The correct choice is:
play an unusually important role in determining the species composition in a habitat.
A keystone species is one that has a disproportionately large effect on the ecosystem relative to its size or abundance. Some key characteristics of keystone species:
• They are not necessarily abundant or dominate the ecosystem numerically or in terms of biomass. But they have an outsized influence on the environment and other species.
• They help maintain the structure, composition, and functioning of the ecological community. Their impact is much greater than would be expected based on their abundance.
• The ecosystem would change drastically if the keystone species were removed. Many other species depend on them to survive, and the whole food web could shift.
• They facilitate the coexistence of other species by reducing competition for resources. For example, sea otters control sea urchin populations which in turn determine the health of kelp forests.
• They are not necessarily primary producers (plants) or restricted to intertidal zones. They can be at any trophic level (producer, primary, secondary, tertiary consumer). Their impact depends on ecological role, not diet or habitat.
• They cannot be removed without major disruptions. The remaining species depend heavily on the keystone species to regulate the ecosystem. Their impact far outweighs their numbers.
The other choices do not accurately describe keystone species:
• Are more expendable: Keystone species fill an essential role and are not expendable. They would greatly impact the ecosystem if removed.
• Occur only in intertidal zones: Keystone species can occur in any ecosystem, not just intertidal zones. Sea otters are an example commonly cited but they are not restricted to intertidal areas.
• Are primary producers: While some keystone species are plants, many others are carnivores, detritivores or other types of consumers. Their role depends on function, not trophic level.
• Can be removed without impact: No, keystone species by definition provide an essential regulating function. Removing them would drastically disrupt the ecosystem.
In summary, the key characteristic of a keystone species is that it has a disproportionately large effect on the ecosystem relative to its abundance. Keystone species help maintain community structure and composition, facilitate coexistence, and fill a crucial regulatory niche. Their impact is essential but extends beyond what would be expected based on their numbers.
Does this help explain keystone species and why choice D is the best option? Let me know if you have any other questions!