What defines a keystone species in an ecosystem?

A keystone species is defined as a species that has a disproportionately large effect on its ecosystem relative to its population size. This means that even if a keystone species is present in low numbers, its presence is critical in maintaining the structure, diversity, and health of the environment. Keystone species may influence a wide range of ecological processes, such as predator-prey dynamics, competition among species, and habitat creation.

For example, the removal of a keystone species can lead to dramatic shifts in the ecosystem, potentially resulting in a decline of biodiversity or a change in community composition. This concept helps ecologists understand the interconnections in ecosystems and the critical roles certain species play in sustaining ecological balance.

The other options do not accurately capture the essence of a keystone species. A species with low population density may not necessarily impact the ecosystem significantly, and a common species may not be crucial to the ecosystem dynamics. Furthermore, having no predators does not define a keystone species; rather, it is the unique role in maintaining ecological stability that distinguishes keystone species from others.