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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-
pite its finality it can be difficult to verify and many frame-
works have been proposed to define formally that extinction
has occurred. For most taxonomic groups and regions there
is no reliable list of species considered to be probably or pos-
sibly extinct. The record of plant extinctions in Australia is
no exception, characterized by high turn-over within lists,
low transparency of attribution and lack of consistency be-
tween jurisdictions. This makes it impossible to evaluate
how many plant taxa have become extinct in Australia.
We present an ecological framework for assessing the like-
lihood of plant extinctions, based on taxonomic soundness,
degree of habitat modification, detectability and search ef-
fort, underpinned by the best available expert knowledge.
We show that, in sharp contrast to both the fate of the
Australian fauna and prevailing assumptions, only of
plant taxa currently listed as or assumed to be extinct are
considered probably extinct, and a further possibly
extinct. Twenty taxa listed as or assumed to be extinct
have dubious taxonomy or occurrence in Australia, and
the remaining taxa are considered possibly extant and
further surveys are required to ascertain their status. The
list of probably and possibly extinct plants is dwarfed by
the number thought extinct but rediscovered since .
Our method can be used for vascular floras in other regions
characterized by well-documented and curated floras and
high levels of expert knowledge, and provides a transparent
platform for assessing changes in the status of biodiversity.
Keywords Australia, conservation assessment, extinction,
flora, taxonomy, threatened species, surveys
Supplementary material for this article is available at
https://doi.org/./S
Introduction
Extinction is the inevitable end-point for all lifeforms
(Koopowitz & Kaye, ; Purvis et al., ), but
the present rate of extinctions globally is estimated to be
one thousand times the natural or background rate as a
result of anthropogenic pressures on habitats and species
(Barnosky et al., ; Pimm et al., ). Ignoring unproven
re-creations, extinction of a species is irreversible and
has biological, evolutionary and emotional consequences
(Koopowitz & Kaye, ; Ryan, ; Collen et al., ).
With every global extinction, a unique evolutionary history
is lost and the web of life irrevocably altered and diminished.
Yet despite the finality and magnitude of extinction, the
actual event can be difficult to verify. Biology is replete with
instances of so-called Romeo errors, in which species are
declared or presumed extinct only to be rediscovered
(Collar, ). Prominent recent examples include the
ivory-billed woodpecker Campophilus principalis of the
Big Woods of Arkansas (Fitzpatrick et al., )and
the enigmatic night parrot Pezeropus occidentalis of the
Australian desert (Dooley, ). There have also been qui-
eter rediscoveries amongst plants (Scott & Yeoh, ;
Wapstra et al., ; de Lírio et al., ; Sochor et al.,
) and invertebrates (Bonham, ; Byk et al., ),
and species whose continued existence remains the subject
of intense debate, sometimes entering the realm of myth-
ology, as for the Tasmanian tiger Thylacinus cynocephalus
(Bailey, ).
The problem of verifying extinctions is a vexed issue
for policy makers and scientists. IUCN guidelines state a
species should be considered extinct when ‘there is no rea-
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
consistently, and will severely underestimate extinctions
(Butchart et al., ; Mace et al., ). In response to
these concerns, the latest Red List Guidelines have an ad-
ditional tag of ‘Critically Endangered (Possibly Extinct)’for
taxa ‘that are, on the balance of evidence, likely to be ex-
tinct, but for which there is a small chance that they may
be extant’(IUCN Standards and Petitions Subcommittee,
,p.). These taxa should not be listed as Extinct
until adequate surveys have been completed and uncon-
firmed reports investigated.
J.L. SILCOCK (Corresponding author) and R.J. FENSHAM* Centre for Biodiversity
and Conservation Science, National Environmental Science Program–
Threatened Species Recovery Hub, University of Queensland, St Lucia, 4072,
Australia. E-mail j.silcock@uq.edu.au
A.R. FIELD* Australian Tropical Herbarium, James Cook University, Cairns,
Australia
N.G. WALSH Royal Botanic Gardens Victoria, Birdwood Avenue, Melbourne,
Australia
*Also at: Department of Environment and Science, Queensland Herbarium,
Brisbane Botanic Gardens, Toowong, Australia
Received July . Revision requested September .
Accepted October .
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Statistical methods have also been developed for infer-
ring extinction based on collection records and survey ef-
fort (Reed, ; Solow, ; Collen et al., ). These
tend to be based on the distribution of sightings or collec-
tions through time, which can be used to calculate the ex-
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
history traits (such as dormant or cryptic stages) and the
potential complexity of a species’ecology, particularly
when it is poorly known. Moreover, they rely on an ad-
equate sample size (typically considered to be .records;
Collen et al., ) and can seldom be applied to species
with only one or two records, which is the case for many
plants that are presumed extinct. Recent methodologies to
estimate extinction probability have incorporated the
timing and degree of threat, and a taxon’s susceptibility
(Keith et al., ), and record reliability and survey data
(Thompson et al., ).
