Organisms and Populations

In developing countries like India, the birth rate (fertility) remains high.
At the same time, the death rate (mortality) has fallen rapidly because of better medical facilities, vaccination, sanitation and food supply.
High birth rate together with low or falling death rate results in rapid population growth (population explosion).
Because many children are born every year, a large proportion of the population is young. The age pyramid is broad at the base (expanding, triangular shape).
So the characteristic features are high fertility, low or rapidly falling mortality rate, rapid population growth and a very young age distribution.

Diurnal variation means the difference between day and night temperatures.
In a desert there is very little vegetation cover and almost no water vapour or clouds in the air. The sandy soil heats up very quickly during the day under strong sunlight, and loses heat very quickly at night by radiation.
So soil surface temperatures can exceed $50^\circ C$ by day and drop close to freezing at night.
In forests, grasslands and shrublands, the plant cover, humus and moisture moderate these changes.
So the diurnal temperature of the soil surface varies most in the desert.