Background extinction rate
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 in 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.

Background extinction rates have not remained constant, although changes are measured over geological time, covering millions of years.

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 is three different ways to calculate background extinction rate..  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.  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.  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.

Lifespan estimates
Some species lifespan estimates by taxonomy are given below (Lawton & May 1995).

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. 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.

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.

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.

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 affected the livelihood of this species. 

Planktonic Foraminifera: These species average lifespan is 7 million years. These species face extinction in cases of glaciation events, hyperthermal event, and climate change.

Cenozoic Bivalves: These species average lifespan is 10 million years. The reason for why this group goes extinct is related to environmental deterioration.

Echinoderms: These species average lifespan is 6 million years. The reason for for why this group goes extinct is related to ocean acidification. Ocean acidification makes it hard for the echinoderms to build their shells.

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.

Five Mass Extinctions 
There have been five mass extinctions since the beginning of time. A mass extinction can be described as when 75% of the species go extinct in a short period of time. A mass extinction can not be classified as a sped up of ecological succession because it completely wipes out most species ending future possibilities for evolutionary and ecological considerations that are associated with the different taxonomic groups. These extinctions are a result of the life conditions no longer being habitable and suitable for all forms of life. The five mass extinctions are the End of the Cretaceous, the Late Triassic, the End Permian, the Late Devonian, and the Late Ordovician.

The Five Mass Extinctions

Late Ordovician: This extinction occurred 447 million years ago. Most major animal groups survived which included trilobites, brachiopods, corals, crinoids, and graptolites. But these groups lost important members, as families of trilobites and graptolites went almost completely extinct. Climate change is what caused the extinction because it cooled the planet and the species could not adjust.

Late Devonian: This extinction occurred 378 million years ago, and affected the reefs of shallow seas in tropical environments. This extinction caused the disappearances of 70% to 80% of all marine organisms. The species included, stromatoporoids, corals, brachiopods, and groups of trilobites went extinct. This extinction was caused by cooling of the climate from the result of a drop of carbon dioxide, as well as tectonic activity, climate and sea level change, as well as impact events.

End Permian: This extinction occurred 252 million years ago, and marked the end of the Permian period. 70% of species were lost during this extinction. Important marine groups were lost, which included trilobites, tabulate and rugose corals, and the land species that were lost were the synapsids. At the end of the Permian a massive scale of volcanic activity led to an eruption, creating carbon dioxide, a greenhouse gas which aids in keeping the planet warm. The global warming increased average ocean water temperatures by 14 degrees fahrenheit. Typically the ocean water would absorb the carbon dioxide but since it had not been introduced yet the ocean could not absorb it in result. 

Late Triassic: This extinction occurred 199 million years ago near the end of the Triassic Period. Both marine and terrestrial species were affected by this extinction. The marine species that suffered the most from this extinction were brachiopods, cephalopods, sponges, and corals, the terrestrial species that were affected were phytosaurs. This extinction was caused by the split up of the continents, in which carbon dioxide was introduced, which lead to a severe event of global warming.

End of the Cretaceous: This extinction occurred 66 million years ago at the end of the Cretaceous Period and is the most recent extinction. It included many marine and land species and habitats, and all dinosaurs except those that are non avian dinosaurs. This extinction is believed to be related to one of two explanations. The first hypotheses being caused by an asteroid or comets, directly hitting the Earth and caused a massive explosion. The second hypotheses being the result of a massive volcanic explosion propelling large amount of volcanic material into the atmosphere. This explosion would have reduced the sunlight, temperature, and productivity in oceans. Organisms that did survive included deposit-feeding and nucleic bivalves, avian dinosaurs, frogs, and salamanders.

Scientist believe that we are currently in a six mass extinction. This extinction is completely different from the others that have happened in the past. The 6th mass extinction is driven from a result of human activity which includes an increase of human population, overexploitation, climate change and global warming, invasive species and disease, pollution, and biodiversity loss.The leading cause of extinction currently is habitat loss and degradation. Being responsible for 50% of avian species, 45% of mammals, 30% of fish species, and 48% of reptiles and amphibians. Overexploitation is the second leading cause of extinction being responsible for 15% of avian species, 35% of mammals, 52% of fish species, and 22% of reptiles and amphibians. Both of these causes are driven by human activities, with the increase in population size it require the use of more land for expansion, and increased productivity rates lead to overexploitation of the lands in the surrounding areas.

References

Further reading
E. O. Wilson. 2005. The Future of Life. Alfred A. Knopf. New York, New York, USA
C.Michael Hogan. 2010. Edenic Period. Encyclopedia of Earth. National Council for Science and Environment. ed. Galal Hassan, ed in chief Cutler Cleveland, Washington DC
J.H.Lawton and R.M.May (2005) Extinction rates, Oxford University Press, Oxford.

External links
  Discussion of extinction events, with description of Background extinction rates

Extinction
Temporal rates