Listing of a species as Extinct has significant conservation
implications for allocation of resources and environmental
reporting (Keith & Burgman, ). Thus it is important
that lists of presumed and possibly extinct species are as ac-
curate and transparent as possible. It has been estimated that
Australia accounts for almost % of the world’s flora that
has been recently categorized as Extinct (Briggs & Leigh,
), a fact that is usually explained by the continent’s
long isolation, high floristic endemism, and rapid and severe
recent land-use changes (Lindenmayer, ; Nipperess,
). However, lists of Australia’s extinct plants are charac-
terized by high turn-over, low transparency of attribution
and lack of consistency between jurisdictions (Keith &
Burgman, ). The first assessment in Australia defined
Presumed Extinct species as not found despite thorough
searching or not collected for at least years, and formerly
known from areas now well-settled, and listed species
that met those criteria (Leigh et al., ). Between
and , taxa were added to the list of extinct
Australian plants, mostly as a result of new evaluations,
and were deleted, mostly as a result of rediscoveries,
but also because of taxonomic changes or corrections
(Keith & Burgman, ). This high turnover has con-
tinued, with numerous species discussed by Keith &
Burgman () either rediscovered or synonymized, and
numerous new species being added to national and state
lists. The number of plant taxa that are actually likely to
have become extinct in Australia is currently unknown,
and without clearly defined criteria the issue will continue
to be a point of confusion for plant conservation.
We present a transparent and uniform ecological frame-
work for assessing plant extinctions, underpinned by expert
knowledge of species and habitats. We provide an estimate
of the current number of plant extinctions in Australia since
and present a revised list of probably and possibly
extinct species. This method can be used for vascular floras
in other regions characterized by well-documented and
curated floras and high levels of expert knowledge, and
provides a coherent platform for assessing changes in the
status of biodiversity.
Methods
We compiled a spreadsheet of taxa listed as Extinct
according to the national Environment Protection and
Biodiversity Conservation Act and lists of threatened
species from individual states and territories. We also
added unlisted taxa that are considered by experts, or
noted in taxonomic treatments or state censuses, to be po-
tentially or probably extinct. Taxa that are considered ex-
tinct in the wild but persist in translocated populations
were included. Taxa that are not currently known from
any living plants but are known to be extant in the soil
seedbank or are considered extinct in one jurisdiction
but still occur elsewhere in Australia were not included.
Subspecies were included, but varieties, generally consid-
ered to be of lower taxonomic status, forms and hybrids
are not considered here. We acknowledge that the distinc-
tion between subspecies and variety is unevenly applied by
taxonomists, but have adopted this approach to filter out
at least some trivial taxa. A separate list of taxa that have
formerly been presumed to be globally extinct under na-
tional and/or state legislation was also compiled, to shed
light on relevant factors behind extinction likelihood and
assessment. Taxa listed as Extinct but that have been re-
cently rediscovered were included.
For each taxon, data were compiled from Australian
Virtual Herbarium records (CHAH, ), Conservation
and Listing Advice, Recovery Plans, peer-reviewed litera-
ture, the assessments of Leigh et al. () and Briggs &
Leigh (), and consultation with experts with field
knowledge of species and habitats. Variables included the
last date collected, survey effort and adequacy (Thompson
et al., ), degree of habitat modification, intensity and
extent of threats (Keith et al., ), detectability and life
history of the species, and any taxonomic issues (Table ).
Taxa for which expert opinion or examination of specimens
indicated they are morphologically close to co-occurring
taxa and may not be taxonomically distinct were removed
from further assessment, as were those whose occurrence
in Australia is considered dubious. Species that also occur
outside Australia were retained, but collecting dates and
habitat notes relate only to Australian records.
To place some transparent and simple criteria around
assignation of extinction likelihood, we devised a key con-
taining relevant factors from Table . Through this key,
each of the taxa currently listed as Extinct in Australia was
assigned an extinction confidence/likelihood: () Almost
2 J. L. Silcock et al.
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certainly extinct, () Probably extinct, () Possibly extinct
(on the balance of evidence, must be considered extinct
but requires further surveys where possible to confirm its
status), and () Possibly extant (surveys undertaken thus
far have not been sufficient to consider the taxon extinct).
In the key, a taxon can be found at any stage from step
. This possibility is not included in the key, as it will of
course lead to removal of the species from any extinction
considerations. It is also possible to locate the target spe-
cies with searching, but for taxonomic uncertainty to pre-
clude definitive identification; this will mean returning
to step . The amount of survey effort before assuming
extinction has occurred increases with lack of spatial pre-
cision, low habitat modification/threat and cryptic lifeform
stages/low detectability. It is impossible to get more certain
than probably extinct unless precision of at least one re-
cord is ,km
,or.% or remaining habitat is sur-
veyed, and detectability is high or surveys span multiple
seasons and years. The cut-off for record accuracy is the
approximate amount of habitat that can be confidently
searched by a botanist in one day. This will vary between
habitats and lifeforms, but km
is considered reasonable
based on ., hours of our own searching for rare
plants. The , and % cut-offs for area of suitable
habitat searched are intended as reasonable guiding
estimates. Assessments of the level of understanding of a
taxon’s life history and optimum survey times relies
upon expert opinion.
