TEMA: UTROTNINGENWe are witnessing the beginning of the sixth mass extinction of species. This time, this dramatic change is being caused by human activity, through climate change, and through the destruction of habitats.

Earth’s flora and fauna have over the last 540 million years seen mass extinction on five occasions, with an elevated rate of extinction and loss of at least 75 % of the species in less than two million years. The Ordovician, Devonian, Permian, Triassic and Jurassic geological periods all ended with a sharp decline in all orders of species. The mass extinctions were caused by a combination of climate change, changes in the composition of the atmosphere, greatly altered habitats, and in the seas and oceans both acidification and lack of oxygen. The fifth mass extinction, which probably took place over only a few years, saw the disappearance of all large dinosaurs. Many researchers claim that we are now standing on the brink of the sixth mass extinction, this time caused by man. Is it so? To answer this question we need to look at the current speed and scale of the extinction.


How fast is the rate of extinction?

The normal rate of extinction has been estimated to be between 0.1 and 2 species per million per year. The number of documented extinctions in history (since 1500) is only about 1,000 species, which does not sound much considering that just over 1.7 million species have been described, but it is still sufficient to be regarded as an abnormal increase in the rate of extinction. Among the animals we know best, the vertebrates, 200 species have become extinct over the last 100 years. With a normal rate of extinction this would have taken up to 10,000 years, not 100. The rate of extinction of vertebrates is already 100 times higher than the normal rate. We know less about other organisms but based among other things on assessments of the number of natural habitats that have disappeared, the current rate of extinction of the Earth’s species is estimated to be between 10 and 1000 times higher than is normal.

An assessment method uses the ratio between number of species and area together with data on known biotope losses to predict the loss of species. 12-24 % of the Amazonian forests are expected to disappear between 2008 and 2050, which would result in the loss of 5-9 % of the plant species. Other methods are based on taking inventory before and after biotope changes and extrapolating to a global scale. All methods have their weaknesses and sources of error, not least because we know about only a fraction of the species that exist. The pattern, however, is clear: some orders of animals and plants now have a much higher rate of extinction than normal. The rate of extinction among birds is 100 times higher than is natural and for amphibians 211 times.

Over 25,000 species are listed as threatened around the world and many of them will die out within the next few centuries, which will further increase the rate of extinction. By about 2100 the rate of extinction among birds will be an estimated 1,500 times higher and among amphibians more than 25,000 times higher. Many of the endangered species are already considered to be unable to survive because too little of their natural habitats remains and they therefore constitute a global extinction debt. Theoretical species-area models applied on losses of forests in South-East Asia indicate that 79% of South-East Asia’s vertebrates will be facing extinction by 2100. If most of the globally threatened species die out within a century the rate of extinction will be at least 1,000 times higher than is natural, which is on a par with earlier mass extinctions, or far above depending on how long it took.


How widespread is the extinction?

The extent of the extinction is also approaching the magnitude of previous extinctions. In many groups of animal and plant species over 25% of the species are threatened. For those groups that are most severely affected by human activities, the proportion of threatened species is even higher: amphibians 42 %, crocodiles 47 %, cycads 63 %, and sturgeon 85 %.

A global loss of species is bad enough but even worse from the point of view of man’s possibilities to take advantage of the ecosystem services the species offer, is the ongoing loss of species locally and regionally, and the decreasing population size and distribution of many species. A large part of the species stock, in particular the large herbivores and predators, can be considered to be functionally extinct. The large-scale pattern is that endemic and specialised species are being annihilated and replaced by global colonists, i.e. the species that are favoured by human activities (including pests and pathogenic organisms). The result is that local sets of species are becoming increasingly similar to each other. The local number of species can thus be maintained, at the same time as the Earth loses diversity among species.

The trend for most threats to species is increasing. There are now 6.7 billion people and this figure will have risen to 9.3 billion already by 2050. All these people need space to live in and resources such as food and energy. This takes living space from animal and plant species. Mankind today uses 25 % of the planet’s primary production, in some regions up to 80 %. Climate change is also expected to become a more serious threat factor over the next 100 years. The “perfect storm” in the form of climate change and substantially altered habitats, including acidification and lack of oxygen in the seas and oceans that characterised the five previous mass extinction periods is already upon us.


The sixth mass extinction is coming closer

As stated earlier we are now standing on the brink of the sixth mass extinction, this time a mass extermination, caused by man. The rate of extinction is already sufficiently high for us to be able to say this, and we are approaching a magnitude of 75 % loss. This mass extinction began already at the end of the ice age, when man contributed to the extinction of the mammoths. If all threatened species of vertebrate die out in the near future and the rate of extinction then reached continues to prevail, 75 % of vertebrates will be gone within 240-540 years. There are several assumptions and rough estimates behind this conclusion, but it is not unreasonable.

The forecasts for future development depend for the most part on how people choose to manage the Earth’s resources. With radical changes, and if they are realised very soon, we can reverse the trend in global extinctions. Scenarios for future developments, however, show a complex relationship between species conservation and climate measures. The scenarios that have the lowest temperature increase are not the best from a biodiversity perspective because large areas would need to be used to produce bio-fuels to replace fossil fuels. The worst scenario, however, is “business as usual”: more severe climate changes and increased land use to feed a growing population.

Following the last five mass extinctions, the diversity of species has recovered but it has taken millions of years. The mass extinction at the end of the Cretaceous period was among the most severe in the Earth’s history, but recovery was in the context very fast; within 300,000 years local species’ diversity had been restored counted as number of species, even though the fauna and flora at that time were quite different. What will come after the sixth mass extinction? Life will probably persist, but will man?


A perfect storm


A ”perfect storm” is an expression that describes an incident in which a rare combination of circumstances drastically aggravates a situation.
Functionally extinct = the species is numerically so small that it can no longer fulfil its ecological role, for example, as predator, prey, saprotroph or competitor in the ecosystem

Read more

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