Which species has a disproportionately large effect on its ecosystem?

• A. Invasive species
• B. Parasitism
• C. Biodiversity
• D. Keystone species

Answer: (D)

A keystone species is a species whose impact on its ecosystem is disproportionately large relative to its abundance. This kind of species helps maintain the structure and balance of the community by regulating other species, shaping habitats, or influencing the flow of energy and nutrients. For example, sea otters keep sea urchin populations in check, which allows kelp forests to thrive and support many other organisms. Beavers similarly engineer habitats by building dams that create wetlands used by a variety of species. This outsized influence distinguishes keystone species from others that are common but don’t drive ecosystem structure. Invasive species can also cause big changes, but the term keystone specifically describes the natural, pivotal role a native species plays in maintaining ecosystem integrity; parasitism is a type of interaction, and biodiversity refers to the variety of species, not a single species’ effect.