TABLE 1 Data compiled for each presumed extinct taxon in the Australian vascular flora.
Data field Description
Conservation status Environment Protection and Biodiversity Conservation Act & state listing
Taxonomy Any taxonomic issues noted, including notes on closely-related taxa; where there is considerable taxo-
nomic uncertainty, the taxon was removed from further consideration
Occurrence reliability Expert assessment about whether taxa actually occurred in Australia; where occurrence is apparently
because of mislabelled specimens, or otherwise considered dubious, taxa were removed from further
consideration
State & bioregion(s) Biogeographical regions as per Thackway & Cresswell (1995)
Habitat summary Best available information on habitat from specimen label; where this was not available, expert opinion on
most likely habitat is given where possible
Last date collected Year of last collection or reliable observation; where this was not available, an estimate is given based on
when the collector was active in the area of collection
Lifeform Tree, shrub, perennial grass, annual grass, perennial herb, annual herb, orchid, fern
Number of collections &
populations
Number of times collected (collections by the same collector on the same day but in different herbaria are
classed as a single record; collections made at the same place by different collectors or by the same collector
at different times are treated as separate collections)
Location precision Precision of most accurate record, in km
2
; calculated by drawing a minimum convex polygon around the
area within which the location description indicates the record lies. If the exact location of the record is
available as GPS-determined coordinates, the precision is 0.1 km. We considered 5 km
2
a reasonable area
that could be searched to confidently rule out the taxon being present at the time of survey, based on
extensive field experience of the authors
Abundance information If recorded
Detectability Low, moderate or high, with cryptic life history stages, disturbance response and other factors noted
Life history knowledge Will inform best times for survey, possible reasons for extinction, microhabitats or land management
regimes where most likely to be found
Targeted surveys Thoroughness & time period of searches
% of suitable remaining
habitat searched
Estimated from vegetation mapping, satellite imagery & expert opinion
Degree of habitat
modification
High (,30% of vegetation type remains as remnant vegetation); moderate (substantial remnants remain
but many highly modified); or low (.70% of vegetation type remains as remnant vegetation, & is con-
sidered to still provide suitable habitat for taxon)
Threat/potential cause of
extinction
Threats that may have driven taxon to extinction; intensity & extent of threat, & susceptibility of taxon
assessed
Extinction likelihood Almost certainly extinct; probably extinct; possibly extinct; possibly extant (see key in Methods)
Notes Relevant to taxon biology, life history or extinction assessment
Source(s) Relevant literature & expert botanist/s interviewed
Assessing extinction likelihood 3
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Key for assessing extinction likelihood in vascular floras
The number of taxa from the presumed extinct Australian species is indicated at each couplet.
1. Subject to taxonomic uncertainty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Revise taxonomy (n=)
1a. Reliable taxon or sufficient certainty to warrant further searching. . . . . . . . . . . . . . . . . . . . . . . . . .
2. Suspected identification or location error, and taxon unlikely to occur in Australia . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Discard unless further information available (n=)
2a. Identification correct and record likely from Australia . . . . . . . . . . . . . . . . . . . . . . . . . . . (n=)
3. At least one collection record able to be located to within km
...................... (n=)
3a. No records able to be located within km
................................. (n=)
4. Long-lived species, no cryptic lifeform stages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (n=)
4a. Ephemeral species or geophyte, cryptic or dormant lifeform stage/s . . . . . . . . . . . . . . . . . . . . . (n=)
5. Habitat highly modified or destroyed AND/OR well-known threatening process . . . . . . . . . . . . . . . (n=)
5a. Habitat not or only lightly modified AND no well-defined threatening process . . . . . . . . . . . . . . . . (n=)
6. Thorough searching at site/s and .% of potential habitat . . . . . . . . . . . . . Almost certainly extinct (n=)
6a. Thorough search effort at site/s and .% of potential habitat . . . . . . . . . . . . . . . . Probably extinct (n=)
6b. Search effort less than a . . . . . . . . . . . . . . . . . . . . . Possibly extinct; surveys needed to confirm (n=)
7. Thorough searching at site/s and .% of other potential habitat . . . . . . . . . . . . . . Probably extinct (n=)
7a. ,%ofsiteandsuitablehabitateffortatsite/s ............Possiblyextinct;furthersurveysneeded(n=)
8. At least moderate understanding of life history/optimum survey times (e.g. seasonal conditions/disturbance events likely
