A mass extinction is a short period of geological time in which a high percentage of
biodiversity, or distinct species
—bacteria, fungi, plants, mammals, birds, reptiles, amphibians, fish, invertebrates
—dies out. In this definition, it’s important to note that, in geological time, a ‘short’ period can span thousands or even millions of years. The planet has experienced five previous mass extinction events, the last one occurring 65.5 million years ago which wiped out the dinosaurs from existence. Experts now believe we’re in the midst of a sixth mass extinction.