Species extinction
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Every species however small may have an important role in maintaining a well-balanced ecosystem. Recent surveys suggest that the number of species (species richness) in an area may enhance ecosystem productivity and stability (1)(2), hence the loss of any species could be detrimental to the ecosystem. Direct effects (e.g. overexploitation, pollution and habitat exploitation) and indirect effects as a result of climate change and perturbations of ocean biogeochemistry have been the major reasons for species extinction. There is evidence that regional ecosystems such as estuaries, coral reefs, and coastal and oceanic fish communities are undergoing rapid losses whether in individuals, whole species or entire functional groups (3).
Problems in Species Extinction
Extinction refers to the loss of species or other taxonomic unit (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 (4).
During the Late Ordovician extinction event, approximately 85% of marine species died. This 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 (5).
Current evidence suggests that few marine organisms have become globally extinct in the past 300 years, compared to land where 829 species have disappeared (6). However, there is little precise information regarding how many species are being extinguished in the marine environment since nobody even knows the numbers of species actually present, and there is uncertainty about taxonomic status and also in defining when the last individual has gone (7). This information is also lacking in other major habitats. However, there can be no doubt that currently, extinction is happening at an alarming rate and faster than it did prior to 1800 (8). Previous mass extinctions evident in the geological record are thought to have been brought about mainly by massive climatic or environmental shifts. Mass extinctions as a direct consequence of the activities of a single species are unprecedented in geological history. 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. Other than invertebrates, species such as Steller’s sea cow (Hydrodamalis gigas), which was driven to extinction by visiting sea-otter hunters, and the great auk (Pinguinus impennis) are examples of recently extinct species in marine environments (9).
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