An
extinction event (also known as a
mass extinction or
biotic crisis) is a widespread and rapid decrease in the amount of life on
Earth. Such an event is identified by a sharp change in the diversity and abundance of
multicellular organisms. It occurs when the rate of
extinction increases with respect to the rate of
speciation. Because the majority of diversity and
biomass on Earth is
microbial, and thus difficult to measure, recorded extinction events affect the easily observed, biologically complex component of the
biosphere rather than the total diversity and abundance of life.