Mass Extinction

because no one knows exactly what defines a species. Species extinction rates, even of background extinction, is not stable over time.

MASS EXTINCTION

Mass extinctions are also referred to as extinction events or biotic crises. It results in a major loss of biodiversity on earth over a short period of time. No one knows the exact number of extinction events there have been, mostly because it is hard to define what is major and what is meant by a rapid change in biodiversity. Extinction events are usually defined based on their effect on multicellular organisms because it isn t known what the effect has been on microbes of the earth. Marine animals are most used to assess these events because their fossil records are the most accurate and stratification is easier to determine in water-based environments.

There have been five major mass extinction events and many more minor events throughout geologic history. We will talk about these events and their possible causes.

The first extinction event occurred at the transition between the Ordovician and Silurian eras about 450 million years ago. There were two events that killed off about 60 to 70 percent of all the earth s species. The earth was warm prior to the events, which were a major fall in sea levels affecting coastal areas and glaciation, which cooled the earth, killing off many marine organisms.

The Late Devonian extinction happened at the transition between the Devonian and Carboniferous eras. There were multiple events that killed off 70 percent of species and that lasted 20 million years. No one knows how many events contributed to this but it primarily affected marine animals. No one knows what events likely caused this extinction.

The largest extinction on earth killed off up to 96 percent of all species. It was called the Permian-Triassic extinction event, killing off insects, trilobites, and many plant species. Mammal-like reptiles died off and it took 30 million years for vertebrates to recover from this great dying event. No one knows if it was due to one or several events but it may have been caused by volcanic eruptions, meteor impact, or climate change causing the release of methane from the oceans.

The Triassic-Jurassic extinction event occurred 200 million years ago, killing off about 75 percent of all species. Archosaurs, large amphibians, and therapsids were largely made extinct. It allowed the dinosaurs to ascend without competition. No one knows what caused it but it caused much of the earth to become much drier.

The Cretaceous-Paleogene extinction event, formerly called the KT extinction. This happened 66 million years ago, resulting in the descent and extinction of the dinosaurs, with the rise of birds and mammals. Seventy-five percent of all species became extinct. It is believed to have been caused by a massive meteor strike near the Yucatan Peninsula.

It is harder to identify the presence and timing of early or older extinction events for several reasons. Older fossils are buried too deeply to evaluate in many cases. Dating older fossils is harder to do. Prehistoric environmental events can damage deposition of deposits. Non-marine fossils are also very hard to evaluate if they are older.

Many also conclude that we are living in what s called the Holocene extinction, which is caused by the industrialization of the earth by humans. Mass extinction has already occurred and oneeighth of all plant and animal species are threatened with extinction.

What hasn t been talked about so far is the Great Oxygenation Event in the Precambrian period. This happened 2.4 billion years ago and was caused by the development of photosynthesis and from the subsequent rise in oxygen levels on earth, which likely disrupted the life cycles of many microbes.

While we think of extinction events as destroying life on earth, it sometimes accelerates the evolutionary process. The new species that emerges isn t necessarily superior to the extinct species but it simply becomes dominant over other species.

Mammal-like and mammals existed from the time of the dinosaurs but they could not ascend because of the niches dominated by the larger vertebrates. The end-Cretaceous mass extinction created vacancies in multiple niches in order to allow the ascendency of mammals. Groups that do survive extinctions don t always ascend but can have a decrease in their overall numbers, going into a gradual and long-term decline themselves.

Extinction events are not generally predictable but there is some periodicity to them, occurring every 26 to 30 million years on average. It is not true that species numbers build up to such a degree that a mass extinction becomes an inevitable thing.

As we have been discussing, the exact cause of mass extinctions is largely unknown or at least may be multifactorial. There is usually some type of shock to the environment that is compounded by a long-term stress in the environment. Either one of these things alone is not usually enough to cause a mass extinction. Finding the exact cause of a mass extinction, however, can be very difficult.

There are three main categories of mass extinction events. Remember that any given extinction can have more than one cause. One of these is called flood basalt events, which usually involve eruptions. The second is reductions in sea level. A third is an asteroid impact, which has only been positively associated with one major extinction event.

Volcanic events inhibit photosynthesis, which can damage the food chain, be emitting dust particles. Sulfur oxides can be emitted, which causes acid rain and the poisoning of organisms. Carbon dioxide emissions can also lead to global warming after the dust dissipates.

Reduction in sea levels can contribute to mass extinctions because they disrupt the continental shelf area life, which is the most productive area in the ocean. This can cause mainly marine extinction but could also trigger changes in the weather pattern. Global cooling itself can cause a reduction in the sea level. This phenomenon has been impactful in all of the major extinction events.

Asteroid impacts of sufficient size can raise dust particles that block photosynthesis, cause large tsunamis, trigger forest fires, and cause acid rain to fall. Outside of the Cretaceous-Paleogene extinction event, the impact of these phenomena is not known in earlier extinction events.

Sustained global cooling can force species toward the equator and will lock up water on earth into glaciers. This is cyclical in nature and isn t believed to have had a major impact on mass extinction events, although they can contribute to the stress in the environment.

Global warming kills temperate species or forces them to the poles. It has the greatest effect on polar species themselves, melting the ice caps and possibly triggering anoxia or lack of oxygen in the ocean. This is believed to have contributed to several minor and a few major mass extinction events. Releases of methane gases in the marine environments worsen the problem because methane is a strong greenhouse gas. Methane can be released from the marine areas by sudden global warming.

Anoxic events happen when layers of the ocean become extremely low in oxygen. Most of these events are due to sudden global warming from sustained and massive volcanic eruptions. This is likely a contributing factor for many of the major and minor extinction events.

Another possible contributor to extinction is an overturning of the saline environments of the ocean. Surface water is more concentrated and sinks to the bottom of the ocean, causing the deeper waters that lack in oxygen to rise to the surface, killing the upper marine life. This is a form of an anoxic event and occurs at the beginning and end of glaciation, during global warming or global cooling.

Plate tectonics and continental drift may somewhat contribute to mass extinctions by changing weather patterns and things like wind and ocean currents. It is not likely, however, that disease infestation could have ever been the cause of mass extinctions because it would have to have affected the majority of the species on earth.