What is a keystone species?

A keystone species is defined as a species that has a disproportionately large effect on its ecosystem relative to its abundance. This means that the presence or absence of this species significantly influences the structure and health of the community in which it resides. For example, a keystone predator may control the population of herbivores, which in turn affects the vegetation and other species within that ecosystem. The interactions of a keystone species often lead to increased biodiversity and stability within the ecosystem, highlighting its crucial role in maintaining ecological balance.

In contrast, other choices do not capture the essence of what makes a species a keystone. A primary producer, though vital, does not fit the keystone definition because it does not describe the broader ecological impact regardless of its abundance. A non-native species might contribute to ecological changes, but it does not inherently affect the ecosystem in a way that justifies the keystone label. Lastly, a species at the top of the food chain may have significant power within the food web, but not all top predators are considered keystone species if their impact does not lead to significant changes in the ecosystem structure or function. Thus, choice C accurately describes the unique role and influence of keystone species in ecosystems.