Species extinction

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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.


Past global species extinctions

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[1]. 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. 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[2].


Present global species extinctions

It is estimated that since the beginning of life on Earth, an average of 0.01-2 out of every million species have gone extinct every year[3]. Current evidence suggests that at least 829 species have become globally extinct in the past 300 years[4]. Extinction ratios in the past centuries are 10-1000 times higher, which is attributed to human activity. The vast majority of extinctions are terrestrial species. There is unequivocal evidence for the extinction of 12 marine species, comprising three mammals, five seabirds and four gastropods[5]. An additional three bird and mammal species are listed as extinct by the World Conservation Union (IUCN) Red List[4], and a recent survey has uncovered evidence to suggest the global extinction in the wild of a further six species comprising two fishes, two corals and two algae[6]. The extinction rate in the marine environment is thus probably more than 10 times lower than in the terrestrial environment. However, the marine figures are not very reliable. We don't really know how many marine species exist. About 240,000 marine species are known, while estimates of the total number of marine species range between 300,000 and 2 million[3]. There is uncertainty about taxonomic status and also in defining when the last individual has gone[5]. However, there can be no doubt that currently, extinction is happening at an alarming rate and faster than it did prior to 1800[7]. 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.

While there are no doubts about the global decline of marine biodiversity, this is less apparent on a local scale. Studies of local marine habitats do not provide clear evidence of a reduction in species richness[8][9][10]. Although these studies may not provide a complete representative picture, they provide a strong indication that changes in biodiversity on a global scale are not automatically reflected on a local scale.


Causes of species extinction

Overexploitation

The intensive exploitation of marine organisms has a short history in comparison with the terrestrial organisms, only commencing in the last few hundred years. 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. In this short period, human exploitation of marine resources has been a major factor of extinction, both through direct mortality of target species and multiple collateral effects on non-target species (bycatch). For example, exploitation is responsible for 55% of the main extinction threat to North American marine fishes[11]. Fisheries are also indirectly responsible for biodiversity loss and ecosystem disturbance by abandoning huge amounts of derelict fishing gear in the ocean, which is deadly to many marine top predator species. See also the article Overexploitation.

Habitat Disturbance

Figure 1: Beam trawling is one of the fishing activities that greatly destruct marine habitats. Photo credit Government of Flanders.

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:

Physical alterations:

Chemical alterations:

Biological alterations:

Climate Change

Recent climate change such as global warming has increased local water temperatures beyond the suitable range of many species. Such changes have made highly 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[12]. See also Climate-induced regime shifts.


Habitat destruction is the primary cause for the decline of biodiversity in the East Indian and Central Pacific marine regions. The annual global loss of coastal habitat has been estimated to be between 1–9% for coral reefs, around 1.8% for mangroves and about 7% for seagrass beds[3].

Overexploitation appears to be the primary cause in the Atlantic and Mediterranean regions. Important, but relatively lesser causes of biodiversity loss are pollution, eutrophication, climate change and invasive species, see Fig. 2.


Fig. 2. Major threats of species loss for different marine regions. From Luypert et al. (2020[3]) Creative Commons licence.


Vulnerability of species

Although every species contributes to the ecosystem function, some species are more vulnerable to extinction and have a greater impact than others. These include:

Species at the top of food chains, such as large carnivores.

Top predators (also called 'apex predators') play a critical role in ecosystems, by regulating directly and indirectly all underlying trophic levels. They need a large territory to get sufficient prey and their abundance is relatively low. Reproduction and growth is slower than for smaller species and so is their recovery after partial depletion. They are a favorite target of the fishery and therefore prone to overfishing[13][14]. Top predators are therefore a vulnerable group of which most species have declined sharply, in some cases more than 90%[15]. For example, even light fishing pressure is sufficient to cause strong population declines in many large shark species[16]. This 'trophic downgrading' has generated widespread concern because of the fundamental role that apex predators can play in ecosystem functioning, disease regulation, and biodiversity maintenance[17].

Specialized endemic species.

Specialized species with a small geographic range are especially vulnerable to disturbance by invasive species and human intervention. These species can fulfill important functions for the local ecosystem, which are lost when they are replaced by non-native generalist species that compete more efficiently on a larger scale[18].

