Answer :
Final answer:
A keystone species plays a crucial role in maintaining the structure and biodiversity of its ecosystem, often having a small population that disproportionately impacts the community. Examples include the intertidal sea star and banded tetra, both of which are vital for the balance of their respective ecosystems. Therefore, the correct statement is that a keystone species is usually a small population.
Explanation:
Keystone Species
A keystone species is one whose presence is key to maintaining biodiversity within an ecosystem and to upholding an ecological community's structure. Unlike species that are abundant in number, a keystone species often has a relatively small population, yet it plays a crucial role in its ecosystem. For example, the intertidal sea star, Pisaster ochraceus, is a keystone species in the northwestern United States. When this species is absent, the population of its prey, mussels, can overwhelm the ecosystem, leading to a reduction in biodiversity as the balance of species is disrupted.
Another example is the banded tetra, a fish found in tropical streams, which contributes significantly to nutrient cycling by providing phosphorus to the aquatic ecosystem. Thus, although a keystone species may not be numerically dominant or endangered, its ecological role is crucial for maintaining the structure and diversity of its environment.
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