19 Causes of Species Extinction

Dr. Felix Bast

1. Learning outcomes

1.1 To learn about how rate of extinction is a crucial determinant of global biodiversity at any point in time

1.2 To know principal causes of natural background extinctions

1.3 To know causes for five big extinction events that happened in past

1.4 To learn about the ongoing sixth mass extinctions and its causes summarized in the acronym HIPPO

2. Concept map

3. Description

3.1. Introduction

It is estimated that more than 99.9% of all species that had ever lived on this planet earth have gone extinct. Out of the rest, 0.1% of extant species (species that are living today) which is currently estimated to be around 1 trillion, we know only one thousandth of one percentage of this, indicating extreme knowledge gap. It is now estimated that almost half of all extant species will go extinct by 2100 even before these species are discovered and characterized.

Extinction, or more specifically ‘species extinction,’ refers the death of the last member of a species. As the definition is centred about the species, all the ambiguities and problems of species concept discussed in speciation module also affects the term extinction. Consider the common ancestor of human (Homo) and chimp (Pan) lineages that lived on planet earth approximately 12 million years ago. Homogenous population of this species might have split into two because of vicariance (the exact cause remain unknown), and each of these two populations evolved to construe common ancestors of either of these two lineages. For example, after this population split into two, the population that evolved into lineage consisting of human beings served as the common ancestor of Homo lineage, not shared with Pan Lineage, and vice versa with the other population. An intuitive analogy for this allopatric speciation is the phenomenon of binary fission (reproduction) that occurs in bacteria. In binary fission, a mother cell split into two daughter cells. Each of these daughter cells grow in size, and split into two again. During this split, can we affirm that the mother cell had died? Even if the mother cell prior to binary fission have become non-existent in the form it used to be, the daughter cells are in part made up of the mother cell. Similarly, when a population of one species split into two and each evolve to different lineages or different species, it is incorrect to call the original species as ‘extinct’ sensu stricto. This phenomenon is sometimes referred as pseudo-extinction or cladogenesis. A rigorous delineation between these pseudo-extinct species and extinct species is nearly impossible to achieve through paleontological studies. For example, if we study the fossils of this common ancestor of Homo and Pan, we would conclude that this species once existed on planet earth (perhaps for a span between 1 million and 10 million) had gone extinct around 12 million years ago. If we reconstruct the evolutionary legacy of these lineages using molecular phylogenetics, of course we would be able to know that this ‘extinct’ species is in fact a pseudo-extinct species.

Typical lifespan of a species is around 10 million years (however, this varies widely between different taxonomical lineages; 1 to 10 million is a better approximation). However, some species greatly outlive this approximation, the so-called ‘living fossils’. One example is an ancestor of lungfishes, Coelacanth. It is thought for a long time that this species have gone extinct at the end of Cretaceous period in the Cretaceous-Paleogene extinction event that also wiped out planet’s dinosaurs. However, in 1938, a South African researcher have found this fish at Chalumna River, South Africa. Such species that thought to have gone extinct but later rediscovers are called Lazarus taxa. Another example for living fossil is the species Ginkgo biloba; this species had virtually no changes for the last 270 million years.

The term extinction is used in ecology to refer the phenomenon of local extinction, when a previously recorded species disappears from a local habitat; this phenomenon is also called extirpation. In many ecosystems, extinction of one species have repercussions on the rest of ecological niche in which the extinct species was part of, causing the knock on effects of several other species. This is especially true if the species that have gone extinct is a keystone species- the species that plays a unique and substantial role in ecosystem function such that without which the whole ecosystem collapses. An example of keystone species is African elephant in a Savannah ecosystem; if this species goes extinct, the Savannah will turn to be a woodland causing widespread effects across the whole ecosystem and initiating a series of extinction events.

An extinct species is often denoted with a dagger symbol (†) in species lists and phylogenetic trees. Many species, including African elephants, are now on the verge of extinction. IUCN (International Union for the Conservation of Nature) Red List often uses the terms threatened and endangered to denote species on the verge of extinction. A number of species that are extinct in the wild already are now conserved through ex-situ mode in zoos and botanical gardens; these species are denoted extinct in wild.

