Abstract
Species go extinct each day, most without notice. The current human-induced extinction rate is up to 700 times higher than the background rate. Extinctions are not different for plants, animals, or fungi, although botanical and invertebrate extinctions are much more poorly documented than those of charismatic vertebrates. In a recent book on extinct plants (Christenhusz & Govaerts, 2023), an overview of botanical extinctions since 1753 was presented, listing which species became extinct and the probable reason for their extinction. As most have a date when they were last documented, a timeline of extinction can also be compiled based on these data. This timeline shows an increase from 1890 to 1940, but a decline in new recorded extinctions after the 1980s, which is likely a result of taxonomic impediment. Extinction rates before 1800 are impacted by the lack of data (here named Berkeley extinction). It can be concluded that extinction is highest in biodiversity-rich areas with high human influence (extinction hotspots). Two new combinations and a new name are proposed here, showing the importance of taxonomy to conservation. Although anthropogenic plant extinction is a global phenomenon, areas of particular concern are the Hawaiian Islands, southern Africa, Australia, the Indian Subcontinent, Southeast Asia, and Brazil. Extinctions have been mainly caused by land clearing for agriculture and urbanization, invasive feral animals, mining, river dams, diseases, and poaching. We predict that the unusual weather patterns associated with rapid climate change may result in more plant extinctions. Reintroduction, even if a species persists in cultivation, is not always possible due to lack of suitable remaining habitat where threats are decreased or removed. Successful reintroduction cannot be guaranteed. It is costly and usually dependent on short-term funding, after which these efforts may be in vain. Protection of species in their natural habitat is much more cost-effective in the long term. Sometimes, rescued plants should be introduced in similar habitats outside their natural range where the threats are absent. This follows the programmes of assisted migration for climate change mitigation, but this can also be assisted introduction to prevent extinction. Protection of critically endangered species that have naturalized outside their native range should also be considered.
Introduction
‘We destroy plants at our peril. Neither we nor any other animal can survive without them. The time has now come for us to cherish our green inheritance, not to pillage it – for without it, we will surely perish.’ David Attenborough in The Private Life of Plants, 1995.
The current geological epoch is characterized by significant human impact on geology and ecosystems and has therefore been called the Anthropocene (Vernadsky 1986, Crutzen 2002). Unfortunately, this term is sufficiently vague that it can be interpreted differently by people with different world views. Some see it as a ‘shame on us’ epoch, whereas others might see it as the epitome of our complete subjugation of nature and ultimate success of our species. Either way it acknowledges the immense impact that humans have made on the environment of Earth.
By roughly 2037, the human population is expected to pass 9 billion individuals (https://www.worldometers.info/world-population/) who will all be seeking water, food, clothing, housing, and many other resources on a planet where we are already irreversibly changing the climate and disrupting the ecology on an unprecedented scale (see e.g. Marsh 1864, Turner II et al. 1990, Etheridge et al. 1996, Steffen et al. 2006). The profound effects we exert on the natural world can benefit society, but sadly more frequently they are done for quick financial gain, resulting in widespread overexploitation, destruction, and pollution. No place on Earth has escaped the influence of humans. From the depth of the oceans to the highest mountains, there is no true wilderness left on our planet.
Biodiversity crisis
If we do not know what we have, it is impossible to quantify how much we are losing. Numbers of species vary from 2–8 million (Costello et al. 2013) to 1 trillion (Locey and Lennon 2016), and it is estimated that most species, especially microbial and fungal species that may be key to healthy ecosystems, remain undiscovered. The biodiversity crisis is thus notoriously difficult to quantify. This ‘taxonomic gap’ (Dubois 2010, Raposo et al. 2020, Engel et al. 2021) needs to be addressed urgently. However, taxonomy is in decline (Baldini et al. 2021 and references therein). Misconceptions about taxonomy and its function as a science have resulted in a dismissive attitude in the scientific community and funding bodies. Many areas in the world remain poorly studied biologically, and we therefore need more taxonomists to better understand global biodiversity.
Extinction is natural, but...
Of the c. 4 billion life forms that we think have existed on earth, roughly 3.96 billion are now extinct. In most cases, we do not know why these species disappeared, but many died out in massive global extinctions. Others perhaps slowly disappeared because they could not compete in a changing environment, or they may have evolved into other species. However, we are certain that in the distant past extinctions were not the result of overexploitation by a single species, as is happening today. The last mass extinction, some 66 Mya, wiped out about three-quarters of all plants and animals (Jablonski and Chaloner 1994). This mass extinction was likely caused by a major meteoroid impact and the resulting climatic changes. Currently we are in a similar, if not greater mass extinction event. Each year, many hundreds, if not thousands of species are estimated to become extinct, but in these cases humanity is the culprit. Undoubtedly, many modern extinctions were completely preventable. Deforestation alone has been estimated to have caused 20% of modern species extinction (Holder 2022), and it is estimated that current extinction rates are 1000 times higher than the background extinction rate (De Vos et al. 2015).
Nature is intimately interconnected, and humans are part of this web of life. In light of the many mass extinctions of the past, wiping out all but a few resilient species, it does not seem reasonable to suppose that our species should be exempt from this wave of extinction. If so, Homo sapiens L. will have one of the briefest lifespans of species, a mere blink in evolutionary time, but a blink with devastating impact on all life, marine and terrestrial.
There are now just under 1000 plant species known to have become extinct in modern times, i.e. since the 18th century when systematic record keeping of plants started (Christenhusz and Govaerts 2023; see also the full updated list in Supporting information, Appendix S1). This number is high, but it only reflects documented extinctions. There will have been many more. We estimate that there are currently around 374 000 known plant species (Christenhusz and Byng 2016), making the extinction rate a mere 0.15%. Compare this with mammals, for which there are c. 60 recently extinct mammals out of c. 6500 species, resulting in almost 1% (or up to 700 times the background extinction rate; Loehle and Eschenbach 2012). However, one should bear in mind that mammals are much better studied than plants and are also much better at leaving evidence, like teeth and bones, after they have perished. There are many more plants that were never recorded by science and left no trace [here named Berkeley extinction, after the philosopher George Berkeley (1685–1753) who posed the question: ‘If a tree falls in the forest and there is no one around to hear it, does it make a sound?’; Campbell 2014]. These species thus cannot be listed as extinct because we never knew they existed. Most botanical extinction is silent, and few people take notice.
