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The Sixth Mass Extinction. Poor Documentation Blamed for Its “Sudden” Appearance
Cienna Lyon
Cienna Lyon
is a student at Ithaca College in New York studying Biology and German and a volunteer science writer for the Paleontological Research Institute.

It is well known that many species of large mammals and birds have been rapidly depleting since humans began expanding their cities and populations. Many of these human-influenced factors are indirect (without intent to cause species declines), such as pollution and the introduction of non-native species. Others are more direct, such as hunting and habitat destruction. Although familiar species such as the rhino, tiger, and many rainforest parrots are critically endangered or extinct, they are not the main source of extinction happening today. In fact, these large, charismatic species account for less than 2% of the total 7% of non-marine species thought to have gone extinct since the Industrial Revolution (late 1700s through the 1800s). The majority of these extinctions are invertebrates, most of which are land snails and other mollusks.

Although we don’t often think about snails, they play an important role in ecosystem processes. Many pond and forest snails are vital to the decomposition of plants, such as leaves and fallen sticks. Those same snails serve as the base of their ecosystem’s food web. With research revealing that many species of snails and other invertebrates are rapidly going extinct, one group of scientists became concerned that an animal with such a vital role was declining, and they wanted to find out why.

This group of scientists, led by Claire Régnier at the National Museum of Natural History in Paris, conducted a study of 200 species of non-marine snails. The results have been published in the June 2015 issue of Proceedings of the National Academy of Scientists. Their goals were to find out why so many species were dying off so quickly, and, more importantly, to show that the official endangered/extinct species list does not begin to account for the massive losses suffered by invertebrates.

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The endangered species lists are controlled and “ranked” by the International Union for the Conservation of Nature (IUCN). The IUCN is responsible for determining the risk of every recorded species for probability of extinction, and placing them on what is known as the “Red List.” After undergoing intense study by scientists and the IUCN board, species are ranked as extant, not threatened, threatened, endangered, critically endangered, or extinct. Another important category is “unable to assess.” When the IUCN finds little to no data on a species believed to be declining, it can be placed on the Red List as such. Species are also placed in this category when their habitat is either destroyed or inaccessible.

The Paris Museum team began by looking at IUCN records, then supplementing these records with museum specimens, shell collections, and actual field work.Of the 200 snails investigated, only nine were previously listed at all on the Red List, all as “endangered.” Most of the other 191 species had either never been recorded again after their first discovery or had not been recorded in over 50 years. Of these, several were endemic species (those that live only in one particular place), making accessing the species for collection quite difficult.


Current IUCN Red List ranks of the 200 snail species studied, with their distribution worldwide. The three columns are (left to right) the present study (“model”), experts (published literature), and the IUCN. Most of the species are listed as “unable to assess” (gray or purple) due to deficient data. Image by Regniera et al. (2015).

Upon final analysis, it was obvious that there were serious differences from the IUCN data. Forty-five percent of the 200 snails selected for this analysis were not threatened at all, and another 40% were unable to be assessed based on a complete lack of data (both in physical collections and in records of field study). The remaining snails were either endangered or extinct. Twenty species were found to be extinct. This is a shocking increase over the Red List’s 9 endangered snail species.


Data gathered for 200 snail species by vigorous study of museum and private collections. The graph shows that in recent years, most of the snails have stopped being recorded in field surveys. Many have only had a few field surveys since their discovery. This is either due to extinction or to the lack of targeted field study for that species. Image by Regniera et al. (2015).

The most important result from this analysis is that there has been a rapid decline of many species and taxa over the past 300 years that has gone virtually undocumented. Snails do play an important role in their ecosystems, but it is more important to note that insufficient data can cause massive underestimates about what is truly going on in our world today. The IUCN has developed an amazingly meticulous method to assess large mammals, but lacks similar efforts for smaller species, such as invertebrates. Although mammals, birds, and reptiles are interesting, attractive, and easier to locate, conservation efforts need to be more evenly distributed across all forms of life if we wish to prevent the sixth mass extinction from progressing further.

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One Comment

  1. Enceladus says:

    The graph descriptions do not match. Your two graphs are reversed. The second one belongs on the top.