To name those lost: assessing extinction likelihood in the Australian vascular flora J.L. SILCOCK,A.R.FIELD,N.G.WALSH and R . J . F ENSHAM Abstract Extinction is a profound biological event, yet des- Introduction pite its finality it can be difficult to verify and many frame- works have been proposed to define formally that extinction xtinction is the inevitable end-point for all lifeforms has occurred. For most taxonomic groups and regions there E(Koopowitz & Kaye, ; Purvis et al., ), but is no reliable list of species considered to be probably or pos- the present rate of extinctions globally is estimated to be sibly extinct. The record of plant extinctions in Australia is one thousand times the natural or background rate as a no exception, characterized by high turn-over within lists, result of anthropogenic pressures on habitats and species low transparency of attribution and lack of consistency be- (Barnosky et al., ; Pimm et al., ). Ignoring unproven tween jurisdictions. This makes it impossible to evaluate re-creations, extinction of a species is irreversible and how many plant taxa have become extinct in Australia. has biological, evolutionary and emotional consequences We present an ecological framework for assessing the like- (Koopowitz & Kaye, ; Ryan, ; Collen et al., ). lihood of plant extinctions, based on taxonomic soundness, With every global extinction, a unique evolutionary history degree of habitat modification, detectability and search ef- is lost and the web of life irrevocably altered and diminished. fort, underpinned by the best available expert knowledge. Yet despite the finality and magnitude of extinction, the We show that, in sharp contrast to both the fate of the actual event can be difficult to verify. Biology is replete with Australian fauna and prevailing assumptions, only of instances of so-called Romeo errors, in which species are plant taxa currently listed as or assumed to be extinct are declared or presumed extinct only to be rediscovered considered probably extinct, and a further possibly (Collar, ). Prominent recent examples include the extinct. Twenty taxa listed as or assumed to be extinct ivory-billed woodpecker Campophilus principalis of the have dubious taxonomy or occurrence in Australia, and Big Woods of Arkansas (Fitzpatrick et al., )and the remaining taxa are considered possibly extant and the enigmatic night parrot Pezeropus occidentalis of the further surveys are required to ascertain their status. The Australian desert (Dooley, ). There have also been qui- list of probably and possibly extinct plants is dwarfed by eter rediscoveries amongst plants (Scott & Yeoh, ; the number thought extinct but rediscovered since . Wapstra et al., ; de Lírio et al., ; Sochor et al., Our method can be used for vascular floras in other regions ) and invertebrates (Bonham, ; Byk et al., ), characterized by well-documented and curated floras and and species whose continued existence remains the subject high levels of expert knowledge, and provides a transparent of intense debate, sometimes entering the realm of myth- platform for assessing changes in the status of biodiversity. ology, as for the Tasmanian tiger Thylacinus cynocephalus (Bailey, ). Keywords Australia, conservation assessment, extinction, The problem of verifying extinctions is a vexed issue flora, taxonomy, threatened species, surveys for policy makers and scientists. IUCN guidelines state a ‘ Supplementary material for this article is available at species should be considered extinct when there is no rea- ’ https://doi.org/./S sonable doubt that the last individual has died and that this is reliant on ‘exhaustive surveys in known and/or ex- pected habitat, at appropriate times...throughout its his- toric range’ (IUCN, ). It has been argued that the IUCN criteria may not be an appropriate onus of proof when the aim is to assess extinctions accurately and J.L. SILCOCK (Corresponding author) and R.J. FENSHAM* Centre for Biodiversity and Conservation Science, National Environmental Science Program– consistently, and will severely underestimate extinctions Threatened Species Recovery Hub, University of Queensland, St Lucia, 4072, (Butchart et al., ; Mace et al., ). In response to Australia. E-mail [email protected] these concerns, the latest Red List Guidelines have an ad- A.R. FIELD* Australian Tropical Herbarium, James Cook University, Cairns, ditional tag of ‘Critically Endangered (Possibly Extinct)’ for Australia taxa ‘that are, on the balance of evidence, likely to be ex- N.G. WALSH Royal Botanic Gardens Victoria, Birdwood Avenue, Melbourne, Australia tinct, but for which there is a small chance that they may be extant’ (IUCN Standards and Petitions Subcommittee, *Also