tohaveresultedingermination).........................................(n=)
8a. Understanding of life history less than .................................. (n=)
9. Thorough searching at site/s and .% of suitable habitat at more than one optimum time/s AND habitat highly
disturbed OR threatening process operating . . . . . . . . . . . . . . . . . . . . . . . . . . Probably extinct (n=)
9a. Thorough searching at site/s AND .% of suitable habitat at more than one optimum time/s; habitat not or lightly
modified AND no well-defined threats . . . . . . . . . . . . . . . . . . . . . . . . . . . . Possibly extinct (n=)
9b. Search effort less than a . . . . . . . . . . . . . . . . . . . . . . . . . . . Possibly extant, surveys needed (n=)
10. Site/s of historic collection + .% of other potential habitat searched across multiple seasons and conditions AND
habitat highly modified . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Probably extinct (n=)
10a. Site/s of historic collection + –% of other potential habitat searched across multiple seasons and conditions AND
habitat highly to moderately modified . . . . . . . . . . . . . . . . . . . . . . . . . . . . Possibly extinct (n=)
10b. Site/s of historic collection not well-searched and/or ,% of potential habitat searched across multiple seasons and
conditions . . . . . . . . . . . . . . . . . . . . . Possibly extant; further surveys in various seasons needed (n=)
11. Habitat not known from record labels; record precision . km
.................... (n=)
11a. Habitat able to be inferred, at least broadly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (n=)
12. General area highly modified and well-surveyed . . . . . . . . . . . . . . . . . . . . . . . Possibly extinct (n=)
12a. General area not highly modified and no major threats identified . . . . . . . . . . . . . . Possibly extant (n=)
13. Habitat highly modified AND/OR documented threatening process/es . . . . . . . . . . . . . . . . . . (n=)
13a. Habitat not heavily modified, or at least large remnants remaining unaffected by major threatening processes . . .
....................................................... (n=)
14. Long-lived species, no cryptic lifeform stages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (n=)
14a. Ephemeral species or geophyte, cryptic or dormant lifeform stage/s . . . . . . . . . . . . . . . . . . . . (n=)
4 J. L. Silcock et al.
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15. .% of potential habitat searched . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Probably extinct (n=)
15b. –% of potential habitat searched . . . . . . . . . . . . . . . . Possibly extinct; further surveys needed (n=)
15c. ,% of potential habitat searched . . . . . . . . . . . . . . . . . Possibly extant; further surveys needed (n=)
16. Good understanding of species, life history/optimum survey times . . . . . . . . . . . . . . . . . . . . (n=)
16a. Poor understanding of species, life history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (n=)
17. .% of potential habitat searched repeatedly at optimum times . . . . . . . . . . . . . . Probably extinct (n=)
17a. –% of potential habitat searched at optimum times . . . . . . . . . . . . . . . . . . . Possibly extinct (n=)
17b. ,% of potential habitat searched at optimum times . . . . . . . . . . . . . . . . . . . Possibly extant (n=)
18. .% of potential habitat searched repeatedly in multiple seasons . . . . . . . . . . . . . Probably extinct (n=)
18a. .% of potential habitat searched repeatedly in multiple seasons . . . . . . . . . . . . . Possibly extinct (n=)
18b. ,% of potential habitat searched . . . . . . . . . . . Possibly extant; surveys in various seasons needed (n=)
19. At least moderately detectable taxon with no cryptic lifeform stages . . . . . . . . . . . . . . . . . . . . (n=)
19a. Low detectability, complex disturbance response and/or cryptic lifeform stages . . . . . . . . . . . . . . (n=)
20. .% of potential habitat searched . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Possibly extinct (n=)
20a. ,% potential habitat searched . . . . . . . . . . . . . . . . . . Possibly extant; further surveys needed (n=)
21. Good understanding of species life history/optimum survey times . . . . . . . . . . . . . . . . . . . . . (n=)
21a. Poor understanding of species life history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (n=)
22. .% of potential habitat searched repeatedly at suitable times . . . . . . . . . . . . . . . Possibly extinct (n=)
22a. ,% of potential habitat searched repeatedly at suitable times . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Possibly extant; further surveys recommended (n=)
23. .% of potential habitat searched across numerous seasons . . . . . . . . . . . . . . . . Possibly extinct (n=)
23a. ,% of potential habitat searched . . . . Possibly extant; further surveys in various seasons recommended (n=)
TABLE 2 Presumed extinct taxa in the Australian vascular flora, and their extinction likelihood as assessed using the methodology described
here. Taxa from different regions are separated by semi-colons.
