Living organisms cope with stressful conditions by any of the following methods:
Method # 1. Regulate:
Homeostasis is maintained by ensuring constant body temperature and constant osmotic concentration.
a. All birds, mammals, a few lower vertebrates and invertebrates have the mechanism of thermoregulation and osmoregulation for maintaining the homeostasis.
b. During summer, sweating occurs profusely and the evaporation brings down temperature of the body.
c. During winter, shivering occurs (a kind of exercise that produces heat) and raises the body temperature.
d. Plants do not have such mechanism to maintain their internal temperature.
Method # 2. Conform:
About 99% of animals and almost all plants cannot maintain a constant internal environment. Their body temperature changes with the ambient temperature.
a. A majority of aquatic animals change the osmotic concentration of their body fluid according to the environment, such animals are called osmoconformers.
b. Thermoregulation is energetically expensive for many organisms. This is specially true for small animals like shrews and humming birds.
c. Heat loss and 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.
d. When it is cold outside, they have to expend much energy to generate body heat through metabolism. Due to this reason very small animals (shrews, humming birds, etc.) are rarely found in Polar Regions.
e. During the course of evolution, some species have evolved the ability to regulate but, only over a limited range of environmental conditions; beyond which they simply conform.
Method # 3. Migrate:
If an organism moves away temporarily from a stressful habitat to a more hospitable area and returns, when the stressful period is over, the process is called migration.
i. Birds underage long distance migrations during winter.
ii. The Keoladeo National Park in Bharatpur (Rajasthan) hosts, thousands of migratory birds coming from Siberia and other extremely cold Northern regions every winter.
Method # 4. Suspend:
a. Some bacteria, fungi and lower plants under favourable conditions slow down their metabolic rate and form a thick-walled spores to overcome the stressful conditions. These spores germinate under the onset of suitable environment.
b. Some organisms are unable to migrate; they might avoid the stress by escaping in time.
c. Those organisms, which cannot migrate, suspend their metabolic functions during the stressful period and resume their functions at the return of unfavourable conditions.
d. The winter sleep of animals such as bear is called hibernation and summer sleep of certain animals like few snails and fish is known as aestivation.
e. During unfavourable conditions, many zooplanktons enter diapause (a stage of suspended development).