There are also many species that have only been found once, and often no botanist has revisited these sites to see if these species are still there, leaving them as data deficient or presumed extinct. Of course, new plant species are still being discovered, approximately 2000 or so each year (Christenhusz and Byng 2016), but many of these are found among historical collections in herbaria that will have been collected decades ago and may no longer be extant (Bebber et al. 2010). An additional problem is that it has become increasingly difficult to do field work (and to obtain funding and the required permits), which results in lack of documentation of where species occur; this is an issue related to the poor understanding by governments, funding bodies, and the public about the fundamental nature and usefulness of biodiversity research. Paperwork and permits for scientific research have become more cumbersome or impossible to obtain, and when easy-to-reach areas have all been converted, areas of nature worthy of botanical exploration are remote, or are under control of companies or people that would not benefit financially or operationally from a biological survey. We can thus conclude that the loss of species is probably much greater than current data show.
Many scientists have also been reluctant to declare species as extinct because this may remove legal protection for these species and their native habitat, a ‘Romeo error’ (Watson 1974, Diamond 1987). Funding for protection or conservation may also dry up, resulting in a lack of resources to continue monitoring and protecting the area. The IUCN category ‘extinct (EX)’ requires that ‘there is no reasonable doubt that the last individual has died’ (IUCN 2001), something that is nearly impossible to verify. Many species have been rediscovered after having been listed as extinct (the so-called ‘Lazarus species’; Jablonski 1986), and this makes researchers reluctant to list them as extinct.
Over 40% of plant species are at risk of extinction due to continued destruction of our natural world (Humphreys et al. 2019, Nic Lughada et al. 2020, Bachman et al. 2024). Most extinctions happen in areas of high biodiversity where simultaneously there is great human pressure, the so-called biodiversity hotspots (Reid 1998, Norman 2003, Barthlott et al. 2005), which are perhaps better renamed ‘extinction hotspots’ as these are correlated (Brooks et al. 2002). Humans will need to create safe spaces for nature in these extinction hotspots and make sure that these do not become an anthropogenic desert.
When is a species extinct?
Not all circumstances of plant extinctions are the same. There are four main circumstances, which we highlight here.
Forever lost
There is always a chance, however remote, that an ‘extinct’ species survives on an inaccessible cliff or lingers deep in an unexplored forest, so it is always difficult to be certain about extinction. Of course, in several cases we can be almost certain that a species is lost forever. If a species occurred in a restricted area now converted to such an extent that there is none of that habitat left and the species has not been seen for many decades, we can be sure that it is forever lost. If the only known place where a species occurred is now in the middle of a city or was submerged by a dammed river, it is also likely that the species is now gone. In cases of once more widespread species, we can also sometimes be sure of extinction, for instance if the habitat no longer exists, maybe because humans drained a marsh for agriculture or the area became an open-site mine. However, in many cases, people keep looking for the plants, and sometimes these become ‘Lazarus species’. Regrettably, this does not happen often. Because of this remote chance of rediscovery, scientists are generally not keen to list a species as extinct on the IUCN Red List, simply because this may remove legal protection from a species or its habitat, which may be devastating for the last remaining plants in case these are still present (Butchart et al. 2006). An example of a plant species lost forever is the Hawaiian Argyroxiphium virescens Hillebr. (Asteraceae), which was last seen in 1945 and was lost due to invasive animal and plant species.
Only found once
Some species have only been found a single time and were described from a single historical collection. It is possible that no one has looked carefully in the place where these species were first found. There are still many poorly explored areas, and many areas are now difficult to reach due to lack of roads, private land, or difficult terrain. As an example, when collecting ferns on Cerro Guayrapurima in the Peruvian Andes in 2002, the first author (MJMC) found a population of Cyathea rufescens (Mett.) Domin (Cyatheaceae; see Lehnert 2011), which was previously only known from a Spruce collection made on the same mountain in 1856 (K000227597). It was not extinct, but the site simply had not been investigated botanically for 146 years.
In other cases, the original collecting locality was not accurately recorded and thus searches cannot be well focused. Even more problematic are species for which the original material was lost, making it difficult to interpret the name.
Also, some new species are already extinct before they can be described. An example of such an extinct species is Vepris bali Cheek (Rutaceae; see Cheek et al. 2018). Specimens from a remnant cloud forest in the Bali Ngemba Forest Reserve in Cameroon had been unnoticed for 67 years in the Kew herbarium. By the time the new species was recognised, the forest where it once grew had been totally converted to agriculture.
Lost and found
The current list of extinct (and extinct in the wild) species includes 962 taxa (as of December 2023; see Supporting information, Appendix S1). Of course, by the time this paper is published, the number may be slightly different. The reader may notice that the table is also slightly updated from the table published by Christenhusz and Govaerts (2023), which was completed in March 2023. In addition, some species now known to be critically endangered may have become extinct, but others may be removed from the list because of recent rediscovery. Rediscovery is not unusual, because publishing these lists highlights these potentially extinct species, which encourages botanists to investigate.
If we still can find new giant palms like Tahina spectabilis J.Dransf. & Rakotoarin. (Arecaceae) on a farm in Madagascar (Dransfield et al. 2008), it is certainly possible that a small herb that was thought to be extinct is still hiding somewhere. Even for small oceanic island endemics with highly specific habitat preferences, rediscovery is possible. A good example is the presumed extinct Ascension Island parsley fern, Gastoniella ascensionis (Hook.) Li Bing Zhang & Liang Zhang (Pteridaceae), which after specific investigation turned out to be still present at one site, hanging precariously off a cliff. It has now been brought into cultivation, and a population has been reintroduced (Clubbe 2013, Baker et al. 2014).
