What defines a keystone species in an ecosystem?

A keystone species is defined as a species that has a disproportionately large impact on its ecosystem compared to its abundance or biomass. The presence or absence of a keystone species can significantly influence the structure and dynamics of the entire ecosystem. For instance, a keystone predator regulates the population of prey species, which in turn affects the availability of resources for other organisms in the ecosystem. This can lead to a cascade of effects that shape the community structure and biodiversity.

In contrast, a species that has little effect on other species would not qualify as a keystone species, as its interactions do not significantly influence the ecosystem's functioning. Similarly, while the most abundant species can play an important role in an ecosystem, abundance alone does not define a keystone species; it's the impact and role that matter. Lastly, migration across ecosystems describes a behavioral pattern rather than the ecological influence a species has within its environment and does not pertain to the concept of a keystone species.