L14 - Mass Extinctions

Question 1

What is the background rate of extinction?

Answer 1

• This is calculated by using the fossil record to first count how many distinct species existed in a given time and place, and then to identify which ones went extinct
• From here, we back-calculate an extinction rate

Question 2

What are the limitations of using the fossil record?

Answer 2

• Fossil record does not accurately represent past species diversity
  
  • Fossil formation is not possible in all environments/habitats
  • Shallow seas are the most conducive environment for fossil formation
  • Soft-bodied organisms do not preserve well (jellyfish, worms)
  • Fossils of land animals are scarcer than those of plants
  • Hard to tell distinctions between species in the fossil record
  • If we find only small fragments of the species…

Question 3

What conditions are required for a fossil to form?

Answer 3

• Fossils can form through freezing, drying or encasement in tar or resin (rare)
• Most are formed when a plant or animal dies in a watery environment and is buried in mud and silt

Question 4

Other issues with using the fossil record to estimate the background rate of extinction…?

Answer 4

• Chronospecies: a single species, changing morphologically, genetically, or ecologically over a long time scale
  
  • Changes occur to such an extent that the original species, and its descendants, are identified as separate species
• Pseudoextinction: when a species is presumed to have gone extinct, but has instead become a different species
  
  • Incomplete fossil record can lead us to believe a species has gone extinct, when it simply evolved into a different species over time.

Question 5

Other issues with using the fossil record to estimate the background rate of extinction…?

Answer 5

• Extinction rates are generally measured at the taxonomic level of family (relative high)
  
  • Not specific enough to differentiate between species
  • Some families are bigger than others: one extinction of a family of 8 species is not as severe as one extinction of a family of 2 species =

Question 6

What are biotic mechanisms of extinction?

Answer 6

• A species can be competitively excluded by a closely related species
• The organisms that a species feeds on may come up with an unbeatable defense
• A new predator may expand its territory
• A species can be wiped out by a disease

Question 7

What are abiotic mechanisms of extinction?

Answer 7

• The niche or habitat the species occupied can no longer support the species due to…
  
  • Temperature or climate fluctuations
  • Extreme sea level changes
  • Impact events (meteorites)
  • Volcanism

Question 8

What is a mass extinction?

Answer 8

• Extinction of a large number of unrelated species over a short period of geological time
• Background rates of extinction are substantially higher
• Extinctions are globally distributed

Question 9

What are the five mass extinctions in history?

Answer 9

1. Late Ordovician
2. Late Devonian
3. Late Permian
4. Late Triassic
5. Late Cretaceous

Question 10

How do mass extinctions affect the evolution of life?

Answer 10

• Very rapid period of speciation among the few species that do survive
• More room for surviving species to spread out and so many niches in the environment that need to be filled

Question 11

Late Ordovician ~ 440 MYA

Answer 11

• At this time, most complex multicellular organisms lived in the sea
• Almost all major taxonomic groups were affected during this extinction event
• Marine organisms suffered the most
• 49-60% of marine genera and nearly 85% of marine species were eliminated.

Question 12

What were the causes of the late-Ordovician extinction?

Answer 12

• Global temperatures cooled and the sea levels fell
  
  • Considering that much of the plant and animal diversity of the time had adapted to shallow warm waters, this was devastating
  • Species could not survive in colder, deeper oceans and many died out
• Land moved to the south pole due to place tectonics
  
  • Allowing for the formation of glaciers
  • Removed water from oceans leading to rapid fall in sea levels
  • Many shallow seas were drained completely

Question 13

Late-Devonian (Age of the Fishes) Extinction ~ 375 MYA

Answer 13

• Land: been colonized by plants and insects
• Oceans: massive reefs built by corals
• Late-Devonian extinction lasted over 20 M years whereas most extinction last 100 000 to 5 M years
• Consisted of extinction pulses – no single cause has been identified

Question 14

Theories behind the Devonian extinction

Answer 14

• During the Devonian, the Earth experienced super greenhouse climate conditions.
• Very warm and lots of CO2 in atmosphere
• As plants expanded onto land to form first forests, they depleted CO2 in the atmosphere (PHOTOSYNTHESIS)
  
  • Rapid reduction in atmospheric CO2 that led to the earth cooling and the extinction of many organisms?
  • Glacial deposits in northern Brazil (near the Devonian South Pole) suggests widespread glaciation at the end of the Devonian

Question 15

Late-Permian Extinction ~ 250 MYA (Largest of all known mass extinctions)

Answer 15

• “The Great Dying”
• 70% of terrestrial vertebrates and 96% of marine species went extinct
• All life today is descended from the few survivors of The Great Dying

Question 16

Causes of the Permian Extinction?

