What defines a keystone species?

A keystone species is defined as a species that has a disproportionately large effect on its ecosystem relative to its abundance. This means that despite possibly being present in relatively low numbers, their role in maintaining the structure and integrity of the ecosystem is crucial. For example, the removal of a keystone species can lead to significant changes in the ecosystem, potentially causing the decline of many other species and altering habitat structures.

This definition emphasizes the importance of ecological interactions and the balance within an ecosystem. Keystone species often play vital roles, such as top predators, herbivores, or providers of essential resources. For instance, sea otters are considered a keystone species in kelp forest ecosystems because they regulate sea urchin populations, which would otherwise overgraze kelp, leading to ecosystem collapse.

Options that refer to commonness, lifespan, or vulnerability to extinction do not capture the essence of what makes a species a keystone. Keystone species are identified by their ecological impact rather than their numbers, longevity, or risk of extinction.