Finally! A Global Documentation of Plant Extinction in the Anthropocene

“Most people can name a mammal or bird that has become extinct in recent centuries, but few can name an extinct plant. This study is the first time we have an overview of what plants have already become extinct, where they have disappeared from and how quickly this is happening. We hear a lot about the number of species facing extinction, but these figures are for plants that we’ve already lost, so provide an unprecedented window into plant extinction in modern times.” (Ledford 2019)

ARTICLES:

Humphreys, A. M., R. Govaerts, S.Z. Ficinski, E.N. Lughadha, and M.S. Votontsova. 2019. Global dataset shows geography and life form predict modern plant extinction and rediscovery. Nature Ecology and Evolution.https://doi.org/10.1038/s41559-019-0906-2.

Ledford, H. 2019. World’s largest plant survey reveals alarming extinction rate. Nature 570, 148-149. doi: 10.1038/d41586-019-01810-6

WE KNEW LITTLE ABOUT PLANT EXTINCTIONS

Extinction of species is a natural part of the evolutionary story of biodiversity on planet Earth. The number of species at any point in time is a balance of past production of new species (by mechanisms of speciation) minus extinctions. Biologists call the long-term average of extinction in the absence of humans the “background extinction rate.”

Humans have changed the magnitude of the extinction part of this equation. Put more bluntly, we have drastically increased the number of species going extinct. This has been clearly documented for a wide range of animals, especially mammals and birds (e.g., Pimm et al. 2014). Extensive estimates of present and future extinctions have also been developed, some of which have been featured in Envirobites (Baumann 2019; Barton 2019). Documentation of plant extinctions, however, has been largely missing, resulting in a glaring vacuum. Not only do plants make up a large portion of the total biodiversity of the planet, but these species form the ecological foundation of terrestrial food webs. In other words, when plants go extinct, other associated species are likely to be lost as well.

 

A NEW ANALYSIS OF PLANT EXTINCTIONS

Botanists at the Royal Botanic Gardens at Kew and the University of Stockholm recently published a comprehensive documentation and analysis of plant extinctions over the past 250 years (Humphreys et al. 2019). On the outskirts of London, Kew is one of the foremost botanic gardens in the world, with plants and records dating back to the mid-1800s (Wikipedia 2019).

In total, the investigators documented the extinction of 571 plant species (Figure 1). Of these, 491 were not on the International Union for the Conservation of Nature (IUCN) Red List of Threatened Species, the most comprehensive and trusted global compilation of species in danger. The botanists documented an average of 2.3 plant extinctions per year over the past 250 years. Since some species were not yet named in the earlier years of this time frame, extinctions were also analyzed separately for before vs. after 1900. Extinctions were twice as high for the more recent times.

Figure 1. Documented pant species extinctions since 1900, assessed in new global plant survey (Humphreys et al. 2019).

How do we make sense out of these numbers? First, the plant extinctions are much lower than documented extinction rates for animals. Bird and amphibian extinction rates were about five times as high as plants, and mammals about ten times the magnitude. (It’s important to note that these comparisons take into consideration the total number of species of each of these groups.) Why the difference? The authors suggest that the time lag between endangerment and actual extinction is longer for plants than for animals. In fact, many plant species still extant are considered at imminent risk of extinction.

One of the most important findings of this study is that plant extinctions in the era dominated by humans is 500 times the background, natural rate before human impacts. The increase is striking: over the past 250 years, we would have expected not the 571 documented extinctions, but only about 1 (Figure 2).

Figure 2. Memorial stone for extinct plant species, Erica pyramidalis, Cape Town, South Africa.Photo credit: Abu Shawka (Wikimedia).

GEOGRAPHICAL AND TAXONOMIC PATTERNS OF PLANT EXTINCTIONS

An additional goal of this project was to examine the patterns of plant extinction across space and among groups of plants. Very similar to animals, plant extinctions were most common in parts of the world with very high biodiversity, especially in tropical and Mediterranean climates. As with fauna, island flora have been especially hard hit, with about 50% of the extinctions coming from those sites. The scientists argue that this pattern makes sense given the many endemic plant species (those occurring nowhere else) on islands. These geographic patterns emphasize the importance of assessing habitat changes on islands and other biodiversity “hot spots” around the world.