Another plant recently discovered is the emblematic Nymphaea thermarum Eb.Fisch. (Nymphaeaceae). Thought to have disappeared from the original site in Rwanda in 2008, it was rediscovered in a nearby site (Thomas 2023), so the species is no longer listed as extinct, albeit still critically endangered. Similarly, Moraea minima Goldblatt (Iridaceae) was found again on a road in the Western Cape (Truscott 2023), after it was last found in 1981. Small geophytes are often easily overlooked when not in flower, and their geographical range is often limited. Of course Moraea minima is still threatened by grazing and habitat destruction.
Some species are ephemeral and appear only briefly when the conditions are correct, and it is possible that some may not have been seen for decades. Their seeds are nonetheless lying dormant in the soil, waiting for the right conditions to germinate and reproduce. A good example of such a plant is Emblingia calceoliflora F.Muell. (Emblingiaceae), a species occasionally encountered in Western Australia in years after fires followed by sufficient rain. It should be noted that such species are vulnerable to climate change, and population trends are difficult to monitor.
Extinct in the wild
Gardens and zoos hold many unusual plants and animals. Some were brought into cultivation long ago and have since become extinct in the wild. Some are the ‘living dead’, i.e. species still with us but drifting towards extinction in no longer viable populations. Horticulture can play an important role in maintaining botanical diversity. If some people did not have the foresight to gather a few seeds, grow them in the garden and share them with their friends and other gardens, species like the Franklin tree (Franklinia alatamaha Marshall, Theaceae) would no longer be with us. Conservation programmes to grow species like the Wollemi pine (Wollemia nobilis W.G.Jones, K.D.Hill & J.M.Allen, Araucariaceae) and dawn redwood (Metasequoia glyptostroboides Hu & W.C.Cheng, Cupressaceae) have resulted in these species now being commonly sold, in some cases helping to fund preservation of the few remaining wild populations (Li et al. 2005, Ashmore et al. 2011).
Is horticulture a solution to extinction?
Cultivation of endangered species removes or reduces collection pressure on wild plants, but these programmes must be done in an organized and controlled manner if they are to be effective. The craze for orchids, cacti, cycads, and carnivorous plants often subjects them to significant threats from poaching. Therefore, only plants grown in horticulture should be traded, and these should be certified as sustainably produced through garden propagation, as has been done for Wollemia, some pitcher plants, and orchids.
If plants that have been reintroduced as part of a restoration programme are subsequently eaten by goats or picked by a plant poacher, reintroduction is doomed from the start. Such reintroduced populations therefore need to be protected from the environmental stresses they face in anthropogenic habitats. We often hear about invasive species escaping from gardens, which is seen negatively, but perhaps ‘assisted naturalization’ can be employed to help critically endangered and extinct in the wild species grow where the pressures that caused decline are absent or reduced. Horticulture thus may give some species a second chance, if not in the wild then at least in gardens and perhaps become established in a completely different part of the world.
A recent study (Albani Rocchetti et al. 2022) investigated which extinct plants could potentially be revived from herbarium specimens. They called these taxa ‘extinct in the wild’, but when taxa are only known from old seeds in natural history collections, they are not necessarily viable. There has been success with germination of old seeds (e.g. Leino and Edqvist 2010, Gros-Balthazard et al. 2021), but these cases are usually for taxa that have long-lived seeds in natural soil seed banks—e. g., desert or weed species—or they are preserved in an exceptional way, e. g., permafrost or in stable temperatures and humidity such as caves. Many historical herbarium specimens have not been kept in optimal conditions for seed storage or have been treated with chemicals like mercury for pest control. These treatments are often not suitable for embryos to survive over long periods. Of course, through embryo rescue techniques, some species may still be revived. This is incredibly promising, but it is important not to lose funding and focus on protecting critically endangered species in their natural habitats. Techniques to revive species are expensive, time consuming, and often create a garden plant with only a few genotypes for which no native habitat remains.
A list of extinct species
Only 129 plant species are currently listed as EX (extinct) and 45 plant species as EW (extinct in the wild) by IUCN (https://www.iucnredlist.org on 11 December 2023), which is the basis of the list of extinct plant species presented here (Supporting information, Appendix S1). Names that are now known to be synonyms of more widespread species have been removed. For instance, Agave lurida Aiton is a synonym of Agave veracruz Mill.; Bourreria veracruziana G.Campos & F.Chiang is included in Bourreria mollis Standl.; Corynanthe brachythyrsus (K.Schum.) W.Brandt is Corynanthe macroceras K.Schum.; Delilia inelegans (Hook.f.) Kuntze is a synonym of Delilia repens (Hook.f.) Kuntze; Erythroxylum echinodendron Ekman ex Urb. is Erythroxylum minutifolium Griseb.; Ilex ternatiflora (C.Wright) R.A.Howard is Elaeodendron xylocarpum (Vent.) DC.; Stachytarpheta fallax A.E.Gonç. is Stachytarpheta cayennensis (Rich.) Vahl, and Viola cryana Royer ex Gillot is part of Viola hispida Lam. (POWO 2023). We then added the names listed as extinct in Plants of the World Online (POWO 2023), which we also used for the taxonomic framework and distribution data.
Due to several detailed studies, some groups of plants have higher numbers of extant species. For instance, the Andean species of Viola L. (subg. Neoandinum Marcussen) received monographic attention, and many species were listed that had not been found since their original description and are likely extinct (Watson et al. 2023). Psychotria L. in the Philippines also received particular attention (Koopowitz et al. 1998, Sohmer and Davis 2007), elevating the number of extinct Philippine species in this genus. National red lists also contributed substantial numbers for several countries (e.g. Sri Lanka: Wijesundara et al. 2020, South Africa: SANBI 2020, Australia: Department of the Environment 2023), potentially skewing the number of extinct taxa listed for these countries. However, it is more likely that the numbers of extinctions in these countries are a more accurate reflection rather than an anomaly. The flora of most countries has not received the same level of attention. A few publications focussing on extinct plants also added some numbers, for instance for the continental USA and Canada (Knapp et al. 2021). The resulting list mostly follows that published by Humphreys et al. (2019) and Christenhusz and Govaerts (2023), with a few additions and deletions of recently rediscovered or recently extinct taxa. Names that are taxonomically unresolved or only known from illustrations were also removed because no reasons for extinction can be given for these taxa. As extinct in the wild (EW), we only classify taxa that are known to be in cultivation, but not seeds stored in herbaria or seedbanks (because there is no evidence that these are viable; but see Albani Rocchetti et al. 2022). The resulting list counts 962 taxa and is presented in the Supporting information, Appendix S1.
