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
3 Climate Change
The largest mass extinction took place in Earth’s history about 250 million years ago. This incident, 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 was first started in the deep ocean area, and then moved up to the upper layers of ocean, killing almost all living creatures (14).
Meanwhile, more recent climate change such as global warming has increased local water temperatures beyond the suitable range of many species. Such changes have made exceptionally productive areas, such as up welling regions, become less productive due to changes in the food web. Lower primary production supports a lower biomass of primary consumers. In the oceans, krill are major primary consumers that support many important ecosystems. Therefore climate change will inevitably impact food webs based on krill and this will be reflected in the reduction of top level consumer such as large plankton-grazing fish and sea mammals (15).
Impact of species Extinction on Biodiversity
An international group of researchers has recently provided the first comprehensive and large-scale assessment of the services provided. They suggest that species extinction has impaired at least three critical ecosystem services: number of viable fisheries (-33%); provision of nursery habitats such as oyster reefs, seagrass beds and wetlands (-69%); and filtering and detoxification services provided by suspension feeders, submerged vegetation and wetlands (-63%). Additionally, the loss of filtering services has the potential to increase the risks of harmful algal blooms (e.g. ‘red tide’), oxygen depletion and declining water quality. Meanwhile, coastal flooding was also increased as a result of species extinction. Although this event is linked to sea level rise, historical losses of floodplain and erosion control provided by coastal wetlands, reefs and submerged vegetation is also responsible (16)(17).
This effect (through extinction) will be more notable with combining effect from species gain (e.g. through invasion). The reason for this phenomenon is because most extinction (approximately 70%) occurs at high trophic level (predators and primary consumers), whilst the lower trophic (70%; plankton feeders, deposit feeders and detritivores) levels are where most invasions occurs. Marine food webs will re-organise following these simultaneous changes; from a normal pyramid capped by a large range of predators and consumers to a shorter structure dominated by filter feeders and scavengers (18).
There are several studies that suggest that marine species biodiversity will be altered solely due to species extinction, without any additional effect by species invasion. Although these studies have been completed in laboratories and may only include relatively strongly interacting species, they are to some extent robust and produce data similar to that from field surveys and fisheries that incorporate the diversity of the whole community. Furthermore, a strong impact on ecosystem function has been recorded even when the invasive and extinct species are not strong interactors (18).
How to Avoid Species Extinction
The importance of biodiversity in maintaining a stable ecosystem implies that we need to avoid species extinction, and there are a number of practices that can be employed to help conserve our marine species.
One of the important approaches to protect marine ecosystems that has been widely applied is Marine Protected Areas (MPAs). MPAs are established in order to protect the richness of marine life and the environment (18). Moreover, these designated areas help to protect depleted, threatened, rare or endangered species and populations, as well as to preserve habitats of critical species. MPAs differ (e.g sandbank, mudflat, lagoon and reef) and so the protected different assemblages of species (e.g. Lamprey, Bottlenose Dolphin and Loggerhead Turtle), based on the needs and natural states in different countries. Currently there are several areas in the UK have been chosen as potential MPAs area, such as Dogger Bank, West Water of Amrum / Sylt, and Western Irish Sea (18, 19). An example of MPAs application is through fishery closures where a sea area is closed to a certain fishing gear or vessel size, or for a certain target species. The closure to fishing activities help to avoid species extinction since it can increase the species richness. Additionally, it can also be a powerful economic tool helping fisheries remain productive and profitable (20).
Other ways to avoid species extinction are through integrated pollution control, and maintenance of essential habitats. Marine pollution could be prevented, or at least, reduced, by integrated coastal and river basin planning limiting the passage of nutrients or other pollutants to the marine environment. On the other hand, crucial habitats could be maintained by limiting physical activities such as fishing and coastal construction, or through mooring systems in sensitive yet important (e.g. coral reef) environments (17).
Above all, formidable efforts should be made to increase public awareness of the urgent need for action. People around the world should understand the causes and consequences of extinctions and the fact that the loss in diversity could be happening everywhere. Furthermore, intensive study of the living biota is very important, as this knowledge will provide evidence of what we might expect to happen in the future. The need of more research is very obvious since at present, data on the rates and direction of biodiversity loss remains scarce and often uncertain. This data is urgently needed in predicting the eventual impacts that will result from extinction (21).
Please note that others may also have edited the contents of this article.