What is an example of a keystone species in an ecosystem, and what are the potential consequences if that species is removed or leaves the ecosystem?

An example of a keystone species in an ecosystem is the sea otter. If sea otters are removed or leave the ecosystem, it can lead to a decrease in biodiversity and disrupt the balance of the ecosystem. This can result in overpopulation of sea urchins, which can then overgraze on kelp forests, leading to a decline in other species that rely on the kelp for habitat and food.