What factors contribute to maintaining the diversity of an ecosystem?

Keystone species, producers, and essential abiotic and biotic factors.

Keystone species are species on which other species rely heavily, such as the gray wolf as it maintains the elk population in Yellowstone and therefore maintains parts of the ecosystem large populations of elk would destroy.

Producers are green plants, and are an ecosystem's source of energy. Without sufficient producers, the ecosystem could be severely limited because there would not be enough energy to support all species.

Essential abiotic factors include temperature, pollution, pH, etc. Essential biotic factors include grass, bacteria, moss, etc. Changes in these factors may influence the ability of species to survive, and if some species cannot survive the ecosystem's diversity decreases.