What defines a mass extinction event?

A mass extinction event is characterized by a significant and rapid loss of a large number of species across various ecological groups in a relatively short geological time frame. This phenomenon typically occurs due to dramatic environmental changes, such as volcanic eruptions, asteroid impacts, or rapid climate shifts, which disrupt ecosystems and lead to the collapse of numerous species that may not have the capacity to adapt quickly enough.

In contrast, the gradual decline of species diversity, the emergence of new ecosystems, and increases in the overall population of species do not capture the essence of a mass extinction event. Rather, they reflect different ecological dynamics that do not necessarily imply the widespread and rapid loss of biodiversity seen during mass extinction events. Thus, the defining feature of a mass extinction is indeed the extensive and rapid extinction of a large number of species.