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.
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.
Barnosky et al 2011
Ceballos et al 2017
Dirzo et al 2014
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