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GLOBAL EXTINCTION
Global extinction refers to the loss of species or other taxonomic units (e.g., subspecies, genus, family, etc.; each is known as a taxon) occurring when there are no surviving individuals elsewhere. The extinction of any species is an irreversible loss of part of the biological richness of the Earth. Extinction can be a natural occurrence caused by an unpredictable catastrophe, chronic environmental stress, or ecological interactions such as competition, disease, or predation. However, there have been dramatic increases in extinction rates since humans have become Earth's dominant large animal and the cause of global environmental change.
At least five major mass extinctions have probably occurred in the geologic past. During the Late Ordovician Mass Extinction events, approximately 85% of marine species died. The mass extinction occurred in 2 phases; at the beginning and in the middle of Hirnantian Age. In the first phase of extinction, changes in nutrient cycling as a result of glacially-forced regression were thought to be responsible. Stagnation of oceanic circulation and post-glacial temperature and sea level rise were the main cause of the second phase of extinction. Meanwhile, both extinction events were thought to be stimulated by the rapid change in climate. The greatest mass extinction in Earth’s history took place about 250 million years ago. This event, commonly known as “the Great Dying” removed up to 95% of life on Earth. It is believed that a gigantic volcanic eruption triggered global warming through the release of carbon dioxide and methane. This mass extinction first started in the deep ocean area, and then moved up to the upper layers of ocean, killing almost all living creatures.
Invertebrates are perhaps the most diverse group of marine organisms, and yet are being lost in the highest numbers. At the beginning of the Cambrian era (about 570 million years ago), numerous animals from this phyla propagated during an evolutionary radiation, but most of them are now extinct. The 15-20 extinct phyla from that period are known from the Burgess Shale of British Columbia.