Extinction Notes

Extinction is the end of a taxon, marking the death of the last individual of the last species.
Over 99.9% of all species that have ever lived are extinct.
Background extinction occurs at slow rates continuously, reflecting the need for organisms to adapt to their environments to reproduce successfully.

Mass extinction events are characterized by global signals of high extinction rates within specific periods of the rock record.
Mass extinctions involve short time frames but high intensity.
They are often related to extreme climate change and habitat loss.
A mass extinction is defined as a sharp spike in the rate of species extinction due to a catastrophic event or rapid environmental change.
Scientists have identified five mass extinctions in Earth's history, each resulting in the loss of more than 75% of animal species.

Loss of 86% of marine life.
Two Main Phases:
- Glaciation Event: Abundant plant life reduced atmospheric carbon dioxide $CO_2$ , causing global cooling and glacier formation, which led to a drop in sea levels and reduced habitat.
- Heating Event: Global warming caused sea levels to rise again. Creatures adapted to cooler climates could not survive the increased temperatures.

About 75% of life died off.
Theories:
- Land plants developed deep roots, releasing an abundance of nutrients into the oceans, which fed algae. Algae blooms consumed vast amounts of oxygen $O_2$ in the oceans, suffocating many species.
- Another global cooling took place, resulting in glaciation and a fall in sea level, leading to habitat loss.

The deadliest extinction in history, with 96% of all life perishing.
Volcanic activity in Siberia released massive amounts of carbon dioxide $CO_2$ into the atmosphere.
Bacteria that thrive on $CO_2$ began producing methane, another greenhouse gas.
Large quantities of both gases warmed the planet and combined with Earth's water, making the ocean and rain acidic, creating a highly toxic environment for life.

About 80% of life was lost.
Theories:
- Volcanic eruptions spewed tons of $CO_2$ into the atmosphere, which trapped heat and acidified the oceans.
- An asteroid or comet impact triggered the extinction.
This extinction led to the dinosaurs' ancestors to take hold.

Wiped out the dinosaurs, along with 60-76% of all life on Earth.
A widely accepted theory is that an asteroid landed in the Yucatán Peninsula in Mexico.
The impact ejected enormous amounts of debris into the atmosphere, causing global temperatures to drop.
The impact may have also caused local fires, earthquakes, tsunamis, and acid rain.

The next mass extinction may already be happening.
The current extinction rate is at least a thousand times greater than the "normal" extinction rate.
The background extinction rate is the average rate of extinction based on the longevity of species through time without human influence, determined by the fossil record.
Scientists believe that human impact on the environment is the leading cause of extinctions today.

During the last 500 million years, Earth has experienced five periods when at least half the living creatures were wiped out.
Estimated probable causes:
- Ordovician: Intense ice age; 60-70% extinction
- Devonian: Drastic drop in oxygen levels; 75% extinction
- Permian: Asteroid impacts, massive volcanic activity; 95% extinction
- Triassic: Intensive volcanic eruptions, asteroids; 70-80% extinction
- Cretaceous: Asteroid impact; 75% extinction
- Ongoing: Homo sapiens

Cumulative extinctions as a percentage of IUCN-evaluated species for mammals, birds, vertebrates, and other vertebrates are tracked over time intervals.
Background extinction rates are compared to observed extinction rates.
Ceballos et al. 2015 is cited as a source.

Species extinction rates are measured in extinctions per million species-years (E/MSY).
$E/MSY = 1$ means that if there were one million species, one species would go extinct every year, or if there was only one species, it would go extinct in one million years.
Recent extinction rates are 100 to 1000 times higher than the natural background rate.
Has been 183 mammal extinctions per million species-years since 1900. This is 1830 times higher than we'd expect.
Background rate prediction: -0.1 extinctions per million species-years.
Species defined as 'probably extinct' by the IUCN are included as species extinctions, according to Pimm et al. (2014).

Study indicates catastrophic declines in populations of both common and rare vertebrate species over the last 20 years.
Examples:
- Cheetahs
  - Acinonyx jubatus
  - 100,000 cheetahs at the start of the 20th century
  - Today, less than 7,000 mainly due to habitat loss
- Giraffes
  - Giraffa camelopardalis
  - Declined 43% in the last 2 decades to 20,000
- Lions
  - Panthera leo
  - In 30 years, numbers fallen 40% to 97,500 due to habitat loss, poaching
  - Have already disappeared in 90% of previous range
- Orangutans
  - Bornean Orangutan (Pongo pygmaeus): 104,700; habitat loss (deforestation), poaching
  - Sumatran Orangutan (Pongo abelii): 14,613; habitat loss deforestation
  - Numbers forecast to shrink to 47,000 in 2025

Mass extinctions are typically caused by major changes in Earth systems, often reflected as climate change and habitat loss.
Key drivers include volcanism, plate tectonics, biological changes, and asteroid impacts.

Graph correlating extinction rate and magnitude/rate of temperature change for the Big Five extinctions (OS, D, P, J, K-Pg) plus the current (N) extinction.

Mountains are orographic barriers that alter wind circulation, precipitation patterns, and migration routes.
Movement of continents toward or away from poles moves available glaciation centers.
Continental configuration can inhibit ocean circulation, which affects climate and oxygenation.

Increased volcanism releases large volumes of gas (carbon dioxide, water vapor, etc.) and particulate matter (dust, ash) into the atmosphere.
Particles cause short-term cooling.
Greenhouse gases cause longer-term warming.