How invasive species can trigger mass extinctions
65 million years ago dinosaurs dominated the Earth. Global temperatures were between six and 14 degrees Celsius warmer than at present and sea levels were over 300 metres higher. 40% of the present land mass was under the sea. Then came the earth-shattering event that brought dinosaur domination to an end. No large land animals survived, plants were greatly affected and tropical marine life was decimated.
Scientists continue to speculate as to the cause of this catastrophe, but hot favourites are the impact of a massive asteroid, gradual climate change or volcanic eruptions. What is not always realised is that this was the latest of five mass extinctions in the Earth's history.
The first of these took place around 434 million years ago at the end of the Ordovician. It is thought that the cause was an intense period of global freezing, leading to a formation of glaciers and a drop in sea levels. This was then followed by a period of global warming and rising sea levels as the glaciers melted. According to fossil records, up to 60% of all sea and land life was exterminated.
The second event came about 360 million years ago and is known as the Late Devonian Extinction. This also had serious consequences for marine life.
About 250 million years ago came the third major event. Again there is speculation as to the cause, but it was certainly the Earth's worst mass extinction, killing up to 95% of all species.
The fourth major event came somewhere around 200 million years ago. This was most likely caused by massive volcanic activity that opened up the Atlantic Ocean and led to a period of deadly global warming. It is estimated that half of all marine invertebrates and 80% of all land quadrupeds became extinct as a result.
Scientists in Ohio have recently been studying the Late Devonian Extinction and their conclusion is that this was unlike any of the others, since rather than being a mass extinction, in reality it was a biodiversity crisis.
During the Late Devonian period amphibians had begun to walk on land and the first forests had appeared. The oceans had the most extensive reef system in the Earth's history, but sea levels were rising and continents had closed in to form connected landmasses. This led some species to gain access to environments that they had not previously inhabited.
The hardiest of these invasive species began to dominate and quickly wiped out the more locally adapted species with a result that the entire marine ecosystem suffered a major collapse. Reefs were destroyed and did not reappear for another 100 million years.
Researchers point to a potential biodiversity crisis in our time that could potential have a similar effect to the Late Devonian Extinction. Human activity has introduced a high number of invasive species into new ecosystems. If we are to avoid another catastrophe on a Late Devonian scale, they say, our only hope is to focus efforts and resources on the protection of new species generation.
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