• Ashley Riane Booth

What's scarier than zombies, ghosts, and witches? A modern mass extinction

When I was a little kid, the things that scared me were a little silly – the slime monster from Ghostwriter, the possibility of my feet falling off, caterpillars, or a sinkhole developing underneath my bed that would swallow me while I slept. While I’ve gotten over these mostly ridiculous fears, being an adult doesn’t mean I am now fearless. Instead, the things that I consider “scary” have shifted. Now, the things that scare me are all too real – things like climate change and mass extinction.

What is extinction?

Extinction is a relatively new concept, first identified by Georges Cuvier in the late 1700’s. During his research on fossilized animal bones, he realized that no living human had ever seen anything like these animals (Figures 1 & 2). Therefore, those particular species must no longer exist. So how did these massive animals go extinct? Cuvier thought a cataclysmic or violent occurrence must have killed these animals all at once in a mass extinction event.

A mosasaurus skeleton hanging in a museum exhibit.
Figure 1. Mosasaurus skeleton like the one Cuvier identified.

Less than a century later, Charles Darwin and his contemporaries introduced the idea of natural selection as the major process driving evolution or extinction of species over long periods of time. Darwin was convinced that extinctions only occurred gradually due to a species’ inability to compete with other organisms or slow changes to available habitat. This concept of background extinction still holds true today. However, the idea of background extinction doesn’t explain mass extinction or why large numbers of animals go extinct in a short period of time.

Fossilized skeleton of an Irish elk that looks similar to modern-day elk.
Figure 2. Irish elk fossil

There have been five major extinction events in the past 444 million years, and during each of these nearly 75% of all species on Earth disappeared. Unlike background extinction, mass extinctions occur much more quickly (over days to a few million years as opposed to hundreds of millions of years) and involve extinction of a high percentage of species. Several of these events were the result of some major event like a volcanic eruption or an asteroid striking Earth that quickly decimated global populations. Some were much slower and the result of drastic changes in climate over several million years.

Are we in another mass extinction event?

A recent Envirobites article explored some major factors causing high rates of extinction. But how do we know if we are currently experiencing a mass extinction? We compare modern extinction rates to what we know about prehistoric mass extinctions. To do this, Anthony Barnosky and several other scientists analyzed rates of background extinction and percent extinctions for each type of organism (e.g. mammals, birds, crustaceans, coniferous trees, etc.) over the past 500 years. When these values were compared to background and percent extinctions for prehistoric organisms, the results were shocking. The speed at which background extinctions are taking place in our current time (the Anthropocene epoch) have dramatically outpaced past mass extinction events. Similarly, high percentages (10-64%) of species have gone extinct in nearly every class of organism over the past 500 years.

According to Barnosky et al. (2011), if our current extinction rates were considered over the same time-scale as previous mass extinctions (millions of years), we would most certainly be in the midst of another mass extinction. However, we have not yet lost 75% of our recent or modern species, in part because we don’t know every single species that exists in the world today. That doesn’t mean mass extinction isn’t possible in our time. If only the organisms currently classified as “critically endangered” went extinct, it “would propel the world to a state of mass extinction that has previously been seen only five times in 540 million years. Additional losses of species in the ‘endangered’ and ‘vulnerable’ categories could accomplish the sixth mass extinction in just a few centuries” (Barnosky et al. 2011).

Why do we care if there is a mass extinction?

Losing a few species here and there may not seem like a big deal in a global context. But species extinction doesn’t mean losing a single organism – in the mid 1850s, there were nearly140 million passenger pigeons in North America. However, by 1914, there was only one left. This loss of a single species is the collective loss of billions (if not trillions) of birds over the course of history.

When a single species goes extinct, this loss can affect both the environment they inhabited and the other species that live in that environment. For example, birds are important for transporting plant seeds are carried beyond the canopy of their parents, thus helping new plants sprout in areas where they can grow. In the past, birds and trees co-evolved so that larger tree seeds were eaten and spread by larger-bodied birds. Now, as bird populations are declining, the spread (and persistence) of some plant species may also be in danger. Emer et al. (2019) modeled the interaction between bird size, tree-seed size, and rates of bird extinction. They found that when large-bodied birds are removed from a habitat, smaller, less-adapted birds are sometimes able to spread the larger tree seeds. However, this spread of the larger seeds by smaller birds is not as effective and may not be enough to ensure survival of large-seeded tree species. Thus, the loss of a single species may impact the survival of other species and environmental function.

Emer’s study was just a small example of a larger problem. On a larger scale, if mass numbers of species go extinct, our environment will cease to function as we know it. That is a large part of why so many scientists are focused on maintaining biodiversity: we need to preserve as many species and functional ecosystems as we can! Unlike my childhood fears, the things that scare me as an adult may be justified unless scientists and the public can work together to value and conserve biodiversity.

To learn more about extinction and how you can promote biodiversity, check out the Envirobites posts listed below:


Barnosky AD, N Matzke, S Tomiya, GOU Wogan, B Swartz, TB Quental, C Marshall, JL McGuire, EL Lindsey, KC Maguire, B Mersey, and EA Ferrer. 2011. Has the Earth’s sixth mass extinction already arrived? Nature. 471:51-57.

Emer C, M Galetti, MA Pizo, P Jordano, and M Verdu. 2019. Defaunation precipitates the extinction of evolutionarily distinct interactions in the Anthropocene. Science Advances. 5:eaav6699.

Kolbert, Elizabeth. 2014. The Sixth Extinction. Henry Holt and Company. New York, New York.

Featured Image: A fish kill at Redondo Beach. Image courtesy of Dana Roeber Murray.

Originally publised on Envirobites.org.

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