Lost taxa: the danger of seemingly harmless concepts
By Branden Holmes. Published on 19 October 2025.
Introduction: the danger of rogue concepts
Evolution and extinction are often treated as entirely distinct phenomena in the popular press. And they are increasingly being deliberately contrasted in order to emphasise the present greatly heightened extinction rate due to the current biodiversity crisis, when compared with the theoretical background extinction rate that more or less occurs during relatively "ordinary" periods in the history of a life-bearing planet. The current anthropogenically-driven biodiversity crisis is so bad that it is actually hindering evolution, through the loss of genetic diversity, demographic changes including skewing of sex ratios and mortality rates, fragmenting of populations (which has a raft of negative effects!), reduction in suitable habitat that reduces carrying capacities, and even the loss of entire functional traits. And that is just from the perspective of the survivors (for now). After all, there are relatively few phylogenetically unique taxa which lack relatives at the ordinal level (i.e. taxonomic rank: order). Most species belong to higher-level taxonomic groups that will survive the loss of individual species, and thus survive the loss of functional traits among their brethren.
Yet evolution itself, when unabated by environmentally-caused extinctions, and when sufficiently cumulative in the absence of geographically-mediated modes of speciation (i.e. the process of production of novel species), will result in the population's genetic and functional traits paradoxically diverging from itself over time. The population remains a single interbreeding conglomerate throughout its history, without fissioning off any evolutionary branches, and slowly changes over time (called anagenesis). The foundational stock will give way to a new species simply through the accumulation of net change after net change, so long as these net changes do actually accumulate, and do not cancel each other out (thus resetting the trait profile over and over again more or less to that of the foundational stock). This kind of in situ extinction, as a species goes extinct in virtue of evolving into a novel species, is known as phyletic extinction or pseudoextinction. In light of the current biodiversity crisis, it is important to understand the concept of extinction in its fullest expression, which can arise out of evolution itself. Indeed, it is important to appreciate all of the relevant concepts at a high level.
In contrast, the IUCN RedList considers the conservation category 'Extinct' to only be applicable to a taxon when no individuals are left:
"A taxon is Extinct when there is no reasonable doubt that the last individual has died. A taxon is presumed Extinct when exhaustive surveys in known and/or expected habitat, at appropriate times (diurnal, seasonal, annual), throughout its historic range have failed to record an individual. Surveys should be over a time frame appropriate to the taxon’s life cycle and life form." (IUCN, 2012:14)
The fundamental problem with this rogue conception of extinction is that it occurs far too late in the extinction process to be meaningful, and worse it is not benign as it raises further problems. Taxa do not technically qualify as Extinct according to the IUCN simply because they have an extant individual or two, even if the population isn't viable. In the midst of a biodiversity crisis, the fact that such ignorance is being perpetuated by a supposed conservation authority does not serve us well. Extinction is a complicated process with many possible causes and timeframes, but it is objectively achieved long before the last individual takes their last breath. A population is viable if it has the potential to survive indefinitely into the future, with no specific expiry date imposed upon the population. But if the population loses enough individuals, or sex ratios become too skewed, or it becomes highly fragmented, or is being hybridised out of existence, or has its habitat utterly destroyed, it can no longer continue into the future with hope, for it has none. It has been earmarked for geologically imminent extinction, and there is nothing left but for the deaths of the last remaining individuals to be actualised. It is therefore a false hope to automatically equate every population of >0 individuals with being meaningfully extant. Especially if we take a holistic approach, taking into account planned habitat destruction (especially damming), geological activity (e.g. volcanic activity), and the effects of global warming, even if we cannot yet precisely predict these events. As long as there is an objective upper temporal bound sufficiently close in time so as to prevent dispersal if they are not already ecologically abandoned to their fate.
There is thus no biological law which says that non-viable populations cannot reproduce. In fact, non-viable populations can persist multi-generationally before succumbing, especially if the cause of impending extinction is geological activity. We therefore need to be extremely careful when making assumptions about the conservation status of taxa simply because we encounter extant individuals, witness reproduction, and observe recruitment of young into the mature population. All three of these individually, and certainly all three in conjunction, clearly take place among viable populations. But they also take place among non-viable populations too. And therefore, diagnosing which populations are viable and which are not is clearly a complex problem, and must include markers beyond the three already mentioned. This is not helped by simplistic approaches such as that taken by the IUCN. On the other hand, we must also understand that not all absences from the scientific record are equally concerning. There are many traits that taxa can possess which predispose them to only being rarely recorded by humans. This includes fossorial and other subterranean ecologies, and the many plant taxa that survive solely in a soil seed bank for extended periods (sometimes many years) before inducive conditions prevail that triggers the germination of this latent genetic material.