Extinction likelihood Region and taxa
Almost certainly
extinct (in the wild)
Norfolk Island group: Streblorrhiza speciosa; Sydney: Allocasuarina portuensis
Probably extinct South-west Western Australia: Acacia kingiana, Darwinia divisa, Picris compacta, Tetratheca fasciculata; Greater
Sydney: Deyeuxia appressa, Gentianella wingecarribiensis, Persoonia laxa; south-eastern South Australia:
Gentianella clelandii, Prasophyllum colemaniarum; Pacific Subtropical Islands: Solanum bauerianum
Possibly extinct South-west Western Australia: Coleanthera virgata, Lepidium drummondii, Leptomeria dielsiana, Leucopogon
cryptanthus, Myriocephalus nudus, Picris wagenitzii, Ptilotus caespitulosus, Ptilotus sericostachyus subsp. roseus,
Scholtzia sp. Bickley (W.H. Loaring s.n. PERTH 06165184), Thomasia gardneri; north-eastern New South Wales:
Euphrasia ruptura, Homoranthus elusus, Indigofera efoliata; South-eastern Australia: Ozothamnus selaginoides,
Pomaderris sericea, Pultenaea maidenii, Prasophyllum sp. Majors Creek (Jones 11084), Senecio georgianus,
Senecio helichrysoides; Brigalow Belt, Queensland: Corchorus thozetii; Christmas Island: Peperomia rossii
Possibly extant Brigalow Belt, Queensland: Amphibromus whitei, Calotis glabrescens; Coastal Queensland: Genoplesium sp. (Raby
Bay), Goodenia arenicola; Gulf Country, Queensland: Ptilotus senarius;Paspalum batianoffii,Persoonia prostrata;
Wet Tropics, Queensland: Embelia flueckigeri, Pseudodiphasium volubile, Marsdenia araujacea, Lastreopsis dis-
secta, Wendlandia psychotrioides; New England Tablelands, New South Wales: Leucopogon confertus; south-west
Western Australia: Conospermum caeruleum subsp. contortum; Greater Sydney/central New South Wales:
Grevillea divaricata, Olearia oliganthema, Pultenaea elusa; Tasmania: Senecio tasmanicus
Assessing extinction likelihood 5
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Results
Overview of presumed extinct plants
Of the Australian plant taxa currently listed as or pre-
sumed to be extinct, (%) are considered to be taxonom-
ically suspect (Supplementary Table ). Nine of these are
known only from the type specimen, which often comprises
incomplete fragments, and the other four are known from
–collections. Seven have not been collected for the past
years. All are very close morphologically to other
co-occurring species, and belong in groups regarded
as taxonomically ill-defined and difficult, for example
Frankenia L., Hemigenia R.Br. and the Orchidaceae genera
Caladenia, Diuris and Prasophyllum.
The occurrence in Australia of a further seven presumed
extinct taxa, all ferns from the Wet Tropics of north-eastern
Australia, is dubious (Supplementary Table ). All are
relatively widespread tropical ferns and their extinction in
Australia would represent local extinctions, but their appar-
ent occurrences in Australia represent highly disjunct, and
sometimes biogeographically unlikely, populations. Three
are known only from collections by Czech botanist Karel
Domin in ,andtwofromDomin’s published descriptions
without specimens. There are large areas of intact rainforest
habitat where they occurred and there has been extensive
survey effort in these areas. Where specimens are available,
they have been verified as being correctly identified, how-
ever a labelling error on this batch of specimens cannot
be ruled out, as Domin also visited areas outside Australia
on the same expedition where the ferns in question
do occur.
There are taxa with sound taxonomy and unambigu-
ous occurrences in Australia that are currently considered
extinct (Supplementary Table ). Although experts consider
them all reliable taxa, there has been some degree of taxo-
nomic confusion with closely-related species in the past
for , and hybrid origins of four cannot be discounted
without further material. Eighteen of the taxa are
listed as Extinct under both the national Environment
Protection and Biodiversity Conservation Act and state
legislation, and a further are listed only under state
legislation. Fourteen are listed as either Critically
Endangered, Endangered, Vulnerable or Priority Flora
(Western Australia) under the Environment Protection
and Biodiversity Conservation Act and/or state legislation,
and five are unlisted, but recent publications, taxonomic
treatments, state censuses and/or expert opinion consider
them potentially extinct. Most families are represented by
one or two taxa, but Asteraceae stands out with nine species
and Fabaceae and Proteaceae with five and four, respective-
ly. Western Australia has the highest number of presumed
extinct taxa (, all bar one from the south-west), followed
by New South Wales () and Queensland (). The
Northern Territory and Australian Capital Territory have
no presumed extinct plants. Three presumed extinct taxa
come from islands. Shrubs are the most represented life
form, with presumed extinct taxa, followed by mostly
short-lived forbs ().
All taxa presumed extinct seem to have been rare and/or
restricted throughout their historic range, with known
from a single collection and all bar five known from ,
collections. Only three are known from $populations.
Twenty-four have not been collected in the last years
and in the past years. Those with more recent collec-
tions have been searched for repeatedly at sites where they
formerly occurred and not found, or the habitat has been de-
stroyed. At least are regarded as difficult to detect, having
tiny growth habits, cryptic life forms and/or complex seed
bank or disturbance dynamics. More than half are known
or strongly suspected to respond positively to disturbance.
Assessing extinction likelihood
Applying our key of ecological and survey factors, only
two species are considered almost certainly extinct in the
wild (although one persists in a translocated population),
are considered probably extinct and possibly extinct
(Table ). The remaining taxa are classified as possibly
extant, primarily because of lack of adequate surveys to
inform their status.
