13.1.2 Responses to Abiotic Factors (NCERT) NEET

13.1.2 Responses to Abiotic Factors 

• Ecology at the organismic level is essentially physiological ecology which tries to understand how different organisms are adapted to their environments in terms of not only survival but also reproduction.
• One would expect that during the course of millions of years of their existence, many species would have evolved a relatively constant internal (within the body) environment that permits all biochemical reactions and physiological functions to proceed with maximal efficiency and thus, enhance the overall ‘fitness’ of the species. 
• This constancy, for example, could be in terms of optimal temperature and osmotic concentration of body fluids. 
• Ideally then, the organism should try to maintain the constancy of its internal environment (a process called homeostasis) despite varying external environmental conditions that tend to upset its homeostasis. 
• Let us take an analogy to clarify this important concept. Suppose a person is able to perform his/ her best when the temperature is 25^oC and wishes to maintain it so, even when it is scorchingly hot or freezingly cold outside. 
• It could be achieved at home, in the car while travelling, and at workplace by using an air conditioner in summer and heater in winter. 
• Then his/ her performance would be always maximal regardless of the weather around him/her. Here the person’s homeostasis is accomplished, not through physiological, but artificial means. 
• How do other living organisms cope with the situation? Let us look at various possibilities.

(i) Regulate:

• Some organisms are able to maintain homeostasis by physiological (sometimes behavioural also) means which ensures constant body temperature, constant osmotic concentration, etc. 
• All birds and mammals, and a very few lower vertebrate and invertebrate species are indeed capable of such regulation (thermoregulation and osmoregulation). 
• Evolutionary biologists believe that the ‘success’ of mammals is largely due to their ability to maintain a constant body temperature and thrive whether they live in Antarctica or in the Sahara desert. 
• The mechanisms used by most mammals to regulate their body temperature are similar to the ones that we humans use. 
• We maintain a constant body temperature of 37^oC. In summer, when outside temperature is more than our body temperature, we sweat profusely. 
• The resulting evaporative cooling, similar to what happens with a desert cooler in operation, brings down the body temperature. 
• In winter when the temperature is much lower than 37^oC, we start to shiver, a kind of exercise which produces heat and raises the body temperature. 
• Plants, on the other hand, do not have such mechanisms to maintain internal temperatures. 

(ii) Conform:

• An overwhelming majority (99 per cent) of animals and nearly all plants cannot maintain a constant internal environment. 
• Their body temperature changes with the ambient temperature. In aquatic animals, the osmotic concentration of the body fluids change with that of the ambient water osmotic concentration. 
• These animals and plants are simply conformers. 
• Considering the benefits of a constant internal environment to the organism, we must ask why these conformers had not evolved to become regulators. 
• Recall the human analogy we used above; much as they like, how many people can really afford an air conditioner? 
• Many simply ‘sweat it out’ and resign themselves to suboptimal performance in hot summer months. Thermoregulation is energetically expensive for many organisms. 
• This is particularly true for small animals like shrews and humming birds. Heat loss or heat gain is a function of surface area. 
• Since small animals have a larger surface area relative to their volume, they tend to lose body heat very fast when it is cold outside; then they have to expend much energy to generate body heat through metabolism. 
• This is the main reason why very small animals are rarely found in polar regions. During the course of evolution, the costs and benefits of maintaining a constant internal environment are taken into consideration. 
• Some species have evolved the ability to regulate, but only over a limited range of environmental conditions, beyond which they simply conform. 

                                 

• If the stressful external conditions are localised or remain only for a short duration, the organism has two other alternatives. 

(iii) Migrate : 

• The organism can move away temporarily from the stressful habitat to a more hospitable area and return when stressful period is over. 
• In human analogy, this strategy is like a person moving from Delhi to Shimla for the duration of summer. 
• Many animals, particularly birds, during winter undertake long-distance migrations to more hospitable areas. 
• Every winter the famous Keolado National Park (Bharatpur) in Rajasthan host thousands of migratory birds coming from Siberia and other extremely cold northern regions. 

(iv) Suspend: 

• In bacteria, fungi and lower plants, various kinds of thick walled spores are formed which help them to survive unfavourable conditions – these germinate on availability of suitable environment. 
• In higher plants, seeds and some other vegetative reproductive structures serve as means to tide over periods of stress besides helping in dispersal – they germinate to form new plants under favourable moisture and temperature conditions. 
• They do so by reducing their metabolic activity and going into a state of ‘dormancy’. 
• In animals, the organism, if unable to migrate, might avoid the stress by escaping in time. 
• The familiar case of bears going into hibernation (winter sleep) during winter is an example of escape in time.
• Some snails and fish go into aestivation (summer sleep) to avoid summer–related problems-heat and dessication. 
• Under unfavourable conditions many zooplankton species in lakes and ponds are known to enter diapause (a stage of suspended development). 

Think and answer

• Considering the benefits of a constant internal environment to the organism, we must ask why these conformers had not evolved to become regulators.
• What is the main reason why very small animals are rarely found in polar regions?
• Define the following- dormancy, sporulation, hibernatio, aestivation and diapause.
• What are partial regulator? explain with example.
• "Heat loss or heat gain is a function of surface area" explain the statement in context of thermoregulation in small animals like shrews and humming birds.