Lost taxa: another rogue concept
The fact that we can observe the three markers already mentioned (i.e. extancy, reproduction, recruitment), and that these are not diagnostic either way with respect to viability, helps to emphasise the fact that extinction is knowledge-independent (and is certainly belief-independent). Our mental lives have not the slightest causal potency in and of themselves to prevent (or cause) extinctions, and there is no metaphysical obligation for taxa to inform us of their extinction. But none of this diminishes the importance of conservation monitoring, in fact it amplifies the need. We need to monitor populations globally so that we can put conservation plans into place if necessary, long before the point at which we can hardly tell whether a population is viable or not: prevention is clearly superior to cure here. But many populations are not merely not being monitored but have not been sighted for decades or even centuries. Given the ease with which taxa can go extinct, especially during the present extinction crisis, it is encumbent upon us to do more to actively monitor the world's biodiversity. In this vein, a small but growing literature has emerged around the idea of 'lost' taxa (Butchart et al., 2005; Tobias et al., 2006; Butchart, 2007; Ladle et al., 2011; Villarroel et al., 2014; Watson & Davis, 2017; Long & Rodríguez, 2022; Martin et al., 2023; Lindken et al., 2024).
Long & Rodríguez (2022) in particular have attempted to standardise the concept:
"A lost species is one not confirmed alive by photographic, audio or genetic information for over 10 years in the wild and has no ex situ population under human care. We consider under human care the maintenance of whole, living specimens, not cryopreserved samples in a biobank. A species’ lost status is independent of its category on the IUCN Red List of Threatened Species but has not met the conditions to be classified as Extinct or Extinct in the Wild. Although a simple definition, it is applicable to all taxa irrespective of life traits or the biome in which they live.
Some have called for a shorter time period to consider a species lost, as much can change in 10 years. We recommend using missing species for those not seen in the wild and not held under human care for > 5 years but < 10. The term long-lost has also been applied (Villarroel et al., 2014; David [sic] Watson & Davis, 2017), and we propose a time span of 50 years for these species. By consistently using these terms we identify a pathway from missing to lost to long-lost that facilitates the catalysis of stage-specific actions to prevent species identified as missing from becoming lost and subsequently becoming long-lost." (Long & Rodríguez, 2022)
This initially seems like a wholly laudable effort, particularly from those papers that publish aggregated lists of such 'lost' taxa (e.g. Martin et al., 2023), making it easier to know which taxa to target. The project of looking for 'missing' (> 5 years but < 10), 'lost' (>10 years) and 'long-lost' (>50 years) taxa is absolutely needed as we know that taxa can quickly go extinct at the best of times, but especially in the midst of the current biodiversity crisis. Threatening processes can quickly and quietly arise and devastate entire populations. So we should prioritise searching for long-lost taxa first and clearing that list, allowing us to move on to the other two categories in succession. Though taxa in the missing and lost categories can be sympatric (i.e. occurring in the same area) with long-lost taxa, so that knowledge of the other two categories is equally important so that we can oportunistically search for them too if we are already in the general area looking for a long-lost taxon. However, there are three major underlying problems with the model being proposed, which all stem from the fact that when lost taxa are finally recorded alive they are automatically considered as rediscovered (Long & Rodríguez, 2022). 1) Having temporal thresholds decreasing in duration allows for massive biases in search efforts and funding allocations. 2) Our conception and understanding of rediscovery is negatively affected by the proposed model. And 3) rediscovery is only valuable when there is conservation potential.
Time for a chat
1) Having temporal thresholds decreasing in duration allows for massive biases in search efforts and funding allocations
Once we have cleared the list of long-lost taxa (>50 years), we will then be down to lost taxa (>10 years) and can then clear that list too. We can then move on to those that are simply missing (> 5 years but < 10). But these temporal categories are far too granular. The shorter the time interval that we get down to, the greater the potential for bias among search efforts. When the turnover time from rediscovery to need to rediscover again is as short as a decade, it allows individuals to ignore plants, fungi and invertebrates in favour of getting their name(s) published alongside those of the giants in the field as having each rediscovered a particular vertebrate taxon. We as individuals, often unjustifiably obsessed with vertebrates, can simply argue that we do not have time to look for those plants, fungi and invertebrates that are also lost when there are so many lost vertebrates constantly in need of rediscovery and which have much larger ecological footprints, especially the so-called 'megafauna'. After all, there is no higher term than rediscovery. To rediscover a taxon means to discover an extant population where there was none known (or in rare cases, was thought to be functionally extinct). So we cannot nominally escalate taxa that receive less conservation attention that way. Instead, under the current model, we simply have to wait until the lapse in time escalates them from lost to long-lost.
