Mass extinction in simple terms is the large scale extinction of species in a very short time period . It may take from thousands of year to a few hundred of thousands of years , for us it look like a huge time period but it really is a short when compared to age of earth. There is a certain rate at which extinction occurs normally and it is called background rate, which says that certain numbers of species will die in a certain length of time . It’s an evolution process, infact as many as 99% of all species that has ever lived are now extinct. If this background rate increases many folds then extinction occurs on large scale.
There could be multiple reasons from huge volcanic eruptions to asteroid collision with earth etc and it is sometimes difficult to ascertain a reason or it could be a series of many reasons which would’ve led earth to its destruction.
There were many mass extinctions that earth has faced during its lifetime but there are 5 that threatened to wipeout all the life from the earth. These are:
1. End -Ordovician
It occurred 443 million years ago. It was perhaps severe ice age which led to fall sea level by 100 meters and when the ice melt it starved the oceans of oxygen and it wiped out almost 60–70% of all species.
It happened around 360 million years ago, perhaps climate change was the reason and it wiped out 70% of all species , including reef building corals.
It occurred around 250 million years ago and it was so severe that it almost wiped out all the life on earth , greater than 95% of all marine life died and on land less than a third had made it. A massive volcanic eruption in Siberia was the reason .
It happened around 200 million year ago, 75% species died , most likely due to volcanic eruptions.
It occurred around 65 million years ago when a giant asteroid collided with earth in Mexico which wiped out 75% of all species including legendary dianosaurs . The collision would have risen the dust cloud around the earth’s atmosphere that would’ve blocked the sunlight hence stopped the photosynthesis , when plants died this extended to the upper food chain. The dust cloud and debris that had encircled the earth was hot and when it fallen back it may have triggered the widespread wild fires.
Some of the scientists believe that we are running through an another mass extinction which they called 6th mass extinction or Holocene ( Holocene is the time since the last ice age ended i.e around 11700 to present) and it is due to human activities .
Reason for this apart from the pollution and over use of resources is that every year 18 millions of acres forests disappear, 17% of Amazon rain forest destroyed in past 50 years , some 80% of world species found in tropical rain forests like amazon. It would take millions of year to reach that level of diversity which earth enjoyed few decades back.
Oceans absorb extra heat trapped in earth due to deforestation , which lead to increase it’s temperature and then sea level because water expand when it heat up and cause habitation loss .
Another reason is alien species, a species originated in an area and brought in other by human , accidentally such as by ships or other transportation or knowingly , have a negative impact on the native species . Earth has made many natural barriers which enabled the species to grow and compete with other if it is taken to other place it would not find competition and perhaps would be hazardous to the native species of introduced place. Since year 1500 1/3 of the extinction occurs due to alien species.
Example: cane toad , it is a native to Northern America and brought to Australia to control crop pest . They produced toxic ooze in defence , while predator in their native place were immune to this toxin , the predators in introduced place are not they died while eating them. With nothing to check them their population exploded and taking tolls on plants and animals of Australia.
Earth has between 9 million to as many as 1 trillion species .There are 26500 species which are threatened to extinction includes 40% of amphibians such as frogs, 33% of reef building corals which sustains more than 25% of marine life and by 2050 to will decline to 50 %, 25% of mammals and 14% of birds. 2.5% of bugs die every year and these are the sources of food for birds , fish and some mammals on which we human depend , 40% of insects declined and at that rate earth maybe free of all insects by 2119 year , this will impact the pollination , more than 30% of vertebrate (those who have back bone) are declining.
According to the Author of the book sixth mass extinction 75% of all species could be extinct within few generations.
Though many scientists believed the opposite that human is not the cause , many other argue that human was behind the extinction of many giant animals of Australia and American continents because it coincided with human migration to these places.
Whatever is the case it is certain that extinctions are occurring whether or not to be called them mass extinction and we should act immediately on this problem as we don’t have any other earth to live.