How can a change to one species trigger a chain reaction that affects many more?

The video discusses the significant impact that certain species can have on ecosystems, particularly those that play a crucial organizing role, known as keystone species. These species have a disproportionate effect on the diversity and stability of the ecosystem. Changes in their population, whether an increase or removal, can lead to dramatic chain reactions affecting the abundance of other species. The ecosystem may collapse without them or thrive upon their reintroduction.

Discuss the relationship between species abundance and ecosystem diversity.

How can the reintroduction of a keystone species affect an ecosystem?