When did species go extinct?
In Supporting information, Appendix S1, the year when species were last recorded in the wild is also given. These dates (see Fig. 1) show that from the mid-19th century the number of recorded extinctions increased. This is partly due to the larger number of discoveries at this time combined with increased exploration, exploitation, and rapid land conversion for plantations and infrastructure development. The true extent of extinction at this time can never be assessed because we estimate that only a fraction of biota was recorded at this time, but the data do show that there was a substantial decline in biodiversity. This decline continued, reaching a peak after the turn of the century, particularly during the 1920s and 1930s when deforestation accelerated and urban expansion increased.
Known plant extinctions per decade. This figure is based on the year when a species was last observed in the wild. Of course, sometimes species may have persisted beyond this date, but we only present the actual recorded dates here.
Even though the number of plant species becoming extinct appears to decline after this period, the extinction rate is no less dramatic. In the post-WWII years, about 40 plant species were documented to disappear each decade, but these extinctions were generally much better studied and documented. Meanwhile funding for field work and taxonomic study has been steadily declining (Engel et al. 2021), and this correlates with the decline of documented extinctions over time and may be the cause of this decline in recorded species extinctions in more recent decades. Most extinctions still happen unnoticed.
In addition to the year of extinction, causes of extinction are also noted, if this is known (from national red lists or species accounts). About 18% of taxa became extinct without the cause being documented. Often, species became extinct due to a combination of factors, making it sometimes difficult to pin down the final cause. Habitat destruction is usually the main cause of extinction.
Discussion
Extinct genera
Even though each extinct species is an irreplaceable loss, the significance of extinction is not always equal. When an apomictic species of hawkweed (Hieracium L., Asteraceae) goes extinct, for instance, there are still many closely related species that could replace this taxon, even though it remains a genetic loss. If a variety of a species goes extinct, but another variety remains, then this is a loss of diversity for a species, but the species itself still remains extant. However, this may make a species more vulnerable to future extinction because it has a reduced genetic and geographic basis. It is much worse if an entire species goes extinct, especially if the genus consists of only a few species.
It is even worse when an extinct species is the only representative of its lineage. If, for instance, Ginkgo biloba L. becomes extinct, then not only the species dies out, but also the order Ginkgoales. This species is unlikely to disappear completely, even though it is included on our list as extinct in the wild (EW). No truly wild populations are known although some people claim that natural stands remain, but these may be naturalized from cultivation. Ginkgo biloba is widely cultivated in most temperate climates of the world and readily grows from seed. There are several genera that have disappeared completely, and a focus on rediscovery or regeneration from dormant seeds is certainly warranted. The extinct genera are: Corsiopsis D.X.Zhang, R.M.K.Saunders & C.M.Hu (Corsiaceae; southeastern China), Euchorium Ekman & Radlk. (Sapindaceae; Cuba), Gyrogyne W.T.Wang (Gesneriaceae; southeastern China), Hesperelaea A.Gray (Oleaceae; Guadalupe Island, Mexico), Neomacounia Ireland (Neckeraceae; Ontario, Canada), Nesiota Hook.f. (Rhamnaceae; St Helena, UK), Nicobariodendron Vasudeva Rao & Chakrab. (Celastraceae; Nicobar Islands, India), Pachalococos J.Dransf. (Arecaceae; Easter Island, Chile), Streblorrhiza Endl. (Fabaceae; Philip Island, Australia), and Trilepidea Tiegh. (Loranthaceae; New Zealand). Franklinia is still in cultivation but cannot be returned to the wild (Enright 2022). Nicobariodendron may still be extant but has not been seen for over two decades. Corsiopsis is a mycoheterotroph and therefore easily overlooked. Hopefully, these last two will be rediscovered.
The saddest story is perhaps that of Nesiota elliptica Hook.f. (Fig. 2; Cronk 2016), the sole species of its genus. On St Helena, a tree once grew that somewhat resembled an olive tree, with dark-green, leathery leaves with a velvety lower side and small maroon flowers. It was never common and was under much pressure from logging for its useful timber. It was long thought to be extinct, but a single large tree was discovered in 1977, and a few fertile seeds grew into seedlings that were planted out in the 1980s but failed to survive. Cuttings were difficult to root, and the last wild tree died in October 1994 from multiple, systemic fungal infections. Tissue was taken to the micropropagation unit at the Royal Botanic Gardens, Kew (Project Popeye; Jackson 1991), but the tissue was so infested with fungi that a sterile culture could not be established. The last surviving cutting was planted out in 1988, where it survived for many years and grew to several metres tall, but eventually it too succumbed to fungal die-back disease in 2002. This is a text-book example of a recent extinction that was caused by human alterations of a poorly understood ecosystem and where last-minute conservation efforts unfortunately failed to secure the species (and genus) for future generations.
Nesiota elliptica. Illustration on page 337 in Melliss (1875).
Philip Island, a tiny Pacific island close to Norfolk Island in the southwest Pacific, was once the home to four endemic plant species, but during colonial times pigs, goats, and rabbits were released and devastated the vegetation of the island. The 5000 feral pigs gave it its nickname, ‘Pig Island’. Among the species that disappeared, there was a spectacular plant, the Philip Island glory pea, Streblorrhiza speciosa Endl. (Fig. 3), which became extinct in the wild by the 1830s. This attractive liana with glossy leaves and an abundance of pink, beaked flowers was by that time widely cultivated in European conservatories, and flowered well when planted out. When planted in a pot, the plants refused to flower, and slowly the plant fell into disfavour. Subsequently, these plants were discarded by many gardens, and eventually it was lost from cultivation altogether. Seeds from an herbarium specimen were sown in the 1980s, but although they imbibed, they did not germinate. Perhaps modern embryo-rescue techniques might have helped to overcome this, but there now are only a few old seeds left, so mistakes cannot be made. Appeals were made to gardeners (in 1996 and 2007) to look for it among their collections, perhaps mislabelled as a Clianthus Lindl. or another vine. We here would like to echo these calls and ask gardeners caring for historical horticultural collections to look for it. If rediscovered, it will be the rediscovery of not just a species, but also a genus.
