What is a keystone species?

A keystone species is defined by its significant impact on an ecosystem that is disproportionate to its abundance. This means that even if a keystone species is not particularly numerous, it plays a critical role in maintaining the structure and health of its ecosystem. For instance, a predator that controls the population of herbivores can prevent overgrazing, thereby promoting a diverse plant community that supports a wide range of other species. The presence or absence of a keystone species can have far-reaching effects, influencing various ecological processes and interactions within the ecosystem.

The other options do not accurately capture the essence of what defines a keystone species. A species with low abundance may not have significant environmental effects, and a species that has minimal environmental impact would not be considered keystone, as its role would not be critical to the stability of its ecosystem. Lastly, a species that is abundant in all ecosystems suggests a broad and even distribution, which does not necessarily correlate with the critical impact characteristics of a keystone species. Thus, the definition of a keystone species revolves around its influential role rather than its numerical presence in an ecosystem.