Exscovery: Rediscoveries that refute global extinctions are special cases

By Branden Holmes. Published on 19 April 2025.

Introduction

The term ‘rediscovery’ in its global sense refers to the discovery of an extant population of a taxon that was not known to survive (or in rare cases, was thought to be functionally extinct). Three broad categories can be distinguished: (1) taxa that simply went unrecorded (hereafter ‘unlocated’) (e.g. Okutani, 1982; Gong et al., 2020); (2) taxa that had been unsuccessfully searched for in the past but were not regarded as (possibly) extinct (hereafter ‘lost’) (Long & Rodríguez, 2022; Lindken et al., 2024); and (3) taxa which were justifiably regarded as (possibly) extinct (e.g. Kandemir, 2009; Biton et al., 2013) as a result of adequate surveys (Fisher & Blomberg, 2011; Penedo et al., 2015). The first category is based on an arbitrary criterion, and the second is minimally informative, whereas the third is of immense importance. The critical distinction between these three categories is failed by the current reliance on the single cover-all term 'rediscovery'. This calls for a new term (‘exscovery’) to distinguish those special (and much rarer) cases of rediscovery in which a justified belief or claim of (possible) extinction is actively refuted.

Relocation

Category (1) treats time since last record as intrinsically meaningful, but this arbitrary criterion has rightly been rejected in its guise as the so-called ‘50-year rule’ introduced by the Convention on International Commerce of Species of Wild Flora and Fauna (CITES) to determine when to declare a taxon extinct (see Penedo et al., 2015). It should be noted that there are also temporal variants of category (2) (see Long & Rodríguez, 2022). There are no unsuccessful searches for unlocated taxa prior to their rediscovery, which are either relocated on the first attempt (e.g. Gower et al., 2006), or relocated through serendipity. There are at least two independent reasons to consider relocated taxa as bona fide rediscoveries s.l.¹ (i.e. in the broad sense). Firstly, it is important to create baselines which will help enable us to detect future shifts and other net changes in biodiversity composition and abundance. But many unlocated taxa are known from very few specimens (Srivastava et al., 2015), which often do not currently allow for reliable population estimates (Lindken et al., 2024). And secondly, the extinction of a particular taxon is not contingent upon human knowledge of its current conservation status or the threats it faces. This is evidenced by the many so-called ‘dark extinctions’ that have presumably taken place (Boehm & Cronk, 2021). Confirmation of the persistence and population health of unlocated taxa is thus important, even if the categorization itself is arbitrary.

Rediscovery s.s.²

Category (2) is minimally informative because even taxa known to be extant in a particular area can fail to be recorded there by methodologically and seasonally appropriate surveys. Moreover, there is no guarantee of the overall quality of the search methodologies employed previously, which may have been rather informal. While many taxa have life history/demography/ecology traits (e.g. fossorial, fire ephemeral) that mean that they are expected to be rarely encountered anyway (Butchart et al., 2006; Fisher & Blomberg, 2011; Bell, 2021). At its closest to category (3), a lost taxon is only one adequate but failed survey away from being considered (possibly) extinct. The possible motivations to search for a taxon and thus disqualify it from category (1) are many and varied. Given our limited resources, it could be argued that it is best to only target those taxa most likely to be at risk of extinction. But the vast gaps in our knowledge means that there may be many unlocated taxa [category (1)] at great risk of extinction that have never been searched for, while lost taxa [category (2)] which have been unsuccessfully searched for may not be at risk of extinction at all. The need for baselines and the fact that extinction is knowledge-independent apply equally to category (2). Confirmation of the persistence and population health of lost taxa is thus important, but no more so by default than those taxa from category (1).

Exscovery

Category (3) is akin to epistemic de-extinction in that it refutes the real possibility (and not merely the hypothesis) of global extinction and is thus of the utmost importance. Yet it also raises questions. Whenever an officially extinct taxon is rediscovered s.l., it needs to be determined whether the extinction criteria themselves or their misapplication was to blame for the premature declaration. But whatever the circumstances that lead to the premature declaration, the fact that the taxon went unrecorded despite extensive surveys means it is almost certainly threatened with extinction and will likely remain that way for generations to come (Fisher, 2011; Scheffers et al., 2011). It may even be functionally extinct. Such taxa should be considered of the highest conservation priority, and yet to date there has been no nominal distinction made between any of the three categories outlined here in the literature. I therefore introduce the new term ‘exscovery’ (a portmanteau of ‘extinct’ and ‘rediscovery’) for taxa that belong to category (3).

