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Why Do Scientists Think We Are in a Sixth Great Mass Extinction?

Updated on July 23, 2017

The Sixth Extinction

Are we in a sixth great mass extinction?

Scientists, chiefly conservation biologists, zoologists, ecologists, paleobiologists, and environmental scientists, are increasingly becoming more certain that humans are causing massive changes to the biosphere, with many claiming we are entering the early stages of a sixth mass extinction event that will occur on earth, also referred to as the "Holocene extinction" or the "Anthropocene extinction". These changes are taking place on a scale that occurred during the previous five mass extinctions events on earth. A mass extinction event is classified as an extinction event where 75% or more of all species on Earth go extinct. That's a whopping figure. To give this some perspective, there's thought to be about 10 million species on earth, and the number of individual animals is much, much higher. According to the fossil record, about 99.9% of all life on earth has gone extinct, due to either evolving into other species or reaching an evolutionary dead-end (this would usually be caused by environmental pressures). So, yes, extinction is a very common occurrence in evolutionary history, there is no need to debate that point. An estimated 1% of species on earth have gone extinct since 1500, and a mass extinction event would take tens of thousands of years if this trend were to continue. The problem is that scientist think that this trend is not going to continue and that we could reach the point of mass extinction much sooner, even in the next century or two.

The most recent mass extinction occurred about 63 million years ago, and this was the extinction event that completely wiped out the dinosaurs. The complexity of life on earth has been slowly increasing for about 541 million years (which was when oxygen first emerged on the planet when the Cambrian explosion occurred), however, the first single-celled organism is thought to have appeared about 4 billion years ago. The most severe mass extinction was the Permian-Triassic Extinction event, also known as the "great dying", which wiped out about 95% of all species on the planet! These mass extinctions usually occur over vast time frames in comparison to human lifespans, with most occurring over tens of thousands of years. Mind you, this is still quite a short time frame with respect to geological time. If the history of the earth since its formation were put onto a 24-hour clock, the history of humanity would traverse about a minute before midnight. Geological time is something we struggle to grasp, as our brains weren't evolved in environments that required us to deal with such large quantities. But this clock metaphor is a good one.


An artists impression of the asteroid that is thought to have wiped out the dinosaurs 65 Ma years ago.
An artists impression of the asteroid that is thought to have wiped out the dinosaurs 65 Ma years ago. | Source

How do we know all of this? Paleobiologists and other scientists have examined the fossil record and can see where mass extinctions have punctuated the evolution of life on earth up until the current geological epoch. By using techniques such as carbon dating and studying the fossil record, these scientists have observed species going extinct but not evolving into other species in huge numbers five times in the past, leading them to conclude that massive environmental changes caused these mass extinction events, and from the evidence examined and from our collective knowledge of science, these causes are hypothesized to include huge changes to the earth's climate, ice ages (also known as Milankovitch cycles), meteor impact, and volcanic activity.

What the fossil record shows, is that absent of these mass extinction events, species tend to go extinct fairly consistently. This is known as the "background rate" of extinction, which is one species per million going extinct every year, or stated another way – if there were only one species on earth, it would go extinct in one million years. The background rate is now thought to be highly elevated due to human activity, and most estimates indicate that it is now about 100 times this rate.

The previous five mass extinction events on earth
The previous five mass extinction events on earth | Source

Since about the year 1500, the ICUN (The International Union for the Conservation of Nature) red list, which is the global data base that states the conservation status of species on earth, estimates that about 1% of all vertebrate species have gone extinct. This is why scientists have concluded that the estimated background rate is highly elevated. For example, the losses in vertebrate species over the last century should have taken roughly 10,000 years to take place. Scientists that research earth's ecological diversity are becoming more concerned that we are not effectively taking into account the full picture of biodiversity decline. Conservationists have done an excellent job in targeting species highly-threatened with extinction and those that are critically endangered, hence the number of species extinctions have been limited, however, there may be a "lag" effect where bigger declines in species extinctions could ensue over the next 50-100 years than what has been seen in the past. These extinctions have been most prominent in the tropical regions of the earth because that is where the highest level of species biodiversity is found, however, all bioregions are experiencing similar declines but this is relative to the level of biodiversity found in each region. Even so, for example, on the Australian continent which is mostly non-tropical, except for its far north regions, has the worst record of mammal extinctions across the globe.

There have even been some remarkable conservation efforts such as the Giant panda (that you see on the World Wildlife Fund logo) being taken off the critically endangered ICUN red list. However, that same year the Australian Koala was newly listed as critically endangered. The trend, overall, appears to be worsening and species extinctions do not appear to be slowing down. Furthermore, what is missing from this picture is the total level of biodiversity, which is largely a function of the population sizes of species (the total number of individual species), species richness (how many different types species there are in our biosphere), genetic diversity (how much the genetic makeup of species vary between individual animals within the same species, but this also includes the genetic diversity between each species), and the habitat ranges of species (how geographically spread out each species is). The World Wildlife Fund and the Zoological Society of London have been publishing what's known as the "Living Planet Index" since 2006, which estimates the total biodiversity and number of individual animals on the earth. In 1992, The United Nations Environment Programme opened the Convention on Biological Diversity for signatures, which has since been ratified by 196 countries around the world. The convention was established to address the global biodiversity decline and states that "the threat to species and ecosystems has never been so great as it is today. Species extinction caused by human activities continues at an alarming rate." The Convention on Biological Diversity uses the Living Planet Index as one of its key indicators that measure biodiversity loss.