Several species such as damselfish (Azurina eupalama), the Mauritius green wrasse (Anampses viridis) and two corals (Millepora boschmai and Siderastrea glynni), the Turkish towel algae (Gigartina australis) and Bennett’s seaweed (Vanvoortsia bennettiana) are thought to be extinct throughout their small distribution areas.

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 is lost.

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.


Consequences of species extinctions at local or regional scales

Species extinction at local or regional scales implies in general a decline in species richness (number of species) and a decline in biodiversity. There is strong evidence that species richness in an area enhances ecosystem productivity and stability [19][20][21][22]. The loss of any species can be detrimental to the ecosystem. This is especially true of the loss of species from the higher trophic levels that suffer the greatest risk of (local) extinction as a result of hunting by humans, loss of habitat or bioaccumulation of toxins. Loss of top predators can result in a so-called trophic cascade - a complete restructuring of the ecosystem and the food web relationships. Reintroduction of lost species is not always possible, as return is prevented by species that have taken their niche in the food web (see the article Trophic cascade for an example).

Invasion of alien species can in some cases compensate for a decline in species diversity (see the article Non-native species invasions). However, most alien invasions are by species from lower trophic levels. The structure of marine food webs then changes from a trophic pyramid covered by a diverse array of predators and consumers to a shorter, squatter configuration dominated by filter feeders and scavengers[23].

A review of global databases related to the impact of biodiversity loss on marine ecosystem services shows that the number of viable fisheries has declined by -33%, the provision of nursery habitats such as oyster reefs, seagrass beds and wetlands has declined by -69% and filtering and detoxification services provided by suspension feeders, submerged vegetation and wetlands has declined by -63% [24]. 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. Moreover, the loss of coastal habitats has also resulted in historical losses of floodplain buffer area and loss of erosion control from coastal wetlands, thus increasing flooding risks to coastal inhabitants[25]. Analysis of the FAO Global Catch Database shows that the rate of fishery collapses, defined here as catches falling below 10% of the recorded maximum, has accelerated over time, with 29% of currently fished species considered to have collapsed in 2003[24].


Mitigation of species extinction

The importance of biodiversity in maintaining a stable ecosystem implies that species extinction should be avoided. Several measures can contribute to this objective.

The most practiced measure to protect marine ecosystems is the instauration of Marine Protected Areas (MPAs). These designated areas help to protect depleted, threatened, rare or endangered species and populations, as well as to preserve habitats of critical species. Currently (2020) MPAs cover about 4% of the oceans; the target of 10% formulated in the Convention on Biological Diversity is still a long way off. In addition, only part of the MPAs offer full protection. These fully protected MPAs, where any kind of resource removal is prohibited, are often referred to as Marine Reserves. A study of 44 Marine Reserves shows an average increase of 23% in species richness. This increase in biodiversity coincides with a sharp increase in fishery productivity[24]. MPAs cover different habitats (e.g sandbank, mudflat, lagoon, mangrove and reef) and therefore provide protection to different assemblages of species (e.g. Lamprey, Bottlenose Dolphin and Loggerhead Turtle), according to the needs and natural states in different countries.

Also crucial to the conservation of species is the elimination, as far as possible, of the various causes of species extinction, by measures such as:

Above all, great 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 loss of diversity is happening everywhere.

Knowledge of life on Earth is far from complete. In the past 250 years of research, taxonomists have named about 1.78 million species of animals, plants and micro-organisms, yet the total number of species is unknown and probably between 5 and 30 million. Taxonomy provides a major foundation of conservation practice and sustainable management of the world living resources. Worldwide research on taxonomy is fostered by the Global Taxonomy Initiative.


Related articles

Overexploitation
Effects of fisheries on marine biodiversity
Measurements of biodiversity
Ecological thresholds and regime shifts
Resilience and resistance
Trophic cascade


References

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The main author of this article is Wan Hussin, Rauhan
Please note that others may also have edited the contents of this article.

Citation: Wan Hussin, Rauhan (2020): Species extinction. Available from http://www.coastalwiki.org/wiki/Species_extinction [accessed on 13-05-2021]