How does one define “extinct”? Is it clearly a black-and-white definition, or are there shades of gray? The Merriam-Webster Dictionary defines “extinct” as “no longer existing,” while the IUCN Red List of Threatened Species defines “extinct” as “when there is no reasonable doubt that the last individual has died.” Both definitions are clear and definite.
Extinction is not everywhere equal, however. A recent study documented plant extinctions over the past 250 years and included a published list of nearly 600 global plant extinctions (Humphreys, et al. 2019). Of the 571 extinct plant species listed in the study, 376 were defined as globally extinct (no longer extant in the wild or in cultivation) and 195 were defined as extinct in the wild (extinct in its natural habitat but known to survive in cultivation). That study was reviewed, tested, and scrutinized several times, and other studies have been published since with the additions of overlooked extinct species and the removal of species reported recently as extant (Knapp, et al. 2021; Rocchetti, et al. 2022). Curious to know how many of these species appear as specimens in the herbarium in which I work, the U.S. National Herbarium (herbarium code US), I matched the list of 571 extinct plants to our online database (US had recently completed a massive digitization project, and the data from its 4.5 million pressed specimens are now available online). I found that the US collection contains specimens of 98 extinct species, with another 48 species extinct in the wild.
The digitization process not only provides label transcriptions but specimen images as well. A close look at the images often reveals some descriptive words that the transcriptions might have missed (or transcriptions difficult to search).
Some specimen labels show that the collector of the specimen was aware of the species’ rarity in the wild. For example, a 1927 specimen of Argyroxiphium virescens shows that collector Otto Degener was very aware of the rarity of this species in Hawaii. For the locality, he writes, “Koolau Gap, Haleakala Crater, East Maui, Fog-swept, moss-covered lava—very rare.” Another Degener specimen, Kokia cookie, states, “On verge of extinction.” (For an even more vivid example of a collector’s awareness of a species’ impending extinction, see “The Last Piece of Evidence,” page 43.)
Gradations of extinction multiply when considering the combined effects of species, collections, and culture. These gradations span a range, from extinct species that have never been discovered, to extinct species that theoretically may one day be resurrected by dried pressed material inadvertently found on herbarium sheets. Does the dictionary definition of “no longer existing” apply when one holds a specimen of an extinct species within one’s hand? While the species may no longer be alive in the wild, the pressed dried plant still “exists.” And what of herbarium sheets that have been destroyed during wars, fires, and floods? I present a flow chart (shown on page 39) that dives into the various gradations of extinction and examines how the representable records of long-ago species provides hope within an herbarium. My explanation of those gradations follows below.
SPECIES NOT KNOWN TO SCIENCE
Dark existence: Extant and undescribed
Estimates of the number of plant species currently present on earth range from 420,000 to 450,000 (Govaerts 2001; Joppa, et al. 2011), yet only ~378,000 accepted species have been described (WTO 2023). Extrapolations suggest that of the 70,000 species still to be described, approximately half await discovery in the wild, while the other half have been collected and are stored in herbaria, but due to the lack of taxonomic expertise or incomplete information have yet to be described (Bebber, et al., 2010). Some of these undescribed species rest as specimens that have been unprocessed in overburdened herbaria, and thus remain unavailable for study. For those specimens that have been processed, new species might be unnoticed, misplaced, or assigned to unidentified material.
Dark extinction: Extinct and undescribed
An immeasurable number of species have gone extinct before botanists gave them a taxonomic name. Boehm & Cronk (2021) use the term “dark extinction” for the extinction of undescribed species. The extinction rate of species today is far greater than in the past (De Vos, et al. 2015; Pimm, et al., 2014; Ceballos, et al. 2015; Cowie, et al. 2022). Thus, in a race against time, we continue to lose species from habitat destruction before species are ever discovered.