Woody plants (shrubs, trees, woody vines) were by far the most likely type of plant to go extinct. In fact, although only 40% of plants of the world is woody, 80% of the extinctions came from this group. This might have a biological explanation: herbaceous plants and grasses often have larger populations and more persistent seed banks (dormant seeds in the soil) than do woody plants. Alternatively, this could simply reflect a human focus on trees. Interestingly, there were no patterns across plant families, that is, extinctions were not clustered in certain taxonomic groups, a finding different than that discovered for animals.

 

NOT DEAD YET!: REDISCOVERY OF SPECIES

One encouraging finding was that some species listed as extinct on the IUCN list have been rediscovered (Figure 3). In fact, 431 plant species thought to be extinct have been sighted in recent times. These tended to be species on continents that are well-studied by scientists, whereas past determinations of plant extinctions from islands and other smaller habitats tended to hold true. The authors remarked that, “This not only improves the accuracy of our understanding of extinction but allows for potential remedial work as well.” (Humphreys et al. 2019).

Figure 3. The Chilean crocus (Tecophilaea cyanocrocus), critically endangered plant species, thought to be extinct but rediscovered in 2001. Photo credit: Kew Gardens.

The study of extinction inevitably comes with caveats. Extinctions of poorly known taxa may go unreported resulting in underestimation of rates; conversely, even for better-known taxa, low detectability may result in rate overestimation, revealed only by rediscovery.

 

WHAT THIS STUDY TELLS US

This article reveals that humans are causing plant extinction at rates far higher than the natural, background rate, especially of woody plants and especially on islands and in the tropics. It also apprises us of possible biases that should be closely considered. On the one hand, we are likely  underestimating extinction rates because of a lack of knowledge about many plant species. On the other hand, low detectability of plants may lead to concluding that a species is extinct when it’s not.

An understanding of plant extinction is critical for the conservation of biodiversity and the healthy functioning of ecosystems. In an interview, Dr. Eimear Nic Lughadha, co-author of the article and a conservation scientist at Kew, remarked that

“Plants underpin all life on earth, they provide the oxygen we breathe and the food we eat, as well as making up the backbone of the world’s ecosystems – so plant extinction is bad news for all species. This new understanding of plant extinction will help us predict (and try to prevent) future extinctions of plants, as well as other organisms. Millions of other species depend on plants for their survival, humans included, so knowing which plants we are losing and from where, will feed back into conservation programmes targeting other organisms as well.” (Kew 2019).

 

REFERENCES

Baumann, M. 2019. The global buzz: A call to restore insect biodiversity. Envirobites, March 6, 2019.

Barton, A. 2019. Fear and Hope: One Million Species at Risk of Extinction. Envirobites, May 24, 2019.

Humphreys, A. M., R. Govaerts, S.Z. Ficinski, E.N. Lughadha, and M.S. Votontsova. 2019. Global dataset shows geography and life form predict modern plant extinction and rediscovery. Nature Ecology and Evolution.https://doi.org/10.1038/s41559-019-0906-2.

IUCN. 2019. The IUCN Red List of Threatened Species. Version 3.1.  https://www.iucnredlist.org.

Kew Gardens. 2019. Almost 600 plants have already gone extinct – Why should we care? Royal Botanic Gardens Kew, 10 June 2019.

Ledford, H. 2019. World’s largest plant survey reveals alarming extinction rate. Nature 570, 148-149. doi: 10.1038/d41586-019-01810-6

Pimm, S.L., C.N. Jenkins, Abell, R., T.M. Brooks, J.L. Gittleman, L.N. Joppa, P.H. Raven, C.M. Roberts, and J.O. Sexton. 2014. The biodiversity of species and their rates of extinction, distribution, and protection. Science 344: 1246752. doi: 10.1126/scie

 

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Andrew Barton

Andrew Barton

Raised in the southern Appalachians of western North Carolina, Andrew Barton is a forest and fire ecologist, science writer, and professor of biology. His research focuses on how forests are responding to changing climate and wildfires in the Sky Islands of the American Southwest. He is the author of the award-winning book, The Changing Nature of the Maine Woods, and Ecology and Recovery of Old-growth Forests in Eastern North America from Island Press. Drew co-founded the Michigan National Forest Watch and the UMF Sustainable Campus Coalition, and was a key player in the Mt. Blue-Tumbledown Conservation Alliance, which protected 30,000 acres of forestland in western Maine. He teaches courses on ecology, conservation, plants, and forests, as well as a travel course on the ecology of Costa Rica. Ph.D. University of Michigan, M.S. University of Florida, B.A. Brown University

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