Biology

The correct answer is: D.They have a disproportionately large effect on the abundance and number of species.

Keystone species play a critical role in maintaining the structure and balance of an ecosystem. Despite their often low abundance or biomass, their influence on the ecosystem is so significant that their removal or decline can lead to drastic changes in the composition and diversity of the entire ecosystem.

The other options are incorrect because:

A.They are always apex predators: While some keystone species are apex predators (like wol...