This has the added benefit of making it far easier to find and track publications (and other materials) that report these highest and most profound rediscoveries s.l. through title and keyword searches via search engines. Instead of having to filter the much larger literature on rediscoveries in general, which includes innumerable non-global cases (i.e. local, in situ, or ex situ) (Holmes, 2024). This will help us to aggregate these cases, because exscovery has the deliberate benefit of allowing direct comparison between the total number of taxa that are currently considered (possibly) extinct with those that have been removed from there through discovery in the wild or ex situ. This is part of the broader project of looking into the causes and extent of turnover in lists of putatively extinct taxa (e.g. Keith & Burgman, 2004). And will ultimately help us to answer one of the most fundamental and perpetual questions in conservation biology: is the current rate of extinction slowing down or not?

Downplaying rediscovery s.l.

Taxa that are not known to survive can nevertheless still be justifiably believed to persist (Ladle et al., 2011), especially if they occur in known pristine habitat. Under the current scheme, we expect a proportionally high rate of rediscoveries s.l. relative to the rate of new declarations of (possible) extinction. The number of taxa for which there have been no records for decades or even centuries is very large (Long & Rodríguez, 2022; Martin et al., 2023; Lindken et al., 2024), with many having not been adequately searched for. Whereas taxa should only be considered (possibly) extinct if there have been adequate surveys that have failed to record them (Penedo et al., 2015). The concept of rediscovery s.l. is thus much broader than that of global extinction, and so there can be no direct comparison between them.

The public do not read the scientific literature for the most part and are therefore unlikely to be sufficiently informed about this. Potentially leading to unjustified public doubts about the severity of the current biodiversity crisis. If the public perception is that there are roughly as many taxa being profoundly rediscovered as are being declared extinct, then there is the risk that the public will think that we are not in fact in the midst of a biodiversity crisis, potentially extending to deep scepticisim of conservation biologists themselves and their competence. Especially if the rapid rate of discovery and description of new species, stories of which are often posted to social media, is additionally mistakenly interpreted to imply that the biosphere must be very healthy otherwise there would be far fewer taxa still extant to even discover and describe in the first place.

Beyond hypothetical scenarios regarding what the public may or may not understand or think, my anecdotal experience of global rediscovery stories posted to social media is that on occasion a user expresses their doubts as to the severity of the alleged biodiversity crisis in the face of “yet another” rediscovery. And since not everybody who reads a social media post interacts with it, this leads me to believe that there may be a significant level of public skepticism regarding the reality and extent of the current biodiversity crisis. By adopting the term ‘exscovery’ for profound cases of rediscovery, and spreading awareness about it, we can avoid potentially damaging public misperceptions of the seeming proliferation of global rediscoveries, all of which may falsely appear to be equally significant (Ladle et al., 2011).

Playing up rediscovery s.l.

Any taxon that has no known extant population can theoretically be extinct, even if its habitat seems pristine. Thus rediscovery s.l. is important so as to refute the extinction hypothesis (Ladle et al., 2011). The paucity of information (for little known taxa) or gaps in our knowledge (for better known taxa) can then be supplemented by new data, and thus we have the opportunity to more accurately assess the conservation status of a taxon. While this can also generate other relevant data on biogeography and population trends (Ladle et al., 2011). On the other hand, there are relevant differences between the three categories. Category (1) can only tell us about the status of the taxon. Category (2) tells us about the status of the taxon, and may (or may not) be able to tell us something about our search efforts. Category (3) tells us about the status of the taxon, and tells us a lot about our search efforts and the false conclusion we reached on their basis (or the interpretation of which). An important conservation lesson given the reality of the current biodiversity crisis.

Conclusion

The rediscovery s.l. of any taxon is important, even if it was believed to still exist despite the lack of records. At the same time, current usage of the term ‘rediscovery’ in the scientific literature (and beyond) is far too broad in the absence of distinct categories. This is artificially inflating the rate of rediscovery relative to global extinction, and risks appearing to the public that the current biodiversity crisis isn’t anywhere near as bad as us scientists are making out. By crafting a new term to distinguish those genuinely profound cases of rediscovery where global extinction is refuted, it helps to explicate the broadness of the term ‘rediscovery’ as currently used in conservation biology relative to the much narrower use of ‘extinct(ion)’. And therefore, the current relative rates of the two are not at all comparable, and thus a high rate of rediscovery s.l. should have little to no bearing on either professional or public perceptions of the severity of the current biodiversity crisis.

Notes

¹ s.l. is an abbreviation for sensu lato, which means "in the broad sense". It may be helpful to think of the lines of latitude across the globe when you see "lato".

² s.s. is an abbreviation for sensu stricto, which means "in the strict sense".

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