Obviously, we can close the gap from lost (>10 years) to long-lost (>50 years) by lowering the long-lost threshold to say 25 years (i.e. by halving it), but then the arbitrariness of these demarcations becomes even more obvious. The closer the gap between lost and long-lost, the easier it is for us to completely ignore these purely temporal thresholds, and thus ignore taxa that we are not personally interested in. Even though there are far more non-vertebrates that are lost than vertebrates. Not to mention the fact that it would make the aggregation of such temporally-demarcated lists far more complicated. We also have to take into consideration the amount of money and resources that we can amass in order to conduct the necessary surveys. The more often taxa are in need of rediscovery, the shorter the onto-epistemic cycle, the more money and resources we need to allocate to them. This then allows for even more bias to enter the fray in terms of the allocation of funds. Given the arbitrariness of these temporal criteria, taxa that have simply gone unrecorded for an extended period should only be considered minimally rediscovered, in contrast to those taxa that were actively searched for many times and on the brink of extinction. I have elsewhere proposed a taxonomy of rediscovery (see my article entitled 'Exscovery' for the technical details) with nominal categories1. The first category of rediscovery is 'relocation', which has only a temporal qualifier. In contrast with the other two categories (viz. rediscovery s.s. and exscovery) for which survey efforts and extinction risk are the qualifying criteria to determine which category applies. Allocation of funding should thus be geared towards extinction risk, which will hopefully help to correct current biases towards vertebrates.
Given the current biodiversity crisis and its Pandora's box of threats, some obvious and static but many difficult to detect and spreading, even in the most absolutely pristine habitats (as far as we are aware) we are obligated to confirm persistence every so often so as to refute that theoretical scenario of extinction no matter how improbable we consider it. However, this temporal threshold should not come with the automatic designation of rediscovered if successful contact is made. One could easily argue that this singular temporal threshold (in contrast to the tri-fold one in the current model) should be 50 years (or even less), but the list of taxa that have gone unrecorded for a century (<1926) is already massive (many 1,000's), and bringing it down to 50 years would expand the list many times over. Thus if we are truly hoping to accomplish something with this kind of temporally-orientated project then we need to carefully engineer the number and values of the temporal thresholds so as to produce a manageable list of qualifying taxa that we can earnestly work through, one that is not marred by bias, including effective publicity for each taxon so as to raise awareness in case they are encountered by chance. The singular concept of a centurion taxon, one not recorded in at least 100 years, is an excellent starting point for such a temporally-orientated project. Though clearly we want to whittle it down absolutely as far as we can in the future. Thus the concept of a centurion taxon is merely an initial stepping stone towards a theoretically ever-shortening temporal threshold. Thus we must excise rediscovery from the picture otherwise the threat of the massive biaes detailed above will roar back into the fray via a slippery slope.
Rediscovering rediscovery
2) Our conception and understanding of rediscovery is negatively affected by the proposed model
If we allow taxa to qualify as rediscovered simply for having gone unrecorded for five years ('missing') followed by a record, then the same taxon can clearly be rediscovered again and again and again. Especially if we retrospectively backdate the policy to include every single scientific record and the negative reality of the gaps present therein. We become accustomed to rediscovery, and even risk normalising it. Making it even harder to stir the public into action to combat the current biodiversity crisis, when the rate of rediscovery is being artificially inflated to many times that of declarations of extinction. Sadly fueled in part by the hitherto failure of conservationists to nominally distinguish between clearly different categories of rediscovery, even though they have long been recognised by conservationists. With the vastly different reasons that taxa may disappear from the scientific record, from mere lack of surveys to total extinction (Ladle et al., 2011), often being lost on those not familiar with the details of the individual cases of rediscovery. Moreover, a taxon may habitually disappear from the scientific record for the same reason (e.g. ecology, cryptic behaviour, difficulty of field identification), leading us to be less concerned about its future disappearances. But if it disappears next time for a different, much more serious reason we may be reluctant to search for it yet again, potentially letting it slip into extinction. Assuming that the reason/s for its disappearance are made known. Because one study found that roughly half (c.50%) of plant and animal taxa assessed as Extinct or Extinct in the Wild by the IUCN RedList used 'long time missing' as evidence, but only 12-28% of accounts mentioned survey/search efforts and 15-27% did not indicate why the assessors evaluated the taxa in those conservation categories (Fisher & Humphreys, 2024). While taxa in these two conservation categories are by definition excluded from being lost taxa (Long & Rodríguez, 2022), this bolsters my claim that there is a broader trend among conservations of treating time missing in and of itself as central to conservation assessments, and also evidences a major shortfall in conservationists publicly conveying the justification for taxon assessments. The latter clearly negatively affects public concern for lost taxa.