Case studies of dark extinctions: Knapp, et al. (2021) lament that “much of the eastern United States was affected by European settlement before botanical exploration began,” and thus, “an untold number of plant species went extinct before discovery.” Lambdon and Cronk (2020) estimate that for the island of St. Helena, there may have been around 10 unrecorded plant extinctions during the period 1502-1771. Similarly, using classical and Bayesian algorithms, Kristensen, et al. (2020) estimate that for Singapore between 304 and 412 plant species have gone extinct since 1822 before they could be discovered.
SPECIES NOT GLOBALLY EXTINCT
Extant and described
This category represents the 381,948 accepted plant species that are currently extant (WFO 2023).
Case studies of extant and described species: Some plant species are on the precipice of extinction, such as the 5,702 species assessed as Critically Endangered (IUCN Red List version 2023-1). Others are thriving in their native habitat, such as the 30,554 species assessed as Least Concern (IUCN Red List version 2023-1). While still others are thriving in introduced habitat as non-native and invasive species, such as the 474 worst invasive plant species that threaten native biodiversity and natural areas (Global Invasive Species Database 2023).
Extant but extirpated
Extirpation, also local extinction, refers to the termination of a species within a chosen geographic area of study. While the species is still extant elsewhere, scientists record when populations or subpopulations go extinct. For instance, species may be extirpated from a specific habitat (forest, bog, island) or an administrative division (country, state, providence, department).
Case study of extirpated species: A study conducted by Ebinger, et al. (2010) on 310 uncommon and rare native Illinois plant taxa found that 87 species are extirpated from the state, that most have not been seen in the state from more than 50 years, and some have not been seen since the mid-1800s.
Possibly extinct: Certain species have been assessed as most likely extinct, but these species require additional study and surveys for confirmation. IUCN states that “if, despite searches, the taxon has not been recorded since the collection of the type specimen, and there are threats in the area, a listing of Critically Endangered (Possibly Extinct) may be appropriate.” The benefits to labeling a species as Possibly Extinct instead of Extinct is that protective measures and conservation funding will continue for a species not believed to be Extinct. The IUCN Red List (version 2023-1) lists 569 species as Critically Endangered, Possibly Extinct CR(PE), an identification tag that can be attached to the CR category to highlight those taxa that are possibly extinct.
Case study of possibly extinct species: A study by Carrington, et al. (2017) highlights the species, Xylosma serrata (Salicaceae), a species assessed at Critically Endangered based on two collections in 1778 and 1980 from the same volcanic sites on Montserrat. The plant has not been recollected nor found during detailed surveys since the most recent eruptions of the Soufrière Hills volcano in 1995. Since a complete survey is necessary but difficult to execute due to the volcanic activity in the area, IUCN has listed this species as Critically Endangered, Possibly Extinct (Bárrios 2015).
Extinct in the wild: IUCN states that when a taxon is extinct in its natural habitat, but it is known to survive in cultivation, in captivity, or as a naturalized population well outside the past range, it is assessed as Extinct in the Wild. This category provided by IUCN also applies to plant taxa when viable seeds exist in storage facilities if there is potential for these seeds to develop into viable reproductive offspring. The IUCN Red List (version 2023-1) lists 45 species as Extinct in the Wild (EW). One species so listed is Franklinia alatamaha, which Rivers (2015) points out “has not been seen in the wild since 1803 despite numerous attempts to relocate it. It is successfully grown in many botanic gardens and arboreta around the world”—including the Arnold Arboretum, which has three living specimens (accessions 2428-3*A, 2428-3*B, and 254-81*C).
GLOBALLY EXTINCT SPECIES
Below are various categories of global extinctions that I crafted (not sanctioned by IUCN) that combine concepts of species, collections, and culture. IUCN’s guidelines for Extinct is when “there is no reasonable doubt that the last individual has died.” It is often quite difficult to confirm that a species has gone extinct, especially for species that are difficult to detect, such as cryptic, inconspicuous, silent, or identification is difficult, of which these examples apply to many plant species. IUCN defines a key aspect of the Extinct category to include “exhaustive surveys” that should be undertaken in all known or likely habitat throughout the species’ historical range, at a variety of times (diurnal, seasonal, annual), and over the timeframe appropriate to the species’ life cycle and life form. The IUCN Red List (version 2023-1) lists 129 plant species as Extinct (EX) while Albani Rocchetti, et al. (2022) identified approximately 360 extinct plant species.