3.2. Causes of natural extinction

Almost the whole of extinctions ever happened in the earth were due to natural causes in contrast to anthropogenic causes. This is partially due to the fact that the humanity came into existence only very recently (3, 00, 000 years ago) in the deep geological history of life on earth spanning 4 billion years. Rate of extinction is an important determinant of biodiversity at any given point of time (along with rate of speciation). Most of these extinctions are ‘natural, background extinctions’; extinction is an eventuality of any species, like death in the case of individuals. Causes of these natural extinctions are multifaceted and often unknown; often there are contrasting hypotheses for the same extinction events.

A major cause of natural extinctions is between-species competition for similar resources. This phenomenon is analogous to natural selection; while natural selection acts on individuals of population, interspecific competition occurs at the level of communities. For example, consider a marine pelagic habitat where the only primary producer are phytoplanktons. There could be competitions between different zooplankton species for this limiting food resource. In this interspecific competition, better-adapted fit species outcompetes the unfit species, and ultimately the fit species drives the unfit to extinction. Species can also lose its competitive advantage over other species when the habitat it occupies changes in response to various and biotic factors. For example, desertification of vegetation leads to dramatic changes in ecological niche; a number of previously well-adapted, fit species would become less adapted to desert ecosystem and ultimately go extinct. However, note that such extinctions are local in nature; same species might be living at some other locations of planet earth.

Many global-scale species extinctions were caused by tremendous catastrophes such as asteroid impact or global glaciation. These massive geological events wipes out all the individuals of species, and thus species goes extinct. Species extinction is an irrevocable event, a loss that can never be reverted. Asteroid impact usually act at a specific location on planet earth, but could have global scale repercussions; for instance, it could spread massive amount of fine dust into the atmosphere such that the whole planet would become covered in thick dust layer and sunlight would become a limiting factor. Without enough sunlight reaching the earth surface, earth would cool down dramatically; plants die out, driving the rest of the trophic levels into extinction.

Extinctions could also be caused by climate changes at the global or local level. Local-level environmental stress conditions would affect endemic species; for instance drying up of a lake housing many endemic species of fishes. As these endemic species are found nowhere else on this planet, even the local level environmental conditions would result in global-level species extinction. Therefore, small speciose habitats housing a substantial endemic species are the most labile ecosystems for the extinctions. For this reason, conservation programmes often prioritize these ‘hotspots’. Biodiversity hotspots are regions with extraordinary level of endemism for vascular plants (1500 species of endemic plants), and it need to have lost at least 70 percentage of its original habitat. While the hotspot criterion is based on endemism and loss of habitat, it is skewed towards vascular plants; it does not account other major plant lineages such as algae or mosses, or other living beings such as animals, fungi, protozoa or bacteria. Global-level climate change would have huge ramifications and drive several species in the world towards extinction, as in the case of Permian mass extinction. During each glacial periods-the last of such events happened around 20,000 years ago, several species at high latitudes would go extinct, as temperate and subpolar regions would become glaciated for a long period.

3.3. The big five extinction events and its causes

While species extinction is an ongoing process as in the case of speciation, some extinction events are truly remarkable and have happened at the global scale resulting a substantial decline in global biodiversity and a large number of species events. These extraordinary extinction events at the global level are called mass extinction events. There are at least five such mass extinction events well characterized by geologists and palaeontologists till date, the big five. These extinction events are so massive such that a well-demarcated rock layer at this period is found across the world that separates comparatively speciose layers above and below this. However note that these mass extinction events are extremely rare in the earth’s deep geologic time; isolated extinction events are much more common. All five extinction events, therefore, mark as boundaries between respective periods above and below the extinction events. Extinction events might not always be due to higher rates of extinction alone; it could be combined with lower rates of speciation, or entirely due to low rate of speciation. While the exact causes for these five events differ, the after effect is mostly uniform. Fluctuations in global climate and sea levels, volcanism and impact events resulting in ejection of toxic gases, ashes and aerosols into the atmosphere causing greenhouse effect, reduction in primary productivity caused by darkened atmosphere, destruction of food chains and anoxia (depletion of oxygen).

