Final answer:
A keystone species has a critical role in maintaining ecosystem structure and biodiversity, but it is not necessarily the most populous.
Step-by-step explanation:
A keystone species is not necessarily the species with the largest population in its ecosystem; rather, it has a disproportionately large impact on that ecosystem's structure and biodiversity. For example, the intertidal sea star, Pisaster ochraceus, is a keystone species along the northwestern coast of the United States.
This sea star preys on mussels, which if left unchecked, can dominate the community and reduce biodiversity. Removing the sea star would lead to an explosive growth in mussel and sea urchin populations, which could drastically alter the ecosystem and potentially lead to its collapse.
Keystone species can be top predators, such as wolves in Yellowstone National Park, or they can provide essential resources, like the banded tetra, which supplies phosphorus to tropical stream communities. Removal of a keystone species often results in significant changes to the abundance and variety of species in an ecosystem, sometimes leading to a loss of biodiversity. Protecting and restoring keystone species is crucial for maintaining ecosystem health and resilience.