Streblorrhiza speciosa. Illustration by Miss Drake in Lindley (1841).
A species missing from many extinct plant lists is the Easter Island (Rapa Nui) palm, Paschalococos disperta J.Dransf. Early reports and pollen records have shown that this isolated Pacific island was once home to a species of palm (Zizka 1991). Exotic nuts were offered to the first European visitors, and traditional huts were covered in palm-leaf thatch. A palm-like glyph looking like a palm with a swollen trunk occurs in the Rongorongo script, and it was said that local women wore headdresses and skirts made of palm leaves. However, by the time Captain Cook visited the island in 1774, no palms were present. It was speculated that the trunks had been used to move the moai statues from the quarry to their platforms and that when the last tree was cut and famine struck, a civil war broke out, resulting in the toppling of the moai. Palms featured heavily in the film Rapa Nui that romanticized this demise. This could have been one of the reasons, but it is equally likely that palm hearts and seeds were eaten by humans, and that the remaining seeds will have been consumed by invasive Pacific rats brought by the first Polynesian settlers.
In 1983, subfossil endocarps of this palm were found on the floor of a cave. The nuts looked like small coconuts and seemed related to the Chilean wine palm, Jubaea chilensis (Molina) Baill. Bite marks on the seeds are evidence that the seeds were consumed by rats, and grazing by feral animals would likely have killed off the last seedlings. We do not know what this palm looked like exactly, because it is only known from its seeds and casts of root bosses. The Rongorongo symbol gives an indication that it may have looked similar to Jubaea.
Extinction hotspots
Regions of the highest extinction rates in the world correlate strongly with the well-known biodiversity hotspots (Barthlott et al. 2005, Le Roux et al. 2019). Places with the highest diversity of course have also the most to lose, but if there are no pressures on these habitats, biodiversity is high and not threatened. However, areas of rich botanical diversity are also suitable for agriculture, forestry, and mining, and thus most areas of high biodiversity equate to extinction hotspots (Le Roux et al. 2019).
Many recent extinctions have occurred on oceanic islands, which are well-defined geographical entities and can often be much more easily and thoroughly studied and surveyed. Although these may not necessarily harbour the greatest species diversity, many species are found only on such isolated oceanic islands. Because they evolved in habitats with no or limited grazing, unlike their continental relatives, they were not adapted to cope with herbivory. When humans invaded these uninhabited island habitats, releasing exotic animals and plants, many species perished, including numerous species that had not been documented (Berkeley extinction).
On continents, species often have had means of escape to other areas or were pre-adapted to grazing or disturbance to some extent. For instance, relatively few botanical extinctions have been documented in Europe where the flora is well known. Also, the agricultural revolution happened long before botanists systematically studied native floras and therefore extinctions before agriculture reached Europe were rarely documented. The varied landscape of Africa provided high levels of diversity, but because humans also evolved here, much of nature was able to adapt to a low level of human disturbance. An exception is the Cape region in South Africa, where the stable mild climate and poor soils resulted in a high botanical diversity. Unfortunately, when artificial fertilizers were introduced, this area also became suitable for agriculture, to the devastation of the local flora. Similarly, the high diversity region of southwestern Western Australia was converted to the ‘wheat belt’, resulting in the spectacular wildflowers being relegated to road verges and the occasional reserve, where invasive plants and unseasonal fire regimes now threaten them. In many regions of Asia, the human population has been large, and documentation of many presumed extinct species is imprecise. There has also been a cultural reluctance in many Asian countries to report extinctions.
The Hawaiian Islands may be thought of as a tropical paradise, but it is a centre of plant extinction with more extinct species per unit area than anywhere else in the world. Many Hawaiian species were already rare when first discovered, and in some cases just a handful of individuals were ever seen. Combined with the large number of invasive species and deforestation for agriculture, forestry, and infrastructure, these species were extremely vulnerable to extinction. However, the steep, often inaccessible volcanic slopes have made it possible for some species to cling on, and some remarkable rediscoveries have recently occurred, particularly with the use of drones (Rønsted and Wood 2020, Nyberg et al. 2023), although these drones have equally helped to confirm presumed extinctions.
In the Americas, extinctions were usually caused by changes in land use. The vast broad-leaved forests of eastern North America were cut by the early settlers for agriculture and timber, and extensive prairies and river cane thickets became grazing land for cattle or were drained and converted to fields of corn and other mass-produced crops. California, another area of high extinction rates/botanical diversity, partially became the fruit and vegetable garden of the United States and has high population growth, especially in the highly diverse coastal regions of southern California. The swamps of Florida and other states on the Gulf of Mexico were drained and converted to citrus and cotton plantations with orange juice being one of the first commodities traded on the New York Stock Exchange. A somewhat unexpected reason for extinction in eastern North America was suppression of regular fires. Although it is well known that the western states experience regular fires, sometimes with disastrous results for livelihoods, it is less known that this used to be also frequent elsewhere in North America. Many species adapted to regular burning of their habitat are now extinct or severely reduced because fires have disappeared or are suppressed in areas that became densely populated. Maybe the best-known example of habitat destruction is the felling of the Amazon rainforest, which continues today at an alarming rate. Even though this has not resulted in many documented extinctions as yet, its disappearance will have profound effects on ecological equilibria. These ‘lungs of the earth’ are one of the main tipping points in climate change (Lenton et al. 2019).
Table 1 shows the countries and territories with the greatest number of recorded extinct plants and extinction as a percentage of the total number of native plant species because larger countries have higher numbers of species, and some countries, like South Africa, lie in areas with exceptional biodiversity. Some large countries unexpectedly do not feature in the list, which is probably due to poor record keeping, outdated information, or lack of reporting for political reasons. This results in other, well-documented countries and islands being placed higher on the list, whereas in some nations species go ‘silently’ extinct.
