Question

Explain the bell-shaped curve of range of tolerance.

Answer 1

Range of tolerance

When the response of an organism to a gradient of a single environmental factor (temperature, light, etc.) is plotted, it gives a bell-shaped curve.

Zones of the curve

1. Optimum zone (centre of the curve): Greatest fitness, growth and survival.
2. Zone of physiological stress (on either side): Organisms are stressed; only a few survive and reproduce.
3. Zone of intolerance (beyond the stress zones): Too little or too much of the factor; organisms cannot survive and are absent.

Limiting factor

Any factor present in the least or excess amount that limits growth is a limiting factor, e.g., low temperature limits plant growth at high altitude.

The bell-shaped curve shows that organisms perform best in an optimum range and decline on both sides, with death occurring beyond the limits of tolerance.