Question

How does biodiversity contribute to the sustainability of an ecosystem?

Answer 1

The more diversity there is in an ecosystem, the more it is likely that several species have the same function in that ecosystem's balance.

Explanation:

In a ecosystem, there are several species interacting with each other.
A good example of those function could be a trophic network (fig 1) with prey, predators and primary productors where every member of that comunity has a dynamic of population with natality and mortality that has a more or less stable balance.

fig 1) a simplyfied trophic network

If for any reason, natural or anthropic, a species is to vanish from the ecosystem and if the said species was bearing a function in the ecosystem, the whole ecosystem could suffer from the disappearance. Indeed if you remove any member of the simplyfied network above, every other spicies would suffer from a lack due to a cascade reaction in the network.

In the orther hand, a richer ecosystem that would have several species bearing the same function would be more resilient to a vanishing species since no function would dissapear.