Background extinction rate, also known as the normal extinction rate, refers to the standard rate of extinction in Earth's geological and biological history before humans became a primary contributor to extinctions. This is primarily the pre-human extinction rates during periods in between major extinction events. Currently there have been five mass extinctions that have happened since the beginning of time all resulting from a variety of reasons.

Overview

Extinctions are a normal part of the evolutionary process, and the background extinction rate is a measurement of "how often" they naturally occur. Normal extinction rates are often used as a comparison to present day extinction rates, to illustrate the higher frequency of extinction today than in all periods of non-extinction events before it.^[1]

Background extinction rates have not remained constant, although changes are measured over geological time, covering millions of years.^[2][3][4]

Measurement

Background extinction rates are typically measured in order to give a specific classification to a species and this is obtained over a certain period of time. There are three different ways to calculate background extinction rate.^[5] The first is simply the number of species that normally go extinct over a given period of time. For example, at the background rate one species of bird will go extinct every estimated 400 years.^[6] Another way the extinction rate can be given is in million species years (MSY). For example, there is approximately one extinction estimated per million species years.^[7] From a purely mathematical standpoint this means that if there are a million species on the planet earth, one would go extinct every year, while if there was only one species it would go extinct in one million years, etc. The third way is in giving species survival rates over time. For example, given normal extinction rates species typically exist for 5–10 million years before going extinct.^[8]

Lifespan estimates

Some species lifespan estimates by taxonomy are given below (Lawton & May 1995).^[9]

• Invertebrates: These species average lifespan is 11 millions years. Reasons why these species go extinct is from habitat loss, overharvesting, pollution, invasive species, and climate change. Invertebrates makes up most of the Earth's biodiversity which is why they do not go extinct as fast as other species.
• Marine Invertebrates: These species average lifespan is 5-10 million years. Many marine invertebrates face extinction because of the high levels of dissolved carbon dioxide in aquatic environments. Seawater chemistry changes with the increase carbon levels which makes it hard for these organisms to survive.^[10] Similar to terrestrial invertebrates, marine invertebrates make up most of Earth's biodiversity which is why they do not go extinct as fast as other species.
• Marine Animals: These species average lifespan is 4-5 million years. Reasons why marine animals go extinct include interactions with fisheries, capturing, pollution, habitat degradation, climate change, and overharvesting.
• All Fossil Groups: These species average lifespan is 0.5-5 million years. The main reason why fossil groups go extinct is because of the changes to their environment. Another reason why they go extinct is that they naturally break down, and dissolve.
• Mammals: These species average lifespan is 1 million years. Habitat loss is the leading reason for why mammals go extinct. Other reasons that follow this is overexploitation, invasive species, pollution, and climate change.^{[citation needed]}
• Cenozoic Mammals: These species average lifespan is 1-2 million years. The main reason for these species extinction was from a direct result of climate change.^{[citation needed]}
• Diatoms: These species average lifespan is 8 million years. Diatoms rely on silica to build their shells, which benefited them when oceans originally started to become more acidic. Now as oceans continue to have chemical changes and become even more acidic this makes it harder for them to continue to thrive. From this information we have concluded that these species are going to extinct due to high rates of ocean acidifications.^{[citation needed]}
• Dinoflagellates: These species average lifespan is 13 million years. It takes a lot for these species to go extinct because they are so promendant in aquatic environments. Dinoflagellates were severely affected during the Triassic extinction, suggesting that that warming of ocean waters can affect the livelihood of this species. ^[11]
• Planktonic Foraminifera: These species average lifespan is 7 million years. These species face extinction in cases of glaciation events, hyperthermal event, and climate change.^{[citation needed]}
• Cenozoic Bivalves: These species average lifespan is 10 million years. The reason for why this group goes extinct is related to environmental deterioration.^{[citation needed]}
• Echinoderms: These species average lifespan is 6 million years. The reason why this group went extinct is related to ocean acidification. Ocean acidification makes it hard for the echinoderms to build their shells.^{[citation needed]}
• Silurian Graptolites: These species average lifespan is 2 million years. Reasons for why this group goes extinct includes climate change, rising sea levels, and loss of habitats.^{[citation needed]}