Having realised that the abiotic conditions of many habitats may vary drastically in time, we now ask–how do the organisms living in such habitats cope or manage with stressful conditions?
But before attempting to answer this question, we should perhaps ask first why a highly variable external environment should bother organisms after all.
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 250C 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
- 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 – 370C. 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 370C, 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.
- 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.
- 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 (Bhartpur) in Rajasthan host thousands of migratory birds coming from Siberia and other extremely cold northern regions.
- Suspend: In bacteria, fungi and lower plants, various kinds of thickwalled 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 date 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 during winter is an example of escape in time. Some snails and fish go into aestivation to avoid summer–related problems-heat and desiccation.
Under unfavourable conditions many zooplankton species in lakes and ponds are known to enter diapause, a stage of suspended development.