Florida Biology EOC Practice Test 2025 - Free Biology End-of-Course Practice Questions and Study Guide

A keystone species is defined as a species that has a disproportionately large effect on its ecosystem relative to its abundance. This means that even if the population of the keystone species is not particularly high compared to other species, its presence or absence can significantly affect the structure, diversity, and health of the entire ecosystem. For example, the removal of a keystone predator can lead to an overpopulation of herbivores, which in turn may overgraze vegetation, drastically altering the landscape and the species that depend on that habitat.

Other options highlight different aspects of species. A critically endangered species refers to those at a high risk of extinction, but this status does not imply any specific role in ecosystem function. The most abundant species in an ecosystem may play a key role, but not all abundant species are keystone species. Similarly, a species with a small geographical range can have limited impact on its ecosystem and does not fulfill the criteria of having a disproportionate effect. Therefore, the essential characteristic of a keystone species is its significant influence on its environment, making the first choice the correct definition.