What is a keystone species?

A keystone species is defined as a species that plays a crucial role in its ecosystem, often disproportionately impacting the structure and function of that environment compared to its abundance or biomass. The presence or absence of a keystone species can lead to significant changes in the ecosystem, including alterations in biodiversity, species composition, and overall ecological balance.

For instance, predators that maintain the population of herbivores can prevent overgrazing, while certain plant species may provide essential habitat or food resources for a variety of organisms. The critical nature of a keystone species highlights how interconnected and dependent various organisms are within an ecosystem, emphasizing the importance of preserving such species for maintaining ecological integrity and health.

The other options relate to characteristics of species but do not capture the unique ecological role that defines a keystone species. Abundance, size, or adaptability to different environments are not what make a species 'keystone'; rather, it is the significant role it plays that is central to this concept.