mass extinctions

• Extinction occurs when death rate is greater than birth rate. They are common and are caused by factors like competition, this is normal and called background extinction.
• After mass extinctions adaptive radiation of species occurs, where new species colonise new habitats. 
• Reasons for mass extinctions:

  • Supercontinent formations
  • Extra-terrestrial impacts 
  • Flood basalt eruptions
  • Methane hydrates and global warming 
  • Rapid and major glaciation events 
• Supercontinent formation, for example the formation of Pangea at the end of the Permian. This causes less continental shelfs, reducing habitats for shallow marine organisms. Pangea caused rapid changes in climate (rapid desertification). There are less nutrients flowing into the sea for organisms and altered salinity. 

  Supercontinents can be on a pole, resulting in a glaciation which reduces sea levels. 
• Asteroids that are 1km diameter hit the earth every 200,000 years. 

  Asteroids locally destroy habitats due to the impact blast. Billions of tonnes of debris are carried around by the atmosphere and cause rapid cooling. Ash blocks sunlight so plants stop growing, this disrupts food chains. 
• Flood basalt eruptions last for 0.5-2 million years and produce enormous amounts of lava, they destroy local habitats and ignite wildfires. Initially ash blocks out light and lowers temperatures, it could also cause glaciation and sea level drop. Lots of carbon dioxide is pumped into the atmosphere which contributes to global warming. Sulphur falls in the form of acid rain and kills vegetation, affecting food chains. It may also acidify oceans impacting marine organisms. 
• Methane hydrates stored beneath oceans and in permafrost are stable at low temperatures. Warming temperatures cause the methane to escape into the atmosphere, accelerating global warming. 
• Supercontinent formation on poles causes glaciation, variations in earths orbit can also trigger glaciation. Glaciation increases earths albedo by reflecting more radiation back to space, further reducing temperatures (positive feedback). Glaciation decreases productivity of primary produces, loss of habitats and potentially a snowball earth. Sea levels drop and thermal contraction on the upper ocean layers.