What is a keystone species?

A keystone species is defined as a species that plays a critical role in maintaining the structure of an ecosystem. This role can be through various ecological functions, such as predation, competition, or providing resources for other organisms. The presence or absence of a keystone species has a disproportionately large impact on its environment compared to its biomass or abundance. For example, sea otters are considered a keystone species in their ecosystems because they control sea urchin populations, which in turn helps maintain kelp forest ecosystems. Without otters, overpopulation of sea urchins can lead to the destruction of kelp forests and the collapse of the entire ecosystem.

The other choices do not capture the essence of a keystone species. While a species that is abundant in a habitat may influence that ecosystem, abundance alone does not define a keystone role. Similarly, being at the top of the food web suggests a predator, but not all top predators are keystone species; some may have little overall effect on the ecological balance. The mention of species being found only in niche ecosystems refers to their geographical or ecological specialization and does not relate to their ability to influence community structure significantly. Therefore, the correct understanding of a keystone species emphasizes its critical influence within the