at: Department of Environment and Science, Queensland Herbarium, Brisbane Botanic Gardens, Toowong, Australia ,p. ). These taxa should not be listed as Extinct Received July . Revision requested September . until adequate surveys have been completed and uncon- Accepted October . First published online August . firmed reports investigated. Oryx, 2020, 54(2), 167–177 © 2019 Fauna & Flora International doi:10.1017/S0030605318001357 Downloaded from https://www.cambridge.org/core. IP address: 170.106.33.19, on 24 Sep 2021 at 03:59:03, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0030605318001357 168 J. L. Silcock et al. Statistical methods have also been developed for infer- extinct species. This method can be used for vascular floras ring extinction based on collection records and survey ef- in other regions characterized by well-documented and fort (Reed, ; Solow, ; Collen et al., ). These curated floras and high levels of expert knowledge, and tend to be based on the distribution of sightings or collec- provides a coherent platform for assessing changes in the tions through time, which can be used to calculate the ex- status of biodiversity. pected rate of sightings if a species were extant. Although these formulae partially account for factors such as species abundance and detectability, they cannot account for life Methods history traits (such as dormant or cryptic stages) and the potential complexity of a species’ ecology, particularly We compiled a spreadsheet of taxa listed as Extinct when it is poorly known. Moreover, they rely on an ad- according to the national Environment Protection and equate sample size (typically considered to be . records; Biodiversity Conservation Act and lists of threatened Collen et al., ) and can seldom be applied to species species from individual states and territories. We also with only one or two records, which is the case for many added unlisted taxa that are considered by experts, or plants that are presumed extinct. Recent methodologies to noted in taxonomic treatments or state censuses, to be po- estimate extinction probability have incorporated the tentially or probably extinct. Taxa that are considered ex- timing and degree of threat, and a taxon’s susceptibility tinct in the wild but persist in translocated populations (Keith et al., ), and record reliability and survey data were included. Taxa that are not currently known from (Thompson et al., ). any living plants but are known to be extant in the soil Listing of a species as Extinct has significant conservation seedbank or are considered extinct in one jurisdiction implications for allocation of resources and environmental but still occur elsewhere in Australia were not included. reporting (Keith & Burgman, ). Thus it is important Subspecies were included, but varieties, generally consid- that lists of presumed and possibly extinct species are as ac- ered to be of lower taxonomic status, forms and hybrids curate and transparent as possible. It has been estimated that are not considered here. We acknowledge that the distinc- Australia accounts for almost % of the world’s flora that tion between subspecies and variety is unevenly applied by has been recently categorized as Extinct (Briggs & Leigh, taxonomists, but have adopted this approach to filter out ), a fact that is usually explained by the continent’s at least some trivial taxa. A separate list of taxa that have long isolation, high floristic endemism, and rapid and severe formerly been presumed to be globally extinct under na- recent land-use changes (Lindenmayer, ; Nipperess, tional and/or state legislation was also compiled, to shed ). However, lists of Australia’s extinct plants are charac- light on relevant factors behind extinction likelihood and terized by high turn-over, low transparency of attribution assessment. Taxa listed as Extinct but that have been re- and lack of consistency between jurisdictions (Keith & cently rediscovered were included. Burgman, ). The first assessment in Australia defined For each taxon, data were compiled from Australian Presumed Extinct species as not found despite thorough Virtual Herbarium records (CHAH, ), Conservation searching or not collected for at least years, and formerly and Listing Advice, Recovery Plans, peer-reviewed litera- known from areas now well-settled, and listed species ture, the assessments of Leigh et al. () and Briggs & that met those criteria (Leigh et al., ). Between Leigh (), and consultation with experts with field and , taxa were added to the list of extinct knowledge of species and habitats. Variables included the Australian plants, mostly as a result of new evaluations, last date collected, survey effort and adequacy (Thompson and were