The 20 countries with the highest numbers of botanical extinctions (and extinctions as a percentage of the total number of plant species in the country) (Butler 2020)
| Country/state | Number of extinctions (and extinctions as a percentage of the total number of plant species in the country) |
|---|---|
| South Africa | 160 (0.75%) |
| Hawaii (USA) | 99 (7.07%) |
| United States (excl. Hawaii) | 76 (0.45%) |
| Australia | 69 (0.44%) |
| Sri Lanka | 61 (1.34%) |
| Brazil | 36 (0.11%) |
| China | 31 (0.10%) |
| Mauritius (excl. Rodrigues) | 28 (4.09%) |
| Philippines | 24 (0.24%) |
| Cuba | 22 (0.31%) |
| India | 20 (0.13%) |
| Mexico | 17 (0.07%) |
| Chile | 16 (0.31%) |
| Madagascar | 14 (0.12%) |
| Italy | 13 (0.23%) |
| Indonesia | 13 (0.07%) |
| Colombia | 12 (0.05%) |
| Rodrigues (Mauritius) | 12 (7.50%) |
| St Helena (UK) | 11 (2.62%) |
| Country/state | Number of extinctions (and extinctions as a percentage of the total number of plant species in the country) |
|---|---|
| South Africa | 160 (0.75%) |
| Hawaii (USA) | 99 (7.07%) |
| United States (excl. Hawaii) | 76 (0.45%) |
| Australia | 69 (0.44%) |
| Sri Lanka | 61 (1.34%) |
| Brazil | 36 (0.11%) |
| China | 31 (0.10%) |
| Mauritius (excl. Rodrigues) | 28 (4.09%) |
| Philippines | 24 (0.24%) |
| Cuba | 22 (0.31%) |
| India | 20 (0.13%) |
| Mexico | 17 (0.07%) |
| Chile | 16 (0.31%) |
| Madagascar | 14 (0.12%) |
| Italy | 13 (0.23%) |
| Indonesia | 13 (0.07%) |
| Colombia | 12 (0.05%) |
| Rodrigues (Mauritius) | 12 (7.50%) |
| St Helena (UK) | 11 (2.62%) |
The 20 countries with the highest numbers of botanical extinctions (and extinctions as a percentage of the total number of plant species in the country) (Butler 2020)
| Country/state | Number of extinctions (and extinctions as a percentage of the total number of plant species in the country) |
|---|---|
| South Africa | 160 (0.75%) |
| Hawaii (USA) | 99 (7.07%) |
| United States (excl. Hawaii) | 76 (0.45%) |
| Australia | 69 (0.44%) |
| Sri Lanka | 61 (1.34%) |
| Brazil | 36 (0.11%) |
| China | 31 (0.10%) |
| Mauritius (excl. Rodrigues) | 28 (4.09%) |
| Philippines | 24 (0.24%) |
| Cuba | 22 (0.31%) |
| India | 20 (0.13%) |
| Mexico | 17 (0.07%) |
| Chile | 16 (0.31%) |
| Madagascar | 14 (0.12%) |
| Italy | 13 (0.23%) |
| Indonesia | 13 (0.07%) |
| Colombia | 12 (0.05%) |
| Rodrigues (Mauritius) | 12 (7.50%) |
| St Helena (UK) | 11 (2.62%) |
| Country/state | Number of extinctions (and extinctions as a percentage of the total number of plant species in the country) |
|---|---|
| South Africa | 160 (0.75%) |
| Hawaii (USA) | 99 (7.07%) |
| United States (excl. Hawaii) | 76 (0.45%) |
| Australia | 69 (0.44%) |
| Sri Lanka | 61 (1.34%) |
| Brazil | 36 (0.11%) |
| China | 31 (0.10%) |
| Mauritius (excl. Rodrigues) | 28 (4.09%) |
| Philippines | 24 (0.24%) |
| Cuba | 22 (0.31%) |
| India | 20 (0.13%) |
| Mexico | 17 (0.07%) |
| Chile | 16 (0.31%) |
| Madagascar | 14 (0.12%) |
| Italy | 13 (0.23%) |
| Indonesia | 13 (0.07%) |
| Colombia | 12 (0.05%) |
| Rodrigues (Mauritius) | 12 (7.50%) |
| St Helena (UK) | 11 (2.62%) |
Taken as a percentage of the total native flora of a country or territory, the highest extinction rates are all on islands or archipelagos: Ascension Island (13.33%), Rodrigues Island (7.50%), Hawaii (7.07%), Mauritius (4.09%), Easter Island (4.00%), St Helena (2.62%), Sri Lanka (1.34%), Tubuai (1.33%), Bermuda (1.21%), French Polynesia (0.96%), and the Pitcairn Islands (0.82%). In general, islands have a reduced flora with higher endemism and relatively high levels of extinction due to invasive species and limited space. The high rate of Sri Lanka is due to recent thorough botanical study (Wijesundara et al. 2020), and we expect this to be the general level of botanical extinction across the tropics, although many of the Sri Lankan species listed as possibly extinct will prove to be Lazarus species. However, a botanical extinction rate of 1 to 2% is to be expected in most tropical and subtropical countries, with the rate being a little higher in the more densely populated Mediterranean climate regions. Some countries have known issues with population growth, and high deforestation rates, but no extinctions are reported. The absence of such countries (e.g. Iraq, Nigeria, North Korea, Paraguay, Russia) in Supporting information, Appendix S1 is probably due to a lack of reporting extinctions.