To be assigned to the almost certainly extinct category, a
species must have a record with high locational precision
(,km
), which has been searched at an appropriate
time/s by expert botanists. A high proportion of other
suitable habitat in the vicinity will also have been searched.
There will usually, although not always, be a demon-
strable threat from destruction or major modification of
habitat. This condition was met for just two species,
Allocasuarina portuensis, which is almost certainly extinct
in the wild but persists as translocated plants at a number
of locations (Office of Environment & Heritage, ) and
Streblorrhiza speciosa, a scrambling shrub known from the
highly modified Phillip Island in the Norfolk group that
has not been seen since despite targeted surveys
(Schrire, ).
The taxa considered probably extinct come from high-
ly modified agricultural and urban areas with little remnant
vegetation, including four from south-western Australia and
three from the Greater Sydney area. All have at least one re-
cord that could be located to within km
(with the excep-
tion of Acacia kingiana, the record only being accurate to
within km
but with a clear habitat description) and
have had .% of remaining habitat searched (.% for
seven taxa). Five are considered to have low detectability, and
most probably respond to disturbance. Paradoxically, species
with more recent records are more likely to be assessed as
6 J. L. Silcock et al.
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probably extinct, as these more recent records are known
from precise localities, whereas older records often have
very low precision, rendering targeted survey difficult.
Although records could be located within km
for only
taxa, habitat could be inferred, at least broadly, for all
except four taxa (Supplementary Tables &). All four
are James Drummond collections from south-western
Western Australia, collected in the s–s from an
area that is now both heavily modified and well-surveyed
botanically. Thus despite lack of location precision, three
are assigned as possibly extinct. A further Western
Australian taxa were assessed as possibly extinct, as well as
six from south-eastern Australia and three from the north-
east of New South Wales (Table ). Location precision and
detectability vary, but nearly all have had at least .%of
potential habitat searched (repeatedly for taxa with cryptic
life forms stages and seasonal fluctuations in abundance).
The habitat of seven is highly modified. Two species, the
short-lived disturbance-responsive forb Corchorus thozetii
from central Queensland and the shrub Pomaderris sericea
from mountainous terrain in Victoria and New South
Wales, have had ,% of remaining potential habitat
searched at appropriate times but are known from at least
one accurate record that has been revisited without the spe-
cies being relocated. In the case of the former, the site where
it was collected in the s has now been cropped, but other
suitable habitat remains unsurveyed. The main reason for
possibly extinct taxa not being assessed as probably extinct
was that their habitats were not highly modified and there
were no threats affecting the entire range of the species.
This often occurred in concert with low detectability and/
or complex and poorly-understood disturbance dynamics.
The remaining taxa assessed as possibly extant have
had ,% of their potential habitat searched, and in
some cases much less, combined with large areas of remnant
suitable habitat and few severe threats. The exception is
Genoplesium sp. (Raby Bay), which would have been ranked
as probably extinct as a result of the high level of habitat
modification, until a possible collection of an infertile
specimen in (Lui Weber, pers. comm., February
). Twelve of these taxa are known or strongly considered
to be responsive to disturbance, with large temporal fluctua-
tions further increasing the demands for repeated surveys to
reliably infer extinction. Five are from the rainforests of
north Queensland, and another five from the sub-coastal
and coastal areas of Queensland.
Formerly extinct plants
More than taxa previously presumed extinct (i.e. listed
under state and/or federal legislation and/or Leigh et al.
() or Briggs & Leigh (), or identified as such in
published sources such as taxonomic treatments) have
been rediscovered in Australia, mostly since the s
(Supplementary Table ). Over half of these () have
been straightforward rediscoveries in the field, but have
involved rediscoveries of specimens collected decades ear-
lier that had not been correctly identified at the time of
their collection. In many cases, taxonomic revisions or ap-
parent field rediscoveries trigger re-examination of collec-
tions. Conversely, specimen rediscoveries sometimes result
in renewed targeted surveys, which in turn locate further po-
pulations. Only five of the specimen rediscoveries have
occurred in isolation from field rediscoveries. The remain-
ing taxa presumed extinct have been removed because of
taxonomic revision revealing them as synonyms of more
widespread taxa (in cases), of hybrid origin or more wide-
spread than previously thought as a result of revision of the
species’concept. Seven of these taxa are still listed on extinct
lists, including five under the Environment Protection and
Biodiversity Conservation Act.
The average time that these rediscovered plant species
have been apparently lost (excluding those with taxonomic
issues) was years, and had not been seen for .
years before they were rediscovered. Western Australia
accounts for .% of rediscoveries (n = ), with from
Queensland. One-third are known from highly modified
habitats, a factor that contributed to their being considered
extinct, but are known from habitats that mostly remain
in remnant condition, with few threats present. Fifty-six are
shrubs, annual or perennial forbs, nine grasses and sedges,
and eight ground orchids. More than were known from a
single recognized collection prior to their rediscovery, while
only six had been known from .collections. Over half
of formerly presumed extinct plants are now known to be
abundant or at least locally common, although .%of
these respond to fire or other disturbance and thus exhibit
large temporal fluctuations. Although remain known
from a single rediscovered population and are categorized
as Endangered or Critically Endangered, are now
known from at least populations. Eighty-two per cent
of rediscoveries and specimen rediscoveries have occurred
since , with over half of these occurring in the s.
