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).
There is unequivocal evidence for the extinction of 12 marine species, comprising three mammals, five seabirds and four gastropods (7). An additional three bird and mammal species are listed as extinct by the World Conservation Union (IUCN) Red List (6), and a recent survey by Dulvy et al. (2003)10 has uncovered evidence to suggest the global extinction in the wild of a further six species comprising two fishes, two corals and two algae.
Although every species has their own importance to the functionality of an ecosystem, some species are more vulnerable to extinction than others (11). These include:
1. Species at the top of food chains, such as large carnivores.
A fairly wide territory is needed by large carnivores to provide them with sufficient prey. Nevertheless, they are to some extent reducing in numbers due to the habitat shrinking as a result of increasing human populations.
2. Endemic local species with a very limited distribution.
Endemic species has limited geographical distribution, and this makes them very vulnerable to local habitat disturbance or human development. Several species endemic to the Galapagos island such as damselfish (Azurina eupalama), the Mauritius green wrasse (Anampses viridis) and two corals (Millepora boschmai & Siderastrea glynni), the Turkish towel algae (Gigartina australis) and Bennett’s seaweed (Vanvoortsia bennettiana) are also thought to be extinct throughout their small geographic ranges
3. Species with chronically small populations.
These species (e.g. Leafscale Gulper Shark, Centrophorus squamosus; and Portuguese dogfish, Centrophorus coelolepis are exposed to extinction given the fact that their reproduction rate is comparatively slow when comparing with other abundance species.
4. Migratory species.
Migratory species need suitable habitats to feed and rest in widely spaced locations. Such species, for example, dugong (Dugong dugon), Loggerhead turtle (Caretta caretta), Hawksbill turtle (Eretmochelys imbricata) and Mediterranean Monk Seal (Monachus monachus) are very vulnerable if one of their habitats’are lost.
5. Species with exceptionally complex life cycles.
Species such as a Tunicate (Ciona intestinalis) and a Brown bryozoan (Bugula neritina) normally need several different elements to be in place at very specific times to complete their life cycles, making them vulnerable if there is disruption of any single element in the cycle.
Mechanisms causing species extinction
1 Direct Take or Killing
For many years, killing by human was a major factor of extinction. Humans kill other species for many reasons including food, recreation, and to protect themselves and their properties. For example, exploitation is responsible for 55% of the main extinction threat to North American marine fishes (12). Initially, marine animals were not obviously threatened by the wave of extinction that land species were subjected to. However, marine species have been put under great pressure since humans became able to travel over the sea. One species, respectively from three major orders of marine mammals (Cetacea, Pinnipedia and Sirenia) were believed to become extinct in North America mainly due to human activities (13).
2 Habitat Disturbance
Biological, physical and chemical factors in most ecosystems are tightly intertwined. Hence changes in one of these factors can result in changes of others. Exploitation of habitat can therefore profoundly influence many components of a system. Examples of habitat destruction are given below (9):
- Marine aggregate dredging
- Commercial development and construction
- Structures for water diversion
- Coastal engineering
- Ocean acidification
- Organic waste
- High concentration of heavy metals
- Industrial and agricultural chemicals
- Plastics and particles
- Introduction of non-native species
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