Extinct without visual record: This category describes when all specimens of an extinct plant species have been destroyed, and there are no other visual representations of that species, such as a painting, illustration, or photograph, beyond the written word. During the course of history, many scientific collections have been partially or fully destroyed, either by natural events such as fires (e.g., Brazil in 2018; Escobar & Vogel 2018) and floods (e.g., Biltmore Herbarium in 1916; Alexander 2007) or by anthropogenic events such as war (e.g., Botanical Museum Berlin-Dahlem during World War II). Likewise, individual collections may have been accidentally or intentionally destroyed, such as when Australian biosecurity officers in 2017 incinerated a collection of plant specimens sent by the Museum of Natural History in Paris that dated back to the mid-1800s (Stokstad 2017).
Case studies of extinct species without visual representation: Rocchetti, et al. (2022) generated a list of 361 globally extinct species. For 30 of those extinct species, Rocchetti, et al., was unable to find any digitized specimens or specimens in the herbaria they contacted. Of those 30, it is unclear if photographs or illustrations remain. One species in the list, Guettarda retusa (Rubiaceae), is a Cuban shrub that has not been located since its time of description by Charles Wright in the nineteenth century. No type specimen was cited in the protologue (new-species description), and thus it’s possible that no specimen exists for this species. Another species in the Rocchetti, et al., (2022) list is Pavetta bangweensis (Rubiaceae), a shrub from Cameroon. According to Manning (1996), the 1934 holotype of this species was destroyed in Berlin, and “no material fitting the description of this species has been found.”
Extinct with non-herbarium record: This category describes when all specimens of an extinct plant species have been destroyed, but visual representations of that species still exist, such as a painting, illustration, or photograph. For example, The Field Museum’s Type Photograph Collection contains digitized images of 71,000 negatives of types that were photographed by J. Francis Macbride before the types were destroyed at the Botanical Museum Berlin-Dahlem during World War II. Likewise, paintings and illustrations give evidence that a species was once extant.
Case study of an extinct species with a non-herbarium record: As the case above, Rocchetti, et al. (2022) was unable to find any digitized specimens or extant vouchers for 30 species in the herbaria that they contacted. One species on their list, Coelogyne acutilabium (Orchideaceae), is known only from a single cultivated plant, now dead, in the Bogor Botanical Garden. Eduard F. De Vogel described this species based on a single dried inflorescence, which he illustrated in Orchid Monographs (de Vogel 1992). The illustration is all that remains of this species.
Classically extinct: Extinct with a herbarium record: The category describes the most common definition of an extinct plant species: when a specimen exists that confirms the identity of a species that once existed as a living plant.
Case study of a classically extinct species: In the Rocchetti, et al. (2022) list of 361 globally extinct species, they found 190 species with specimens that did not bear seeds or fruit. One such species is Weinmannia spiraeoides (Cunoniaceae), which is known from a single sterile specimen collected in 1840 from Ovalau, Fiji (see image on page 42). No material on this herbarium specimen can resurrect this species.
The Last Piece of Evidence
The starkest example of a collector knowing that a species is about to go extinct can be seen ona specimen of Santalum fernandezianum (shown above). This tree, commonly known as Chile sandalwood, was endemic to the Juan Fernández Islands off the coast of Chile. It was overharvested over a century ago for its aromatic wood. When Carl Skottsberg collected a specimen from this tree in 1908, he knew he was looking at the living dead. In his penmanship, he writes, “Branch from the last tree.” This specimen is the last piece of evidence that this tree ever existed.