Often we do not know precisely why a taxon has disappeared from the scientific record, with so many knowledge shortfalls at play (Hortal et al., 2015). Many taxa are widespread and common, but many others are highly localised and often inherently difficult to study. But when we do have a reasonable idea they should not just be triaged in accordance with best practice and cutting-edge knowledge, but that this should be adequately conveyed to the public. Or at least made accessible to them should they wish to consult the relevant online resources. After all, it is increasingly being recognised that citizen scientists can (and do) play an important role in biodiversity monitoring and recording. Notably, this includes population trends, migratory behaviour, range extensions, range expansions, and even the discovery of novel taxa and the rediscovery of others. Through surveys, oral and written reports, and photographic records via online platforms like iNaturalist and BugGuide.net. But the potential of citizen scientists to meaningfully contribute to conservation biology will not be fully realised until we build frameworks for them to actively participate in conservation work beyond mere opportunism. This includes educational resources that intellectually lift them up sufficiently for them to be able to autonomously read the peer-reviewed literature. Unlike many other subjects, the sheer importance of the natural world means that conservation biology should be the earnest remit of every single person on Earth. It is thus essential for us to get across the fact that rediscovery is a spectrum with three distinct zones.
The first zone, relocation, relates strictly to temporality. It is met when a taxon is recorded on the first search attempt, or prior to this serendipitously. Perhaps as part of a broader biological survey of the area. While the other two zones pertain more objectively to previous failed survey efforts and extinction risk. The second zone is occupied by rediscovery s.s., which minimally requires at least one failed taxon-specific survey to have taken place for the taxon. Though it could have been rather informal in nature, and therefore it is variable as to how much we can infer about the actual conservation status of taxa from such failed surveys. Thus in our search for lost taxa that have been missing for more than a decade, it may take us multiple rounds of surveys to actually record them alive, in which case relocation clearly does not apply. Although taxa that we know are extant in a particular area can also be missed by methodologically appropriate surveys. It is only when surveys are exhaustive that we can have a clear picture of the conservation status of a taxon, assuming that all potentially suitable habitat has been surveyed. But rediscovery, when it occurs, is hopefully the start of a conservation process as many rediscovered taxa remain highly threatened long thereafter (Fisher, 2011; Scheffers et al., 2011). But there are many lost taxa that have never been searched for and likely are not threatened, especially those from pristine habitats (Ladle et al., 2011). If we do not start to meaningfully distinguish between the different categories of rediscovery then cases of mere relocation, including potentially many instances involving the same taxon, will flood the internet and other information sources, making it seem like cases of rediscovery in which a well documented historical decline to near extinction has taken place are much rarer than they are. This will give the completely false impression that the biosphere is much healthier than it is. The true significance of rediscovery lies in the conservation potential that it uncovers, and can show that a taxon thought lost forever (or nearly lost) can in fact bounce back to healthy population levels given the right conservation plan, funding and conservation actions. Whereas many relocated taxa were never really thought to be extinct (or even non-viable), and thus their rediscovery is far less important (though still significant) as nobody seriously doubted the taxon's viability and thus the conservation potential of the population/s.