Answer 16

• Series of great volcanic eruptions
• Total volume of eruptions and intrusions was enough to cover a region the size of the US in km-deep magma
• Siberian Traps erupted at the right time, and for the right duration, to have been a likely trigger for the Permian extinction.
• Eruptions would have released CO2 into the atmosphere resulting in a GH effect – heating up the atmosphere and acidifying the oceans
• Eruptions would have also released aerosols and lead to dust clouds, which would have blocked our sunlight and thus disrupted photosynthesis both on land and in the oceans, causing food chains to collapse
• Eruptions may have also caused acid rain when the aerosols washed out of the atmosphere

Question 17

How long did it take for life to bounce back from the Great Dying?

Answer 17

1 M years after Permian Extinction

Question 18

Late-Triassic extinction ~ 200 MYA

Answer 18

• ~76% of all species went extinct
• Mass extinction on land
• Vacated ecological niches on land, allowing the dinosaurs to assume the dominant roles in the jurassic period

Question 19

Causes of the late-Triassic extinction?

Answer 19

• Several hypotheses…
  
  • Asteroid impact: Rochechouart Crater in France has been dated to 200 MYA but at 25 km across it appears to be too small
  • Massive volcanic eruption: Observed elevated mercury concentrations in extinction-aged
    rocks from around the world
  • Since volcanism is the main nonanthropogenic source of mercury in the environment

Question 20

Late-Cretaceous Extinction ~ 66 MYA

Answer 20

• 75% of plant and animal species went extinct
• No tetrapods (4-legged vertebrates) larger than 25kgs survived
• Exceptions were leatherback turtles and crocodiles
• End of the dinosaurs
• Followed by the rise of mammals (rapid adaptive radiation for mammals to fill the empty niches of the dinosaurs)

Question 21

Causes of late-Cretaceous extinction?

Answer 21

• Impact from comet or asteroid 10-15km wide
  
  • Impact would have ejected enormous amounts of dust/ash into atmosphere, blocking the radiation from the sun
    
    • Zero sunlight reaching surface of the Earth for ~ 1 year
    • Caused a global winter
    • Prevented photosynthesis by plants on land and plankton in oceans

Question 22

What is the evidence for the impact hypothesis?

Answer 22

• 150 km wide crater dated precisely to 66 million years ago.
• High concentrations of iridium (extremely rare element on Earth but common source in asteroids and comets) in sedimentary layers deposited globally during end-Cretaceous (as much as 160 times above expect levels).
• Shocked quartz found in the sedimentary layer of late-Cretaceous.

Question 23

Other causes of late-Cretaceous extinction?

Answer 23

• Increased volcanic activity?
  
  • Deccan Traps: Likely produced many of the same effects seen during the extreme volcanism of preceding mass extinctions

Question 24

Which occurred first? Volcanism or impact?

Answer 24

• Did impact cause volcanism?
• Or was volcanism already occurring and impact followed?

Question 25

What is press/pulse theory?

Answer 25

- When 2 events occur... - First event stresses an environment - Second event then occurs as a result of the environmental stress caused by the first event AND has a greater-than-expected impact on the environment - Alone, either event may not have been severe - Combination of deadly sudden catastrophes - "pulses" - with longer, steadier pressures on species -"presses."

Question 26

The sixth mass extinction: Anthropocene Extinction

Answer 26

Based on our definition of mass extinction, some scientists believe we are currently undergoing a mass extinction

Question 27

What is the justification?

Answer 27

- Historical rates of extinction is 0.1 species per million species per year. - Current rate of extinction is 100 extinctions per million species per year. - ~100 to 1000x the natural background rate of extinction

Question 28

Causes of 6th mass extinction?

Answer 28

- Unintended consequences of human activity - Habitat loss - Invasive species - Overhunting

Question 29

What are planned extinctions?

Answer 29

Ways in which humans are deliberately driving a species to extinction

Question 30

What was the planned extinction of the passenger pigeon?

Answer 30

- 19th century: most abundant bird in North America (possibly in the world) - Passengers had all the right biotic traits to avoid extinction - North American farmers started blaming them and declared them agricultural pests - Local bounty put on passenger pigeons - since they flew in large flocks, it was easy to kill them in large numbers - Species was killed into extinction

Question 31

De-extinction of the passenger pigeon?

Answer 31

- Using DNA samples from taxidermy pigeons to revive the passenger pigeon

Question 32

Gene editing?

Answer 32

- Gene editing technology: CRISPR/Cas9 - Can now guarantee any gene we insert into an individual can and will spread rapidly though the population