Reasons for plant extinction
By closely examining documented plant extinctions from around the world, we can be certain that extinction does not only happen in a few biodiverse places far away, but it is happening in every country, habitat type, and climate (Fig. 4). Although most modern extinctions happen because of habitat destruction, the reasons for this can vary, from overexploitation, climate change, and flooding to volcanic eruptions. Humans have always cleared natural vegetation to grow crops, build cities and roads, mine ore or coal, divert streams for irrigation, and harvest natural remedies. With the increase of the human population now estimated to be 8 billion people (an increase of 6 billion since 1930; https://population.un.org/wpp/), the pressure on land is immense. There are now only 19 km2 per person in the world, but most of this is uninhabitable desert or is covered by ice, and in many countries a single square kilometre is shared by hundreds or even thousands of people (e.g. Singapore with 8033 people per km2; Bangladesh c. 1200/km2; Lebanon 656/km2; Rwanda 470/km2; Haiti 428/km2; Netherlands 423/km2; India 418/km2; UK 271/km2; China 145/km2; USA 87/km2). All these people on such a small land surface have of course an enormous environmental impact. Little nature remains in areas of high human population density, causing many extinctions (Luck 2007).
A, Map of number of plant extinctions per country (based on Supporting information, Appendix S1). B, Map of plant extinctions as a percentage of total plant diversity (based on Supporting information, Appendix S1 and Butler 2020).
There are several hypotheses about the likely reasons underlying extinction. Extinction can be completely random (the so-called ‘field of bullets’), or it can be that species poorly adapted to changing environments simply disappear (‘fair game’; Monarrez et al. 2021). However, during mass extinction events caused by rapid climate change, the two types of extinction appear to be combined resulting in random extinction, in which occasionally better adapted species perish and apparently less well-adapted species may survive. We cannot explain the reasons for many extinctions, apart from the most obvious cause: habitat destruction. Why habitats disappear and some species cannot adapt quickly enough whereas others adjust is often a mystery. The difference in the Anthropocene mass-extinction is that we do know what causes habitat destruction: human activity in nearly all cases.
The fact remains, when a habitat is lost, we do not lose one species but all species that depended on that habitat, from trees of the forest to fungi in the soil. Therefore, recreating a lost habitat is often impossible simply because the network of species can take centuries or longer to re-establish all its interconnections. Recalibrating the balance between species necessary for the ecosystem to function properly remains a highly challenging task.
Based on the list presented here (Supporting information, Appendix S1), the main reason for extinction of plants is invasive species (N = 280), which includes the impact of all invasive animals, plants, and fungi. Feral or free-roaming grazing animals (goats, sheep, cattle, horses, camels, rabbits, squirrels, deer, etc.) are among the worst culprits, but increasingly, invasive insects, slugs, snails, fungi and other pests and pathogens are causing havoc among native plant populations. Invasive plants are problematic due to exclusion of the native vegetation, prevention of regeneration, or provision of fuel for hotter and more frequent wildfires.
Wildfires that can benefit some species also may result in extinctions (N = 19), which climate change may exacerbate. Other climate-change related issues such as drought (N = 6), erosion (N = 5) and landslides (N = 4) are also on the rise, as well as rare weather events (N = 5). Climate change is likely to soon become a much bigger problem for plant survival, especially because of the unpredictability of weather patterns. A sequence of closely spaced droughts, floods, fires and frosts can easily wipe out even widespread species before they have time to recover.
Deforestation and land clearing (N = 210) are followed closely by agriculture and forestry (N = 180); the second and third most common reasons for extinction. It is obvious that when all vegetation is removed, native species cannot survive. Therefore, it is essential that critically endangered plant populations are adequately mapped and their habitats protected.
This is in direct relation to extinctions caused by urbanization and infrastructure developments (N = 137), quarrying (N = 17) and recreational activities (N = 5). These extinctions were all avoidable if these human activities had been properly planned. Therefore, the whereabouts of critically endangered species should be known to local councils and administrators. Related water management issues are also to blame and can be prevented if plants are rescued from new dams (N = 12), drainage improvements, and stream diversions (N = 15).
Overcollection, harvesting and poaching (N = 26) of rare species is another major problem. Even though there are regulations in place to prevent international trade of endangered species (e.g. CITES), these are largely ineffective (Cooney 2001). Legislation generally seems to have little effect, especially at the national scale, and the best strategy to save some species may be a combination of local protection and flooding of the market with artificially propagated material. If a species is easily obtained from cultivation, wild plants are generally left alone (Phelps et al. 2014).
It has been exceptionally rare for natural phenomena to wipe out entire species. During the Anthropocene, only one plant species has been destroyed by a hurricane, and four species died out due to volcanic eruptions. These last extinctions would not have been preventable. Of course, these species might already have had reduced geographical ranges due to human activities.
The future of extinct plants
Seed banks store seeds to preserve the genetic diversity of plants for the future. Seed banking has become more commonplace since the 1970s (Curry 2022). Even though not all seeds can be stored successfully, many can persist for a long time, especially at low humidity and temperature (-20 °C). The aim is to be able to store species for future usage, varying from genetic studies to reintroduction. There are now more than 1000 seed banks around the world, varying in type, size, and focus.
However, preserving genetic diversity is only part of the conservation story. The seeds also need to be cultivatable. For many species, germination protocols, a science of trial and error, have yet to be developed. Thus the use of banked rare plant seeds to test their viability and cultivation requirements is problematic, especially if limited seeds are available for trialing. If there are only a few available seeds, one must be more careful and perhaps carry out trials with more common, related species first. Every seed is invaluable in cases when there are very few seeds left.
Seeds are also preserved in herbarium collections, but viability of these seeds is less certain than properly banked fresh seeds. Herbarium specimens may have been treated against pests, generally freezing or poisoning, or preservation in the field with alcohol before they are dried. Therefore, seeds are not always viable, but when a species is extinct or very rare, it is certainly worth trying to revive them under laboratory conditions. However, a single individual does not make a species. Genetic diversity is likely key to survival, and thus when seeds are collected for conservation and seed banking, they are taken from as many individuals and populations as possible. This leaves us the question of how the germinated plants are to be used. Are they meant for a garden or is there a natural habitat for species reintroduction?
The entire procedure from banking of seeds to reintroduction in the wild has not yet been fully established, but progress is being made. Of course, ecological restoration is more expensive than simply preserving what we have, so we should aim to protect as many intact ecosystems as possible. Simply having banked the seeds does not mean we can recreate nature when it was lost, and a restored habitat will always be a mere shadow of the complex natural ecosystem it once was.