Conservation potential
3) Rediscovery is only valuable when there is conservation potential
One of the issues we face when organising a search for a lost taxon is the accuracy of locality and ecological (including phenological) data. Knowing where to look, and when, is vital to the success of such searches, assuming that the targeted taxon is still extant. Taxonomic data can also be very important, allowing us to know exactly what we are searching for (e.g. Balchin, 2007:7). All of this relates to locating those population(s) of lost taxa that have previously been recorded, but it is also perfectly relevant to locating additional populations which may exist but have not yet been discovered by science (though many are surely ethnoknown). The 2019-2020 bushfire season here in Australia was one of the worst on record, so bad in fact that there was widespread discussion of possible global extinctions as part of the devastating effects. According to Marsh et al. (2023), 382 invertebrate species had their entire known range destroyed in the fires, but because these taxa were only known from one or two records the true overlap of the fires with their actual ranges is not currently known. It is likely that many of the 382 species are extinct, since their true range even if much broader than that known may still have been encompassed by the enourmous fires. But many may still be extant in the form of hithero undocumented populations, each of which is of conservation importance if they are viable. Thus we do not simply want to rediscover lost taxa, we want to rediscover each and every known/suspected population of every taxon, and to also search for any undiscovered ones too. Yet the overly thin concept of lost taxa does not adequately take into account the individual populations that collectively comprise a taxon. Rediscovering a lost taxon by showing that one or two individuals from a single population technically survive, does not show that the taxon is viable.
The study of these "first" individuals may help our understanding of the biology, physiology, ecology, behaviour, habitat preferences, etc., of the taxon, depending upon our prior level of knowledge, and thus help inform search methodologies for other possible surviving individuals. But until there is sufficient evidence of viability, including the three markers already mentioned, any rediscovery (whether reported or not) is arguably nominal and premature (see 'Introduction' above). Because it is epistemically dangerous to assume that other individuals must exist, given that we are in a global biodiversity crisis. It is true that taxa exist in a non-viable state for a geological instant when compared with their long evolutionary history, rendering it statistically near certain that it would be viable if we were to visit it on a random day along its evolutionary path. However, it does not follow that almost all of the taxa extant at a given time are viable, because of the potential for clustering of non-viability across many taxa simultaneously, either stochastically or during a biodiversity crisis. The only real concern can be as to how likely it is that we would ever find any individuals from these non-viable taxa2, given that their population levels would be inherently tiny. But methodologically appropriate taxon-specific surveys are highly efficacious by their very nature, as they explicitly target a single taxon by taking into account it's biology, ecology, phenology, etc., and creating an ideal method of detection. Thus the failure of a given search effort for these stipulated last individuals is not down to any theoretical deficiency with the concept in general, but rather with practical factors such as its application to a particular taxon for which we lack key knowledge, funding, thoroughness, etc. In reality, we do not know which lost taxa are viable and which are non-viable (and which are totally extinct), so that if we do succeed in finding a few individuals we need to continue searching to see whether these are the last of their kind, or an arbitrary few from a much larger population. We need to find out whether there is conservation potential present, and if it turns out that there is, then and only then should we declare a rediscovery. Lest we give false hope, do public damage to conservation biology, and continue to be misguided and thus potentially make other mistakes.
Lost taxa are by definition not considered Extinct, but Long & Rodríguez (2022) nevertheless uphold the IUCN RedList's 'Extinct' conservation category, and thus by implication the rediscovery of lost taxa rules out the applicability of that conservation category. But until the taxon is shown to be viable, there has been no meaningful rejection of functional extinction, which is clearly the pertinent sense of extinction. The discovery of a couple of extant individuals from a functionally extinct population may allow for genetic samples to be taken, which can have both epistemic and ontic benefits, but these are of extremely limited utility in service of the conservation of the taxon. To argue otherwise is to double down on the thinnest possible notion of 'Extinct' beyond all possible ecological soundness. To maintain that functionally extinct populations are not meaningfully extinct is to claim that there is still conservation potential within these very last individuals. The only logically possible potentials being, (a) deliberate hybridisation to salvage some modicum of their genetic legacy (e.g. Garnett et al., 2011), (b) cloning to increase the population (e.g. Folch et al., 2009), or (c) de-extinction. But none of these is a major answer to the unfolding crisis. The widespread loss is happening at many biological levels: alleles, genes, populations, taxa, functional traits, ecosystem services, etc., confounded by changes in abundance and composition, and subject to extinction debt. So bringing back a few dozen vertebrate taxa that make ideal candidates for de-extinction is not going to meaningfully contribute to its avertion when the plants and invertebrates they interacted with are forced to stay extinct. Assuming that de-extinction is even metaphysically possible in the first place, thus it is not clear that there is any real conservation potential involved. While hybridisation deliberately drives pure individuals extinct in order to ensure that some of their genetic makeup survives, and thus at best constitutes quasi-conservation potential. That leaves cloning, which by its very nature produces genetically identical individuals that are especially susceptible to diseases, and inhibits adaptation which is needed in the face of climate change. Thus none of the three re-viability processes reliably injects conservation potential back into non-viable taxa and their doomed populations.