At times, the collector is so certain that a species is about to go extinct (or already has) that when they give the plant a scientific name, they’ll use a form of “extinct” in the new name. An inspection of the International Plant Names Index (IPNI) finds 13 instances in which a plant’s name contains the Latin extinctoria, extinctorium, or extinctoriformis. It should be noted, however, that in instances in which this is found, the reference is to describe the plant’s morphology as to be “fire extinguisherlike” as in the look of a candle- or wick-snuffer. So, surprisingly, these plant names did not refer to the potential demise of the species.
In contrast, three plant names contain the Latin extinta or extinctus. Gasteranthus extinctus, for example, is a species that was given its dramatic specific epithet by L.E. Skog and L.P. Kvist. The authors write, “all four collections come from … [a] forest [that] has been totally cleared, likely causing the extinction of this species” (Skog and Kvist 2000: 67). Things take a strange turn two decades after this species was given the extinctus name. During a hunt for rare and threatened species, Gasteranthus extinctus was rediscovered in the wild (Pitman et al. 2022). This extant species now has to carry the baggage of an unfortunate name as long as it stays extant.
POST-EXTINCTION
While lists have been drawn up to record which species are extinct, there are occasions when a species might be removed from that list. The removal may be due to the practical reason of rediscovery, or if the theoretical science of resurrection becomes a reality, the removal from the list will be due to the de-extinction of that species.
Rediscovered: Sometimes, conservation biologists make mistakes. Sometimes, taxonomy changes. There are cases when species once assessed as extinct have been rediscovered in their native range (Scheffers, et al. 2011) or in an ex situ facility, such as a botanic garden or arboretum. There are also cases when improved taxonomic knowledge leads to the species becoming a synonym of an extant species. In these cases, extinct species are rehabilitated and removed from the list of extinct species and can then be assessed as Extinct in the Wild or extant (either as threatened or as not threatened).
Case study of rediscovery in the wild: Drone surveys on extremely remote vertical cliffs of Kaua’i conducted in 2019 documented three individuals of Hibiscadelphus woodii (Malvaceae), a species noted as extinct in 2011 (La Vigne, et al. 2022), but now listed as Critically Endangered.
Case study of rediscovery in a botanic garden: Abeli, et al. (2022) determined that 12 plant species once deemed to be extinct are more accurately extinct in the wild or extant in a study that used Botanic Garden Conservation International (BGCI) PlantSearch database and through contacting ex situ institutions.
Case study of taxonomic changes: Syzygium gambleanum (Myrtaceae) from India was listed as extinct in 1988 (Oldfield, et al. 1998). However, S. gambleanum (an illegitimate name) is now treated as a homotypic synonym of S. microphyllum, a species currently assessed as Endangered (World Conservation Monitoring Centre 1998).
Resurrected: What if we can bring extinct plants back to life? Resurrection science may seem like something of the future, but recent studies are looking into de-extinction events (Shapiro 2017; Rocchetti, et al. 2022). Ethics aside, what if a plant specimen has viable seeds on the sheet? What if the specimen has extractable DNA and that plant can be cloned? This category describes species that are classically defined as extinct, but viable seeds or extractable DNA happen to exist on a specimen sheet. Can these species be resurrected?
Theoretical case studies of resurrected species: Rocchetti, et al. (2022) identified 161 extinct species and 556 herbarium specimens that appear to have available seeds, but their viability has yet to be tested. At present is a lack of optimized germination and in vitro culture protocols for utilizing the herbarium specimens for de-extinction. Since the seeds preserved on herbarium specimens of extinct species are “precious and cannot be wasted,” scientists studying these specimens are using the utmost care in testing seed vigor, dormancy-breaking, and germination performance.
Gary A. Krupnick is head of the Plant Conservation Unit within the Department of Botany at Smithsonian’s National Museum of Natural History in Washington, DC.
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