In the Earth’s 4.6 billion year history, over 99% of all the species that have ever existed have become extinct. In the majority of cases, these extinctions are due to competition between species for space, food, or other resources; or that a species did not adapt sufficiently or quickly enough to changes in the environment. Extinction is a natural and on-going part of the biosphere’s engineering that continues at a background rate. This constant cycle of extinction, adaptation, and evolution means that as one species meets its demise, another will take its ecological niche. Over long time periods (thousands or millions of years), this can cause vast changes in the biosphere. One obvious example is the stark comparison between the species that inhabited planet earth during the dinosaur era, and the species that we see around us today. On-going research at the Natural History Museum, London, seeks to explore the causes and consequences of extinctions over time.
Through studying the fossil record and evidence of past environmental change, scientists now know that there have been instances of mass extinctions. This is where large numbers of species become extinct worldwide within a relatively short period of time. There have been 5 mass extinction events:
End-Ordovician – 440 million years ago. Life was dominantly marine-based at this time, and around 85% of all species became extinct.
Late Devonian – 375-359 million years ago. A major environmental shift caused a long-lasting extinction event that saw the demise of major fish species and prevented the formation of coral reefs for c.100 million years.
End-Permian – 252 million years ago. This is the largest mass extinction event in earth’s history and up to 97% of species observed in the fossil record are believed to have been wiped out.
End-Triassic – 201 million years ago. A rapid mass extinction event saw a power-shift from the dominance of amphibians and mammal-like reptiles to the rise of the dinosaur!
End-Cretaceous – 66 million years ago. After a long-reign of the biosphere, an asteroid or comet impact is often regarded as the cause of the dinosaurs’ demise.
Have there been other major extinctions more recently?
Late Quaternary extinctions
Since the end of the Late Quaternary Ice Age (about 110,000 to 15,000 years ago), a large number of large mammals (mega-fauna) became extinct. This included the extinction of species such as woolly mammoth, woolly rhinoceros, giant deer (Irish elk), cave bear, cave lion, and spotted hyena. Whilst the scale of this extinction does not feature in the same scale as the ‘Big 5’, the Quaternary extinctions occurred all over the world, and scientists are investigating the causes of such widespread mammalian extinction. There are currently two major hypotheses for the cause of Quaternary megafaunal extinction:
- Environmental change
- Over-hunting by humans
Research undertaken by the Natural History Museum, has focused on fossil bone and tooth remains in Europe, northern Asia, and North America. This allows scientists to map the former distribution of these Quaternary mammals and identify their habitats. Radiocarbon dating is used to establish the age of the fossils and tell us the time at which the animal was once roaming the earth. Other collaborators at the University of Southampton are collating thousands of archaeological records of human presence over the same period, to establish if range shrinkage or extinction of the mammals was related to the spread and population density of prehistoric humans.
Through compiling this information, scientists can establish two things:
- The point at which there is no more fossil evidence of a particular species – this is used to infer the time at which the species became extinct.
- The distribution of fossils across the world – by identifying where the most recent fossils of a particular species are found scientists can understand the pattern of their extinction
The timing and spatial pattern of species extinction can be compared with records of past climate and vegetation change. This can be used to explore the relationships between species movement and extinction with environmental fluctuations and the spread of human populations.
Research has discovered that the dramatic shifts of vegetation at the end of the last ice age, as climate began to warm, had major effects on the range of many Quaternary mammalian species. As many of these species were adapted to cold climate conditions, they were often forced towards higher (and colder) latitudes – such as Siberia. This meant that populations shrunk to small, often isolated communities which left them vulnerable to human predation. This reveals that neither environmental nor human influences alone would have caused such widespread extinctions.
The Woolly Rhino
Before 40,000 years ago, the range of the woolly rhinoceros covered much of northern Eurasia.
During the last glacial maximum, between 27,000 and 20,000 years ago, the species still occupied a wide range but had retreated from Western Europe.
As the climate began to warm, between 15,000 and 14,000 years ago, the species’ range contracted further to the East.
By 13,000 years ago, as forests replaced its grassland habitat, the woolly rhinoceros was extinct.
The Spotted Hyaena
The spotted hyaena is another species that forms part of the late Quaternary mega-faunal extinction event. Its distribution through space and time, however, varies considerably when compared to the woolly rhino.
The spotted hyaena is alive and well today across many areas of sub-Saharan Africa. During the Quaternary, the species also covered much of Eurasia and southeastern Asia. A recent compilation of 100 radiocarbon dates from spotted hyaena remains indicates that the species was extinct in Central Europe and Russia by 40,000 years ago, and in north-west and Southern Europe by 31,000 years ago. This suggests that as the climate cooled, the cold intolerant hyaena was progressively forced to the warmer climates of the Mediterranean. An unfavourable climate, coupled with a concentration of the population (which leads to an increase in competition for prey), eventually led to the demise of the spotted hyaena in Eurasia. This extinction history is quite the opposite of the cold-loving woolly rhino, which saw its demise at the onset of climatic warming.
It has become apparent that not all Quaternary mammalian species became extinct at the same time, or indeed due to the same causes. Scientists are now investigating a number of species that survived the Quaternary extinction phase – such as the reindeer and the bison – to examine whether the ecological tolerances of different species can be used to explain why some species (the mammoth, woolly rhinoceros, cave bear) became extinct and others did not. By investigating these past extinctions, we can more fully understand the impacts of future environmental change on habitat loss and species vulnerability.
These mass extinctions seem like regular occurrences…will there be another one?
Whilst the previous five mass extinctions are attributed to natural environmental changes, many scientists now believe that human activity and environmental exploitation may be leading to a 6th mass extinction event. The International Union for Conservation of Nature maintains a detailed database of the current ecological status of over 50,000 species from across the world (IUCN Red List). Their most recent assessment suggests that species extinction rates are currently operating at 1,000 times that of natural/background levels. This is due to habitat destruction, climate changes, pollution, poaching and the spread of invasive species. Invasive species include those that are non-native to a particular habitat. Their introduction (increasingly by human activity) means that they can out-compete native species, forcing them out of their ecological niche and sometimes into extinction. With continued environmental change from human-induced pressures, it is possible that we may see a large scale extinction event.
This article was written by Climatica editors, in consultation with Prof. Adrian Lister.