Ordovician-Silurian (O-S) Extinction event is the second largest of the five mass extinction events. This happened approximately 450 million years ago, and resulted in the extinction of nearly 85% of marine species. A major cause of this extinction event was massive glaciation, the most severe glaciation in the current eon Phanerozoic. The glaciation resulted in the extinction of most of the warm-water species. Among those species that could seek glacial refugia in limited supporting habitats, very low population size of these founding population along with high extent of genetic drift caused elimination of entire ecological niches and extinction of many species. During glaciation, sea levels drop, while during interglacials, it rises again, causing extreme fluctuations. A contrasting hypothesis for O-S extinction event postulate as a massive gamma ray burst from a hypernova 6000 light years ago from earth. Even a modest ten-second gamma ray burst would have removed half of the ozone layer, which resulted in extinction of several plant species by resulting UV exposure, wreaking havoc at the primary trophic level, affecting entire ecological niche and driving many species to extinction. Yet another contrasting hypothesis states that the cause of global glaciation was volcanic eruptions across the globe. These eruptions deposited silicate rocks around the world, and created entire orogenies (mountain ranges) such as Appalachian Mountains, with predominant siliceous rocks. When these siliceous rocks weathered, it sequestered (removed) CO2 from the atmosphere, that lead to lower greenhouse effect, global cooling and ultimately the glaciation.

Second of the big five is Late Devonian extinction that happened approximately 370 million years ago, around the boundary between Devonian and Carboniferous period. This extinction event resulted in extinction of almost 70% of species worldwide, although in terms of extinction intensity, this event was the lowest among the big five. One of the major trigger for this extinction event is now considered to be the plant evolution in Devonian period; from relatively simple plants at early Devonian measuring 30 cm in height with rhizoids that penetrated merely few centimetres of soil layers, vascular plant evolution in Devonian Period resulted in massive trees up to 30 meters in height. Expansive forests of these trees covered much of the landmasses, with two effects. The first and foremost, these deep-rooted trees mobilized deeper soil layers causing weathering of siliceous rocks as well as nutrient run off to ocean. As explained in the O-S extinction event, weathering of silicate rocks removed CO2 from the atmosphere, leading to a major glaciation event, Late Devonian Glaciation. The nutrient run-off to ocean and limnetic systems resulted in eutrophication (explosive bloom of algae) and finally anoxia wiping out many aquatic species. Second consequence of planet-wide forestation was CO2 sequestration by these trees through formation of massive coal deposits. As atmospheric CO2 was removed, global cooling began, that finally culminating in the late Devonian glaciation. Resulting ‘icehouse’ phenomenon would extend much of the subsequent Carboniferous and Permian periods.

The third mass extinction event, Permian Extinction, happened 252 million years ago, and is the most catastrophic among all five. This ‘big dying’ event drove almost 96% of all marine species extinct; the biggest in the history of earth til date. There are several contrasting hypotheses on the possible causes of this event, however, no consensus exist on the exact cause. As the extinction was such a devastating level, most probably this extinction was triggered by a catastrophic event, such as a bolide impact or volcanism. Currently the most popular among the hypotheses on the causes of Permian extinction is that an impact by a massive bolide, approximately 250 km in diameter. As the earth’s surface is mostly ocean, covering 72%, probability of asteroid impact occurring at ocean is far higher than that occurring at land. However, oceanic plates keep on moving, the phenomenon of tectonic subduction is analogous to movement of conveyer belts such that the ocean floor recycle once in every 200 million years. This recycling happens everywhere; crater resulting from any such a massive bolide impact would have been erased by now. An indirect existence of such an impact crater near Falkland Islands near Antarctica was discovered in 1992, as revealed by a circular anomaly in gravity. Several follow-up researches published later corroborated this early suggestion. Adding on, the landmasses of the world was united into supercontinent Pangea at this time, and such an oceanic impact would have resulted in more severe extinctions in the now-contiguous ocean rather than on land. In fact this event wiped off 96% of all marine species while only 70% of terrestrial vertebrate species. This impact might have triggered extreme volcanic activities at the antipode of Impact site, in the Russian Siberia. One of the most expansive igneous deposits on the planet earth, Siberian Traps, had been dated to be around this time, perfectly in agreement with bolide impact hypothesis. Extreme volcanism at Siberia might have ejected massive amount of ash, toxic gases and CO2 into the atmosphere. It also caused ignition of massive coal beds that resulted in further rise in CO2 levels and more atmospheric ash. The dense ash layer might have completely blocked out sunrays reaching the earth, collapsing ecosystems of both ocean and land. Other hypothesis include release of methane from methane clathrate deposits of ocean floor, or by methanogenic bacteria inhabiting benthic zone, global oceanic anoxia, release of hydrogen sulphide and so on. However, almost all of these events could probably be the after effects of initial bolide impacts, and subsequent volcanic eruptions, rather than the initial cause.

