Responses to abiotic factors

Every winter Keoladeo National Park (Bharatpur, Rajasthan) hosts thousands of migratory birds from Siberia and other extremely cold northern regions.

At high altitude the atmospheric pressure is low, so less oxygen is available.
Over time the body acclimatises: it increases red blood cell production, decreases the binding affinity of haemoglobin (so oxygen is released more easily to tissues) and increases the breathing rate.
After six months these people are acclimatised, so they have more RBCs and haemoglobin with a lower binding affinity; altitude sickness occurs only in the first few days.

The graph plots the internal level of a factor (e.g., body temperature or osmotic concentration) against the external level.
Line a is horizontal: the internal level stays constant whatever the external level, so the organism maintains homeostasis. It is a regulator (e.g., birds and mammals).
Line b is a straight diagonal: the internal level changes in direct proportion to the external level. It is a conformer (about 99% of animals and nearly all plants).
Line c stays constant over a limited range of the external level and beyond that range changes with it. It is a partial regulator.
Hence a is regulator, b is conformer and c is partial regulator.