Lost taxa and ex situ care
Not all Extinct in the Wild taxa have equal potential to be reintroduced to the wild (ex situ) or to have heavy management removed (in situ), nor is either a guarantee that a viable population will remain so. Thus the conservation status of a taxon in the wild (and not being heavily managed) is vital, irrespective of whether or not it exists in captivity or is being heavily managed. Yet Long & Rodríguez (2022) explicitly preclude Extinct in the Wild taxa from being lost taxa, even if there is no real chance of reintroduction or removal of management. This fails to appreciate the importance of unmanaged wild populations, and helps to prevent non-researchers from learning a valuable conservation lesson. This is one last way in which the concept of lost taxa is ecologically unsound, in addition to the three major problems arising from its deep association with rediscovery.
Conclusion
The broader concept of looking for taxa that have not been recorded by science for years, and often for many years, is itself highly laudable. However, this does not mean that any approach whatsoever will achieve the desired goal. There is more to consider here than just the target taxa. Public perceptions of the current biodiversity crisis can be impacted by many factors, but especially science communication in its various forms and via different mediums. Unfortunately, the concept of lost taxa does not bode well for public understanding of the current biodiversity crisis because it suffers from three major flaws. Firstly, it treats arbitrary temporal threshholds as preeminent, which allows for biases in search efforts and public, private and research funding. It also takes focus away from the real metrics: threats and extinction risk. Secondly, it artificially lowers the bar for rediscovery to take place by introducing a new minimum criterion, merely five years without a record, with no further justification needed. This unjustifiably broadens the concept of rediscovery, which negatively affects our understanding of rediscovery by skewing the relative rates of mundane versus serious cases in favour of the former. And thirdly, it glosses over the importance of viability/conservation potential by allowing for rediscovery to take place even if only a single individual is found with no evidence of any others. It therefore openly welcomes impostor cases of rediscovery masquerading as legitimate rediscoveries, which provides us with false hope and does damage to the public reception of conservation biology.
It does not bode well for some scientists either, who have not only proposed such a flawed model but actively promoted it as well. This flawed thinking and action is part of the reason why the current biodiversity crisis is so grim. Our response as a species has been completely inadequate, from theory right through to practice and beyond. We are continually making innumerable localised attempts to solve a global problem, with so much division within conservation biology and beyond that is slowly sawing off the very branch we are sitting on of the Tree of Life. And it all starts with the philosophy of conservation biology. Making sure that we have a sound understanding of the basic concepts at play, and upholding these through experimental design, practical research, technical publication and science communication. Central to all of this is ecology, the study of how species make their living from the wider environment. Any factor that can negatively affect ecology (or our understanding of it) needs to be carefully delineated, described and studied. Central to this is rediscovery, which shows that a particular taxon is either more ecologically robust than we thought, or we prematurely gave up on it. And everytime we write off a taxon only for it to re-enter the biosphere, we should be reminded of how careful we need to be when proclaiming the conservation status of taxa. But instead of distinguishing between different categories of rediscovery, which are vastly different in nature and implications, the idea is being promoted of consistently rediscovering taxa decade after decade. Scientists are potentially crying wolf about many taxa, so that when they really do disappear nobody believes them. And there is no evidence that we can really de-extinct any species, or scientific careers for that matter.
Notes
1 To date, prior to my own attempt to create a taxonomy of rediscovery and nominally distinguish them, I am only aware of the attempt of Ladle et al. (2011) to do so. But their typology was simply from (1)-(4), "based on decreasing level of certainty that the rediscovered species was extinct" (Ladle et al., 2011). Unfortunately, there has been no real uptake of their system of more than a decade ago, possibly because it was numeric rather than nominal, although that is pure speculation on my part. Given the fact that taxa can be prematurely declared extinct, there is no real distinction between their category (1) and category (2), leaving three categories like my own. They do suggest a possible fifth category, namely those unrecorded/lost taxa that are elevated from sub-species to full species. But regarding the latter, I think we should rename subspecies as proto-species, firstly because this is more accurate from an evolutionary point of view. And secondly, this will help to bridge the taxonomic and conservation attention gap between species and subspecies.
2 This concern is logically equivalent to the assumption that whenever we find a tiny number of individuals of a taxon, others exist. Because from this it follows that there are no cases where we find an individual but there are no other individuals extant. Thus the two are logically equivalent. But putting it the way I have makes the discussion and answer much clearer.
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