to live in water, while those living in the deserts are adapted to live in hot and arid conditions of the deserts.
Limiting Factors and Law of Tolerance:
Every organism requires unique set of environmental conditions or factors for its survival. Both abiotic and biotic factors influence the growth, reproduction, abundance, distribution and survival of organisms. Some factors exert more influence than the other. Any factor that tends to slow down the rate of metabolism or potential growth in an ecosystem is said to be a limiting factor.
The factor that controls the survival of an organism is said to be regulatory factor. Different ecosystems have different combinations of limiting factors. Temperature, light, soil, humidity, carbon dioxide and oxygen, pressures are important limiting factors.
When the factor remains constant and abundant and if the organism has a wide range of tolerance, the factor is not a limiting factor. But if the factor fluctuates and the organism has a narrow range of tolerance, the factor can be called a limiting factor.
The mechanisms of limiting factors can be explained by two main laws:
a. The Law of Minimum:
It was proposed by Liebig in 1840. According to this law, growth and reproduction of organisms are dependent on the factor that is present in minimum quantity in the environment.
b. The Law of Tolerance:
It was proposed by Shelford in 1913. According to this law, organisms are exposed to a variety of environmental factors such as light, temperature, nutrients, etc. Each organism survives well only at a particular range of intensity of the factor. This is called tolerance. Every environmental factor has two zones – the zone of tolerance and zone of intolerance.
In the zone of tolerance, organisms survive well.
This zone is further divided as follows:
i. Optimum Zone:
The zone where growth, reproduction and survival capacity is high. Maximum numbers of organisms are found in this zone.
ii. Stress Zones:
The stress zone is found on either side of the optimum range in which the activity slows down. These are the zones of physiological stress and only a few organisms are found in these zones.
In the zone of intolerance or lethal zones, the intensity of the environmental factor is too low or high. No organism can survive in this zone. When the species has a narrow range of tolerance, the prefix ‘steno’ is added to the factor.
For example, stenothermal means an organism that can tolerate a narrow range of temperature; when the species has a wide range of tolerance the prefix ‘eury’ is added to the factor. For example, eurythermal means an organism that can tolerate a wide range of temperature.
When the activity of an organism in response to a range of environmental factor is plotted on a graph, a bell shaped curve is obtained. When an environmental factor shifts beyond the tolerance of an organism, it can become dormant until the return of favourable conditions, or migrate to a place with favourable conditions or it can acclimatise.
Organisms possess physiological adaptations that help them to respond quickly to a stressful situation. The feeling of nausea, headache and general discomfort, an individual feels, when he visits places at high altitude is known as altitude sickness. This happens because the body does not get enough oxygen due to the low atmospheric oxygen.
But usually within a week, the body adjusts to the change and stops experiencing altitude sickness. This process of adjustment is known as acclimatisation. The body compensates for the low oxygen availability by increasing oxygen production, decreasing the binding capacity of haemoglobin and by increasing breathing rate. But people who live at high altitudes possess higher lung capacity and large number of red blood corpuscles to compensate for the low oxygen levels.