The fourth mass extinction event, Triassic-Jurassic (T-J) happened around 200 million years ago that lead approximately 70-75% of all species go extinct. In terrestrial ecosystem, an obvious after effect of this event was that as it wiped out competitors for earliest dinosaur species, it paved way for the great expansion of dinosaurs in Jurassic Period. In contrast with the rest four, only T-J event among the big five was the result of decrease in speciation rate rather than an increase in extinction rate, as revealed by a number of modern research. Background extinction rates remained same during T-J while the rate of speciation tremendously decreased. While a number of causes were hypothesised, as of now no consensus exist. The event was sudden, however, not due to extinctions, but rather a decrease in speciation rates, posing great difficulty for the palaeontologists who address the possible causes. Amongst the hypotheses are gradual climate change along with ocean acidification (ditto of the current day climate change and ocean acidification phenomena), Asteroid Impact, Volcanic eruptions. A possibility is that series of volcanic eruptions initiated great changes in global carbon cycle that resulted in comparatively high intensity climate change. An immediate aftermath of this extinction event is the break-up of supercontinent Pangea.

The fifth mass extinction event, Cretaceous-Paleogene (K-Pa, also called Cretaceous-Tertiary K-T) occurred 66 million years ago. This event caused extinction of about 75% of all species of earth that include all non-avian species of dinosaurs, and caused rapid diversification of mammalian lineage. As this event occurred comparatively recently, there seems to be a good deal of consensus among geologists and palaeontologists about the cause of this event. They key discovery happened in 1980 when Nobel Laureate Luis Alvarez found a layer of Iridium with Iridium concentrations as high as 160 times that found in nature, in the K-Pa layers from various sites across the world. As iridium is very rare in earth’s surface, he proposed that a major asteroid impact was the initial cause of K-Pa event. In addition, clay layers of K-Pa layer contained microscopic spheres of rock that by all probability formed when molten rock recrystallized inside the clay, suggesting a planet-wide extreme heat immediately after the impact. It is now accepted that a giant asteroid of approximately 180 km in diameter stroke the earth at Chicxulub near Yucatan coast of Mexico. This created the now famous the oval submarine Chicxulub crater. Consequences of this impact were tremendous; the entry of asteroid itself would have caused a massive IR irradiation instantly killing most of the organisms inhabiting planet’s surfaces. The burning released immense amount of ash and soot that blocked-out sun, a phenomenon known as nuclear winter. The impact released an energy, which is more than a billion times combined energy of two atomic bombs that decimated Hiroshima and Nagasaki in 1945. As the impact site was rather rich in sulphur, the impact released very large sulphur aerosols into the stratosphere, resulting in planet-wide acid rains, ocean acidification and global cooling, that continued for the next 32000 years. Mean ocean temperature hovered around 7°C. Ocean acidification killed phytoplanktons, organisms having calcareous shells and devastated marine ecosystems. The impact also resulted in megatsunami that devastated coastal regions around the world. Simultaneous volcanic eruptions at various regions around the globe that happened around this time is now considered to have been triggered by this initial bolide impact. Amongst these volcano eruptions is the famous one that resulted in Deccan Traps in India.

