Getting to Grips With the Sixth Mass Extinction

Featured Image: It is well-understood that the Earth’s biodiversity is in severe decline. However, it is less clear if this decline can now be called a mass extinction. Public domain image via. The Wilderness Society.

Paper: The Sixth Mass Extinction: fact, fiction, or speculation?

Authors: Robert H Cowie, Philippe Bouchet & Benoît Fontaine

Human-driven emissions and land use changes have impacted Earth’s biosphere greatly, causing global extinction rates to climb fast. However, does the current undeniable biodiversity crisis meet the requirements to be called a mass extinction? 

Mass extinctions are defined by a threshold in species loss (usually >75%) over a short geological timespan. This definition works well for geological examples but breaks down when evaluating the current crisis. By definition, the current extinction event can’t be confirmed as a mass extinction until after it has already happened.

A more useful definition is to use the rates of species loss. When global extinctions dip beneath the rate at which evolution results in new species, or when it vastly outweighs the natural background extinction rate, mass extinction is likely to occur. This applies to the current extinction crisis. Cowie and co-workers explore the uncertainties that come with such an enormous task as cataloguing species losses and calculating the exact scale of the current extinction crisis. Improving our ability to track species losses more accurately is essential to reducing extinctions moving forward. 

The most widely cited reference guide on the status of a species is the International Union for Conservation of Nature (IUCN), also known as the Red List. This list is an extraordinary effort to compile the extinction risk levels of species from around the world, yet it is far from perfect. Cowie and co-workers summarise five major problems:

  1. The extinction of marine species is less understood compared to amphibians, mammals, and birds.
  2. Since the IUCN is primarily a conservation group, they are hesitant to declare a species extinct. Doing so would reduce funding and efforts to protect a species.
  3. The extinctions listed by the IUCN only include species that have become extinct since the year 1500. This makes it hard to understand the influence of the human species before industrialization, even though records do exist.
  4. Some species go extinct before we ever discover them.
  5. Invertebrates are underrepresented so we lack an understanding of their extinction rates.

Summary of Red List Species accounted for in terms of raw number and percentage of those evaluated for the Red List compared to the total estimate.

Cowie and co-workers’ estimation for global extinctions is based on surveys of invertebrates, like terrestrial mollusks and insects, as they make up 95% of all species on Earth. Their results show that 7.5-13% of all species since around 1500 have gone extinct. That is 150,000-260,000 of the roughly 2 million species known. This gives a rate of extinction of 150-260 extinctions per 10,000 species per 100 years (or one million “species years”). An enormous value that far exceeds background rates of extinction based on natural causes alone.

The researchers conclude that the sixth mass extinction is highly likely to be happening today. Moreover, they outline where future work must focus to better make use of the finite conservation resources we have. Understanding the patterns and characteristics of extinctions is vital to aid efforts that address the current biodiversity crisis. If we improve our understanding, especially focusing on weak spots identified in this paper, we can improve how we use resources and help the most vulnerable species recover.


Getting to Grips With the Sixth Mass Extinction by Jordan Healey is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

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