Define extinction rate.

The extinction rate refers to the frequency at which species disappear from Earth over a specific period of time. This metric is crucial for understanding biodiversity and the health of ecosystems. A high extinction rate indicates that species are vanishing at an alarming pace, which can have significant implications for ecological balance, conservation efforts, and the overall health of the planet.

Other options do not pertain to extinction. The formation of new species is related to speciation, which is the opposite of extinction. The growth rate of existing species and the reproductive rate of species deal with population dynamics, focusing on how species increase in number rather than disappear. Understanding extinction rates is essential for conservation biology as it helps in assessing the impact of human activity and environmental changes on various species and can guide efforts to preserve biodiversity.