The Thylacine or "Tasmanian Wolf" is a well-known species that has gone extinct due to humans, with the last confirmed sighting in 1933
The Thylacine or "Tasmanian Wolf" is a well-known species that has gone extinct due to humans, with the last confirmed sighting in 1933 | Source
The giant panda is no longer listed as critically endangered.
The giant panda is no longer listed as critically endangered. | Source

The Living Planet Index is the largest database of its kind and is often cited in academic research papers. In the most recent edition, published in 2016, the report states that there has been a 58% decline in vertebrate species between 1970-2012. This index is composed of the three different types of ecosystems on earth, and it shows that terrestrial populations have declined by 38%, freshwater populations have declined by 81%, and marine species have declined by 36%. These massive population declines are therefore occurring at orders of magnitude faster than individual species extinctions. What scientists are concerned about is that massive population declines usually precede mass extinction events. It has also been documented that the loss of coral reefs in the oceans due to ocean acidification, which is occurring now, has accompanied the previous five mass extinction events – coral reefs are impacted the hardest during a mass extinction event. According to the World Resources Institute and Columbia University, "Ten percent of coral reefs have already been damaged beyond repair, and if we continue with business as usual, WRI projects that 90% of coral reefs will be in danger by 2030, and all of them by 2050." Invertebrate species and plants are also showing similar declines that vertebrate species are experiencing. If entire ecosystems start to decline rapidly, the ecosystem services that are provided by them that humans need in order to survive will start to break down and the benefits humans derive from them will also be lost. The ecosystem services and benefits that humans derive from ecosystems include crop pollination, the maintenance of healthy soil through nutrient cycling, the regulation of the climate, providing clean air and water, food for eating, medicines (the majority of our medications are derived from nature as opposed to synthetically manufactured), recreation, spirituality, aesthetic value, and many others.

A recent paper published by the leading American scientific journal PNAS, which was authored by the highly distinguished Professor Paul Ehrlich, who is currently president of the Center for Conservation Biology at Stanford University; Rodolfo Dirzo, Professor of Biology also at Stanford University and Senior Fellow of the Stanford Woods Institute for the Environment; and Dr. Gerardo Ceballos, a distinguished Senior Researcher at the Institute of Ecology of the Universidad Nacional Autónoma de México, have written that we need to reexamine the decline of the earth's biodiversity more critically and take it more seriously: "The strong focus on species extinctions, a critical aspect of the contemporary pulse of biological extinction, leads to a common misimpression that Earth’s biota is not immediately threatened, just slowly entering an episode of major biodiversity loss. This view overlooks the current trends of population declines and extinctions. Using a sample of 27,600 terrestrial vertebrate species, and a more detailed analysis of 177 mammal species, we show the extremely high degree of population decay in vertebrates, even in common 'species of low concern'. Dwindling population sizes and range shrinkages amount to a massive anthropogenic erosion of biodiversity and of the ecosystem services essential to civilization. This 'biological annihilation' underlines the seriousness for humanity of Earth’s ongoing sixth mass extinction event."

"The resulting biological annihilation obviously will also have serious ecological, economic, and social consequences. Humanity will eventually pay a very high price for the decimation of the only assemblage of life that we know of in the universe... we emphasize that the sixth mass extinction is already here and the window for effective action is very short, probably two or three decades at most."

Jane Goodall, David Attenborough, Richard Dawkins, and Richard Leakey debate how we should tackle the issue of saving our own planet.

What can you do about the sixth mass extinction?

Anthony Barnosky, Professor of Integrative Biology at the University of California Berkeley says, "with all the gloomy predictions being thrown about, you may not know that the sixth mass extinction is not a done deal. Yes, it’s true that about a third of species we’ve evaluated are threatened with extinction, and that we’ve killed about half of all our wildlife in the past forty years. But it’s also true that so far we’ve only lost less than one percent of the species that have ridden the planet with us for the last twelve thousand years. That doesn’t mean species aren’t in trouble – more than 20,000 of them are – but it does mean that most of what we want to save is still out there to be saved."

He writes that we can stop the sixth mass extinction by doing the following things:

  • Spreading the word to others.
  • Reduce your greenhouse gas emissions – since climate change is predicted to be a leading threat to biodiversity into the future.
  • Buy sustainably produced products and avoid products that exacerbate biodiversity loss such as palm oil.
  • Buy only sustainably farmed fish.
  • Eat less meat – deforestation, carbon and methane emissions caused by cattle farming are putting excess pressure on the biosphere.
  • Never buy products made from endangered species such as ivory.
  • Spend time in nature so that you see the value of biodiversity and nature as an end in itself, rather than a means to an end.
  • Volunteer as a "citizen scientist".
  • Use political action and vote for parties that enact policies that protect biodiversity.
  • Don't give up – as holding apathetic attitudes towards the environment will not help stop this extinction crisis. Humans have generally been quite good at coming together to stop catastrophic events occurring once the will is there.

Yes, it's true that the earth will recover no matter what we humans do to it. After a few million years, even if humans were to go extinct, biodiversity will likely be at levels that surpass the current levels, which is what has happened after every mass extinction event in the past. Chris Thomas, a Professor of evolutionary biology at the University of York is arguing exactly that in his recently written book titled Inheritors of the Earth: How Nature is Thriving in an Age of Extinction. He claims that we are creating many new hybrid species, climate change is pushing species to new habitats, and many species have moved around the globe that we categorise as "invasive species". He wants us to rethink conventional wisdom with regards to biodiversity measurements.

This is quite a contrarian point of view with respect to biodiversity conservation since the majority of conservation biologists are of the opinion that we are in a mass extinction event. It's early days now to see how well-received Chris's work will be, or whether it will even register as making any impact on those who study biodiversity. He doesn't think that we are off the hook either with regards to conservation issues but wants us to rethink what we count as biodiversity. A voice worth considering.


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