Final answer:
A keystone species is one whose absence affects many organisms in a community, shaping ecological structure and biodiversity.
Step-by-step explanation:
The statement that best describes a keystone species is: "Its absence would affect many organisms in the community." A keystone species has a disproportionately large impact on its environment relative to its abundance. These species help to maintain the structure of an ecological community and biodiversity. Examples of keystone species include the intertidal sea star, Pisaster ochraceus, which prevents mussel populations from outcompeting other species, and the banded tetra which provides an essential nutrient, phosphorus, to its ecosystem.
A keystone species is one whose presence is key to maintaining biodiversity within an ecosystem and upholding an ecological community's structure. It plays an especially important role in its community, and its absence would affect many organisms in the community. For example, the intertidal sea star, Pisaster ochraceus, is a keystone species that controls the population of mussels in its habitat. If the sea star is removed from the community, the mussel population would increase dramatically, affecting the entire species composition and reducing biodiversity.