There has been an average of one rediscovery per year
over the past decade across Australia, the latest being the
ferns Oreogrammitis leonardii and Hymenophyllum whitei,
both collected for the first time in years in on remote
mountain tops in far north Queensland, and the shrub
Acacia prismifolia, collected beside a busy highway in
south-western Australia in , having not been seen
since .
Discussion
The systematic assessment of extinction likelihood amongst
the Australian vascular flora reveals that the true extinction
Assessing extinction likelihood 7
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toll is far less than previous assessments, in sharp contrast to
the general perception that the number of identified extinc-
tions languishes behind the true number that have occurred
(Butchart et al., ; Collen et al., ). This is partly a
result of the listing of many plant species that are now con-
sidered taxonomically dubious or that never occurred in
Australia, and partly a result of the vastness and inaccessi-
bility of some largely unmodified habitat types, the tiny
proportion able to be feasibly surveyed, and the cryptic
life form stages of many species (a response to Australia’s
variable climatic conditions). The history of rediscoveries
and taxonomic changes reinforces the fact that taxonomic as-
sessment and adequate surveys must precede any declara-
tions of extinction, especially if large swathes of habitat are
still intact and threatening processes cannot be identified.
Species with unresolved taxonomy, and those with dubi-
ous Australian occurrences, should be prioritized for taxo-
nomic revision and specimen re-examination to clarify their
status (Wege et al., ). The species assigned possibly
extant and possibly extinct should be searched for where
possible, with a focus on accurate recording of search effort,
and seasonal and habitat condition (Keith, ; Silcock
et al., ). We suggest that taxa assessed as possibly extinct
should be retained on lists, or added where necessary, and
those that are listed as possibly extant should be removed
pending further survey. This treatment would leave taxa
as presumed extinct in Australia, albeit with two-thirds
ranked as possibly rather than probably extinct. Two-thirds
occur in highly modified habitats with well-documented
threats, particularly where habitat clearing for agriculture
and urbanization coincides with high levels of narrow-range
endemism (Burgman et al., ). Grazing and hydrological
modification seem to have played a part for numerous taxa.
The remainder occur in habitats that are not substantially
modified, including four that seem to have become extinct
in the absence of any known or suspected threat.
Extinction is a natural process and the possibility exists
that some taxa may have become extinct independent of
human activities (Richman et al., ; Williamson, ),
although undoubtedly most extinctions are anthropogenic
(Regan et al., ; Barnosky et al., ).
More than species formerly considered extinct
have been rediscovered in Australia since the mid s,
many of which eluded detection for more than a century
(Supplementary Table ). This cautions against declaring
species extinct, particularly for those that may be ephemeral
in response to disturbance (Kirkpatrick & Gilfedder, ;
Duretto et al., ). Most species listed as Presumed
Extinct in Leigh et al. () were known from single
or few, low-precision pre- records, and were not redis-
covered until concerted flora survey and taxonomic studies
since the s. Many of these were initially assessed as hav-
ing a slim chance of being rediscovered, primarily because of
severe habitat loss and modification (Leigh et al., ). Even
species that occur in close proximity to well-collected urban
centres can remain undetected for many years, as demon-
strated by recent rediscoveries in the Perth and Sydney
metropolitan areas (Davis, ; Duretto et al., ).
Rediscovery of presumed extinct taxa, particularly those
that are considered to have always been rare, could be con-
sidered parallel to the continuing discovery of new species.
Between and , an estimated mean of . taxa
of flowering plants per annum have been described as new
(Chapman, ). Some of these are mere formalizations of
long-known or suspected informal taxa, but many represent
genuine new discoveries. The discovery of a new species is
closely akin to the rediscovery of a species known by only
one or few historic collections. In this light, we should not
be surprised or suspicious of rediscoveries of presumed
extinct species.
Our review highlights the differences in assessing flora
compared to fauna extinctions (Boyd & Stanfield, ;
Collen et al., ). Plants often have persistent seedbanks,
increasing the effective generation time and making it
difficult to confirm extinction on the basis of the species’
absence in the above-ground plant community (Silcock
et al., ). There are usually fewer records for plants,
often only the type specimen. This makes it difficult to be
confident of taxonomic status, as well as rendering it im-
possible to infer declines. However, most plant sightings
are reinforced by collections, thus removing the issue of as-
sessing the veracity of sightings, which is a vexed issue for
inferring animal extinction (Boyd & Stanfield, ;
Solow, ). The task of assessing extinction likelihood
has been made more streamlined by the digitization of
many herbarium collections, including the Australian
Virtual Herbarium (CHAH, ).
