What is the basis of variations in the physical and chemical conditions of different habitats?

The variations in the different types of habitats is due to the following factors,

$(a)$ Abiotic factor : Temperature, Water, Light and Soil.

$(b)$ Biotic factor : Pathogens, parasites, predators and competitors of the organism with which they interact constantly.

The abiotic factors are the non-living factors which affects the environment of the habitat. These are as follows :

Temperature : It is the most ecologically relevant environmental factor.

The average temperature on land varies seasonally, decreases progressively from equator towards the poles and from plains to mountain tops.

- It ranges from subzero levels in polar areas and high altitudes to $>50{ }^{\circ} \mathrm{C}$

In tropical deserts in summer.

The temperature in the unique habitats such as thermal springs and deep-sea hydrothermal vents exceed $100^{\circ} \mathrm{C}$.

The temperature affects the kinetics of enzymes and through it the basal metabolism, activity and other physiological functions of the organism.

A few organisms can tolerate and thrive in wide range of temperature(Eurythermal).

- Majority of the organisms are restricted to a narrow range of temperature (Stenothermal).

Water : Next to temperature, water is the most important factor influencing the life of organisms.

- Life originated in water it is unsustainable without water.

The productivity and distribution of plants is also heavily dependent on water.

- Organisms living in oceans, lakes and rivers also faces the water related problems.

For aquatic organisms the quality (chemical composition, $\mathrm{pH}$ ) of water is important.

- The salt concentration (measured as salinity in parts per thousand). Is less than $5$ in inland waters, $30-35$ in sea and $>100$ in some hypersalinic lagoons.

Some organisms are tolerant of a wide range of salinities (Euryhaline).

A Some animals are restricted to a narrow range of salinities(Stenohaline).

- Many freshwater animals cannot live for long in sea water or vice-versa due to osmotic problems.

Light : Light is important for autotrophs since plants produces food through photosynthesis which is possible due to sunlight.

- Many species of small plants (herbs and shrubs) growing in forests are adapted to photosynthesise optimally under very low light conditions because they are constantly overshadowed by tall,

canopied trees.

Many plants are dependent on sunlight to meet their photoperiodic requirements for flowering.

$\Rightarrow$ Light is important for Animals in maintenance of diurnal and seasonal variations in light intensity and duration (photoperiod)as cues for timing their foraging, reproductive and migratory activities.

The availability of light on land is closely linked with that of temperature since the sun is the source for both.

However in deep $(>500 \mathrm{~m})$ in the oceans, the environment is perpetually dark and its inhabitants are not aware of the celestial source of energy called Sun.

The spectral quality of solar radiation is also important for life.

The $UV$ component of the spectrum is harmful.

$\Rightarrow$ All the colour components of the visible spectrum are not available for marine plants living at different depths of ocean.