How does a diversity of organisms increase the chances that some will survive a major change in environment ?

This can be explained with an example.

Let’s lake an example of two species 1 and 2.

Species 1 has a wide genetic diversity due to which organism are also diverse in phenotype. Their phenotype are:  

Black, dark brown, brown, dark red, red, light red, pink, skin color.

From left to right organism are becoming less adapted to high temperature relatively.

For species 2:  This species has less genetic diversity and its phenotypic diversity is as follow:

Black, red, Skin color.

From left to right organism are becoming less adapted to high temperature relatively.

Now if there is the sudden high temperature in the environment; 4 Types of phenotype will survive are black, dark brown, brown, dark red.

While there will only two types of organism in specie 2 which will survive are black, red.

So that’s how the diversity of organisms increase the chances that some will survive a major change in environment.