Despite our analysis pointing to fewer extinctions than
previously thought, there may also be undocumented extinc-
tions, where species disappeared before they were collected.
The relatively small number of botanical collectors in
Australia prior to broad-scale habitat modification would
not have collected a comprehensive sample of the entire
vascular flora (Keith & Burgman, ). Undocumented ex-
tinctions are especially likely to have occurred in rapidly
modified habitats that were not well-surveyed prior to modi-
fication, especially those that contain endemic and restricted
species, for example Great Artesian Basin springs (Rossini
et al., ) and the Western Australian Wheatbelt. There
are also numerous taxa that are not listed as extinct but
have not been seen for many years (Gibson, ). Some
occur in highly modified habitats, and surveys are urgently
required to assess their status.
Even accounting for possible undocumented extinctions,
almost times as many taxa are listed as being at
risk of global extinction (i.e. Critically Endangered or
Endangered under federal and/or state legislation) than
have become extinct since . This could be a result of
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the inherent resilience of the flora, whereby species can be-
come extremely reduced in range and abundance but are
able to persist. In some ancient landscapes such as south-
western Australia, long-term landscape stability has resulted
in a propensity for flora to occur in small disjunct popula-
tions and have genetic systems to cope with this (Byrne &
Hopper, ; Hopper et al., ). The relative resilience
of Australia’s flora in comparison to its fauna is highlighted
by total extinction rates. Over % of Australia’s terrestrial
mammals (Woinarski et al., ) and .% of its birds
(Szabo et al., ) have become extinct in the past
years, compared to .% of its flora.
Alternatively, the discrepancy between the extinct and
threatened lists could highlight a looming extinction debt
(Vellend et al., ; Kolk & Naaf, ; Halley et al.,
). Under this scenario many plants listed as threa-
tened are functionally extinct, having fallen below the
critical number to sustain their populations in the long
term, or with populations dominated by mature indivi-
duals with minimal recruitment. Of the species listed
as Critically Endangered or Endangered at a national
level, are known from , individuals, and
from ,, well below numbers considered to represent
long-term viable populations (Traill et al., ). Many of
these are now restricted to tiny remnants that are inher-
ently vulnerable to degradation and within which eco-
logical processes, particularly those driving recruitment,
no longer operate.
The list and accompanying data presented here
provide a baseline for tracking plant extinctions in
Australia. The reasons for assigning a species to each ex-
tinction likelihood category are explicit, through assem-
bling the best available information on species biology,
ecology and search effort, and working each species
through the key. This avoids the uncertainties and biases
that tend to plague decisions on assigning extinction
status (Keith et al., ). It will now be clear whether ad-
ditions or removals from the presumed extinct list are a
result of taxonomic revisions, specimen rediscoveries or
real changes to status (i.e. documented extinctions or
rediscoveries). The use of multiple explicitly evaluated ex-
tinction likelihood categories provides a more meaningful
measure for reporting on changes over time and between
regions than the variable and changeable presumed ex-
tinct lists, as they are currently compiled. By accurately
evaluating, understanding and acknowledging extinctions,
we come closer to preventing more species being added
to the list of the lost.
Acknowledgements We acknowledge the many botanists who shared
their knowledge of presumed and possibly extinct species, and their efforts
to find them, and who delved into herbarium specimens and often murky
taxonomies: Greg Keighery, Mike Hislop, Kevin Thiele, Neil Gibson,
Barbara Rye, Dave Coates, Sarah Barrett, Barbara Rye, Andrew Webb,
Bree Phillips, Anne Harris and Carolyn Wilkins (Western Australia);
Lachlan Copeland, Peter Weston, Bob Makinson, Amanda Jowett, Liz
Tasker, Mark Clements, Tony Auld, John Briggs, Steve Douglas, Keith
McDougall, Lachlan Willmott (New South Wales); Rigel Jensen, Bob
Jago, John Thompson, John Neldner, Bill McDonald, Lui Weber, Bill
O’Donnell, Keith McDonald, Paul Forster (Queensland); Bob Bates,
Doug Bickerton, Peter Lang, Dan Duval, Greg Guerin (South
Australia); Paul Foreman, Jeff Jeanes, Gary Backhouse, Neville Scarlett
(Victoria); Richard Schahinger, Jamie Kirkpatrick (Tasmania). Neil
Gibson commented on the text and checked Appendices for Western
Australia. John Woinarski alerted us to numerous additional species
that are or have been considered extinct. This work was funded
by the National Environmental Science Program’s Threatened Species
Recovery Hub. ARF’s examination of material in European and US
herbaria was supported by an IPID4all Germany Academic Exchange,
Technische Universität Dresden Graduate Academy Project 2015_43
and a Queensland Smithsonian Fellowship 2017.
Author contributions Research design, data collation, writing: JLS;
discussion of ideas and writing: RJF; data contribution, discussion of
ideas and writing: ARF, NGW.
Conflicts of interest None.
Ethical standards This research abided by the Oryx guidelines on
ethical standards.
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