Is the 6th Mass Extinction Here?

By Kristiina Joon

With numerous stories of yet another extinction of some beloved  species making headlines in recent years, one has to wonder about the extent of this ecological crisis we are causing. Could it be considered the sixth mass extinction?

The geological record shows us that there have been five far-reaching global extinction events over the past 500 million years. These events are defined as relatively short intervals of time, up to a few million years, over which at least three quarters of all terrestrial and marine organisms disappear due to environmental stressors, such as global warming or cooling, meteorite impacts, or ocean acidification and deoxygenation. Based on the rock record alone, though, it is difficult to disentangle what exactly the triggers and mechanisms were that resulted in such large-scale losses of biodiversity, as the geological record is imperfect and not all organisms end up being preserved. 

A sixth extinction?
The numbers of lost animals in the human era are alarming. From the conservative estimate of 5 to 9 million animal species which inhabit the planet, we are likely losing between 11 and 58 thousand per year — a figure difficult to imagine.

• Since 1500, we have recorded the extinction of more than 300 land vertebrates (not including any plants, insects, worms). This figure may seem small compared to the rate of loss noted above, but this is because much of the focus of conservation efforts so far has been on specific vertebrates, particularly mammals. When one considers that most invertebrate species have not even been described yet, let alone classified as ‘vulnerable’ or ‘endangered’ by the IUCN (International Union for Conservation of Nature), the disparity makes more sense. 
• One particularly notable recent extinction of a terrestrial vertebrate was that of the northern white rhinoceros: the last male died in 2018. According to the Global Assessment Report on Biodiversity and Ecosystem Services, 75% of all land areas have been significantly altered by human activity. 
• The situation is no less dire in the marine realm, where populations have seen a nearly 50% decline since 1970. Many species are struggling to keep up with the increasing hunting pressures, which change the careful balance of the ecosystem. A good example is oyster catching along the East Coast of the US in the late 19th century, which led to a shift towards a microbially dominated ecosystem where the oysters themselves struggled to thrive. 

Is it our fault?
Humans are putting increasing pressure on various groups of organisms via direct exploitation, hunting, the introduction of invasive species, and habitat destruction. However, in the majority of cases, it can be argued that we are not causing extinction per se. Instead, we are driving a decrease in the number of animals within certain species, especially of those located at the tops of food chains. This causes the links in the ecosystem to weaken, eventually leading to a collapse, accompanied by the extinction of the more vulnerable species involved. The old network of intricate food webs is replaced by a new, typically much more simplistic, system as was the case with oyster catching, where the multitude of estuarine species are now effectively gone due to the over-harvesting of only one, fundamental, organism in that ecosystem. Humans start the chain of changes but do not directly cause all the extinctions. 

​Considering all this information, are we then in a mass extinction? Many scientists would argue ‘yes’  but the answer to this question is slightly more complex. When people think about extinction, they tend to think about the number of species that have been lost. To understand the severity of the current ecological crisis, however, we must also consider the rate at which the species are lost. If species continue to be lost at a similar rate as they have been over the past few centuries, we would be approaching extinction magnitudes similar to the previous five mass extinctions — at least 75% of all organisms lost — in just a few centuries. 
Regardless of the terminology we use, it is inarguable that humans are putting severe pressures on the biosphere, and that we are approaching a point after which recovery to the previous state becomes unlikely. 

References: 
Barnosky et al 2011
Ceballos et al 2017
Dirzo et al 2014
Jackson 2001
IPBES
WWF