Botanical gardens are where species from wild, known origin are cultivated and can be the only refuge for threatened, rare, and extinct-in-the-wild species. Unfortunately, there is still little long-term coordination among botanical collections. A species may be brought into cultivation as part of a particular project, but when the project ends, collections may be neglected or discarded, thus often wasting earlier efforts. Plants are often accessioned into botanical garden collections haphazardly and not always shared with other parks and gardens, which may be because a species is difficult and expensive to propagate or because the plants had collection licences that do not allow the sharing of material.
Gardener wisdom says, ‘If you want to keep a plant, give it away’. Cases in which plants were not shared have often resulted in loss. Successful conservation is more likely when species are shared among many gardens to prevent losses due to pests, wars, climate change, lack of funding, or changes in fashion.
Species can survive in horticulture, but we must be mindful of genetic drift in horticultural settings. Over time, a species may no longer be suitable for reintroduction into the wild (Ensslin and Godefroid 2019). For instance, the Franklin tree, Franklinia alatamaha, originated in Georgia, USA, but material from horticulture does not seem to grow well in its original swamp along the Altamaha River or in gardens nearby. Perhaps the plant or the local climate has changed.
Once a plant has been grown in horticulture and gardeners know how to propagate it, it may be acceptable to reintroduce such species to the wild if genetic diversity is tracked and maintained. Therefore, a sufficiently large number of genetically diverse individuals may need to be grown so that cross-pollination is possible. In many cases, nearly extinct species went through genetic bottlenecks because they were reduced to a single or few individuals. This narrow genetic base often makes a species less competitive in a natural setting. A good example of a genetic bottleneck is the toromiro tree, Sophora toromiro, a legume species from Easter Island. Once the dominant tree on the island, the bark was stripped by sheep, and by the 1880s it had become rare. A single group remained in the 1930s and eventually seeds from the last individuals were taken in the 1950s to Sweden by Thor Heyerdahl and to Chile by Efraín Volosky. These persisted in cultivation in botanical gardens while the last wild tree was cut down for firewood in 1960. All material now grown originated from these few individuals. Material from cuttings, seeds, and micropropagation were distributed to other botanical gardens in the 1970s, and several reintroduction attempts on Easter Island were made in the 1980s. A Toromiro Management Group, funded by various Chilean and foreign organizations was initiated, and in 1995 nearly 200 plants from Bonn and Gothenburg were brought to the island. Sadly, this reintroduction failed, probably due to continued grazing pressure by horses and reduced fitness of the species. The management group was eventually discontinued due to lack of funding (TMG 1995, Maunder et al. 2000, Christenhusz and Govaerts 2023). Sometimes there are ways around bottlenecks, but in general, reintroduction of plants from horticulture is challenging and expensive. Success often depends on continuous funding for genetic study, maintenance, and proper documentation of the cultivated plants, genetic testing and expensive propagation units, and shipment and long-term monitoring to ensure survival in the field.
To save species from extinction, they need a place to grow. It may be impossible to reintroduce a plant to an area where it was once known to occur due to habitat alteration in the intervening years. Sometimes similar habitats can be identified, but we will never know if all the variables are the same in a new place. In some cases, it may even be desirable to introduce a species into a completely novel environment, simply because no original habitat remains. A disease or pest may have killed off the original population, and reintroduction will then only succeed in places where this threat is absent. Such introductions outside the natural range of species are controversial, but these may be needed if we wish to preserve our natural heritage, the biodiversity of this planet.
With climatic changes causing more floods, droughts, and wildfires, increasing pressure on the land will certainly result in more plant (and animal) extinctions. Which ones and where these occur cannot be predicted, but prevention is always better than reintroduction. Efforts should be made to protect all remaining natural habitats, bring critically endangered species into horticulture, and use assisted migration to establish or fortify populations outside areas where they are threatened.
Supporting information
Supplementary data is available at Botanical Journal of the Linnean Society online.
Supplementary Appendix 1. List of species known to have become extinct since the 1750s. A list like this is constant flux, with more extinct species to add and others being rediscovered. This list, compiled in December 2023, is therefore incomplete. Several species will have disappeared that were not recorded, and some will have been rediscovered. There are also numerous species known only from historical collections but these have not been retrieved or studied in the field recently. Some regions are also better known than others. This list merely functions as an overview of global plant species extinction.
New combinations
There are a three species that, based on taxonomic study, need to be nomenclaturally amended.
Adonis paryadrica (Boiss.) Christenh. & Govaerts, comb. nov. Basionym: Adonis cyllenea var. paryadrica Boiss., Fl. Orient. [Boissier] 1: 16 (1867). Note: because the two varieties of A. cyllenea Boiss., Heldr. & Orph. are separated by 1300 km of unsuitable habitat (one in Greece, the other in Turkey). Because there is no gene flow between these populations, they are thus better treated as species, which will also help conservation efforts of both taxa.
Ceropegia witzenbergensis (C.A.Lückh.) Christenh. & Govaerts, comb. nov. Basionym: Huernia witzenbergensis C.A.Lückh., Stapelieae, ed. 2. [White & Sloane] 3: 890 (1937). Note: the genus Huernia is now included in Ceropegia, but this extinct species had not yet been transferred.
Elatostema srilankana Christenh. & Govaerts, nom. nov. Replaced synonym: Elatostema lineolatum Wight var. petiolare Thwaites ex Trimen, J. Bot. 23: 243 (1885), not Elatostema petiolare W.T.Wang. Note: Thwaites (in Trimen, 1885) suggested this may need to be treated as a species rather than a variety. We agree and for this reason, it is elevated to species here. It differs from E. lineolatum in having petiolate leaves and lacking cystoliths on the upper leaf surface.
Acknowledgements
We thank the publisher Sterck en De Vreese for supporting the publication of our book Uitgestorven, especially Katrien De Vreese, who helped with conceptualisation and publishing of the book, which formed the basis of this article. Professor Mark Chase has been instrumental in discussions and commented on drafts of this manuscript. We thank him for his help.
Conflict of interest
None declared.
Data availability
The data underlying this article are available in the article and in its online supplementary material.
