Extinction of Animals

Extinction — the permanent disappearance of a species — has been a common, arguably inevitable, feature of the history of life, because species cannot always adapt successfully to large or rapid environmental change. The impact of an extinction event is conventionally considered at three levels. Species extinction eliminates a single entire species, typically triggered by a localised environmental event (such as a flood) or a biological event (such as disease, or the loss of half or more of the species' food resource). Mass extinction eliminates half or more of the species within a whole region or ecosystem, often following a large-scale event such as a volcanic eruption; five major mass extinctions are recognised since the Cambrian period (Table 6.2) — at the end of the Ordovician, Devonian, Permian, Triassic and Cretaceous periods — with the end-Permian event being the most severe, wiping out an estimated 84% of marine genera and 96% of marine species. The end-Cretaceous event is commonly called the K-T extinction (Cretaceous–Tertiary extinction), marking the boundary between those two periods and including the disappearance of the non-avian dinosaurs. Global extinction is the most severe level, eliminating most species across an entire continent or the whole Earth, and larger taxonomic groups along with them — a 'Snowball Earth' glaciation event and extinction driven by a sharp rise in atmospheric CO2 are cited as examples. Importantly, extinction events are not purely destructive for life as a whole: by clearing out established species, they open up vacant ecological habitats, which can in turn trigger a burst of adaptive radiation among the s …