3.4. HIPPO, or the sixth mass extinction at the Anthropocene

There seem to have a consensus that the planet earth is currently undergoing a major, sixth mass extinction, caused almost entirely by human activities. Effect of human activities since the beginning of industrial revolution in 17th century is so tremendous that resulted in an extraordinary spike of global CO2 levels along with other greenhouse gases. The series of events lead geologists, climatologists and palaeontologists to propose the beginning of new epoch, Anthropocene at the end of Holocene that began around 11500 years ago. There are at least 784 confirmed species extinctions since 1500s. The main causes of this event can be summarized in the acronym HIPPO. HIPPO stands for Habitat Loss and Destruction, Invasive Species, Pollution, Human Population, and Overharvesting. Because of human population expansion and agriculture, a number of pristine habitats have now completely altered. Habitat loss is especially prone in the wetland ecosystems of the world, and a number of endemic species of the wetlands have become extinct. Invasion or introduction by exotic species exerts tremendous pressures for the local species and drive much of the local species to extinction. One famous example is accidental introduction of brown tree snake to the island Guam from Australia during World War II. As the Guam until then had no snakes and therefore no snake predators, the exotic snake species could thrive there that resulted in extirpation of most of the local bird species. Other examples include introduction of feral dog Dingo to Australia 4000 years ago that drove extinction of several marsupial species, and introduction of rats into Mauritius by Dutch sailors that ate eggs of endemic flightless bird Dodo. Pollution, especially by synthetic chemicals pesticides, caused extinction of a number of key amphibian and insect species throughout the world. World’s aquatic fauna is especially prone to overharvesting; it is now estimated that by 2150 almost the entire species of marine and freshwater fishes would go extinct. Albeit its threatened status, a number of whale species of the world continue to be hunted by countries including Japan and Norway. Human population explosion caused almost the all of these associated problems, including habitat loss, species invasion, pollution and overharvesting.

Summary

4.1. Despite the public perception, most of the extinction that have happened in the history of life on this planet was due to natural causes; process of extinction along with speciation determines biodiversity at any specific time.

4.2. One species splitting into two is technically called cladogenesis or pseudo-extinction; the extinction is death of the last individual of a species worldwide. Extinction is also different from ecological extinction or extirpation, which is the local disappearance of a species from a habitat. However, local extinctions would cause species extinction if the species in question is an endemic species-the species that occur nowhere else on the planet.

4.3. Natural background extinction events are caused by a number of factors; ecological, climatological, or natural catastrophes. Vast majority of species extinctions happened at isolated places. Although rare, four out of five mass extinctions resulted in the extinction of a vast number of species en masse.

4.4. Each of the five mass extinctions had its own causative factors; in general these factors include asteroid (bolide) impact, volcanic activities, changes in global carbon fluxes, climate change, ocean acidification, extreme fluctuations in sea levels and blockade of sunlight due to ash and soot deposition in atmosphere resulting in global cooling.

4.5. There is a consensus among geologists and palaeontologists that since the beginning of industrial revolution the earth is undergoing sixth mass extinction, that delineate the present epoch, Anthropocene, from the preceding epoch, Holocene

4.6. The present mass extinction is caused by five principal factors; Habitat loss, Invasive Species, Pollution, Human Population and Overharvesting, summarized in the acronym HIPPO.

you can view video on Causes of Species Extinction

Further e-resources and learn more

YouTube videos:

Cardillo, M., Mace, G. M., Jones, K. E., Bielby, J., Bininda-Emonds, O. R., Sechrest,
& Purvis, A. (2005). Multiple causes of high extinction risk in large mammal species. Science, 309(5738), 1239-1241.
Smith, K. F., Sax, D. F., & Lafferty, K. D. (2006). Evidence for the role of infectious disease in species extinction and endangerment. Conservation biology, 20(5), 1349-1357.
Johnson, C. N. (1998). Species extinction and the relationship between distribution and abundance. Nature, 394(6690), 272.
Gurevitch, J., & Padilla, D. K. (2004). Are invasive species a major cause of extinctions?. Trends in ecology & evolution, 19(9), 470-474.
Clavero, M., & García-Berthou, E. (2005). Invasive species are a leading cause of animal extinctions. Trends in ecology & evolution, 20(3), 110.
Milner-Gulland, E. J., Bukreeva, O. M., Coulson, T., Lushchekina, A. A., Kholodova, M.V., Bekenov, A. B., & Grachev, I. A. (2003). Conservation: Reproductive collapse in saiga antelope harems. Nature, 422(6928), 135.