The abiotic conditions of many habitats are never constant and keep changing significantly with time. These changes in abiotic conditions also affect the organisms living in that particular habitat. To sustain life, many species go through a process called homeostasis to maintain the constancy of their internal environment. Homeostasis is defined as a physiological process by which an organism regulates its internal environment in response to the fluctuating external environment.
There are various ways by which organisms can respond to adverse external environments. They can regulate, conform, migrate, hibernate, aestivate or suspend themselves under such circumstances.
Organisms that are capable of maintaining homeostasis by physiological or behavioural means are called regulators. The mechanism by which organisms regulate their constant body temperature irrespective of the external temperature is called thermoregulation. Similarly, the mechanism by which organisms regulate a constant osmotic concentration irrespective of the external osmotic concentration is called osmoregulation. A few lower invertebrates and vertebrate species along with birds and mammals are capable of regulation. Evolutionary biologists believe that the success of mammals is greatly due to their ability to maintain a constant body temperature, which enables them to survive in extreme climates.
Some organisms that are unable to regulate their internal environment may partially regulate their internal environment or conform to external conditions. These organisms are known as conformers. The body temperature of such organisms changes according to the ambient temperature. Such animals are likely to cope with the unfavourable conditions by undergoing hibernation during winter to preserve energy and aestivation in summer to avoid heat-related problems and desiccation.
Interestingly, many zooplanktons in lakes and ponds are known to enter diapause – a state of suspended development during unfavourable conditions. Although these mechanisms are seen in animals, plants too show various responses to stressful external conditions. They usually suspend themselves in unfavourable conditions. In the case of bacteria, fungi and lower plants, thick-walled spores are produced, which help them survive in unfavourable conditions and germinate in favourable conditions. Similarly, in higher plants, seeds and other vegetative reproductive structures are produced, which help overcome stressful periods besides helping in dispersal. This is done by reducing their metabolic activity and undergoing a dormancy period. In favourable conditions, these seeds and vegetative reproductive structures germinate to form new plants. In this way, we see that to respond to stressful abiotic conditions, various organisms regulate, conform, migrate, hibernate, aestivate or suspend themselves.