No viability? No rediscovery!

By Branden Holmes. Published on 27 April 2025.

Introduction

Given the rapidly declining state of the world’s biodiversity, any global rediscovery s.l. of a taxon is very important, not just for its conservation but for other reasons too (Diesmos et al. 2005; Ladle et al. 2011; Vilela et al. 2022). Yet reporting that one or two individuals of a taxon that is considered (possibly) extinct are in fact extant should not immediately overturn its assigned conservation status. Every genuine disappearance from the human record is caused by a population decline whose precise severity is unknown, while rediscovery itself does not abate threats, so until the population is shown to be viable it is not clear that the taxon has actually avoided a singular path to extinction. Such premature pronouncements of global rediscovery aren’t meaningful, may provide false hope, and artificially decrease the gap between the current extinction rate and rediscovery rate. The assumption that other individuals exist is epistemically dangerous.

The critical importance of biological viability

Intrinsic viability here refers to any inter-individual biological unit (species, subspecies, population, clade, etc.) that theoretically has the intrinsic potential to survive indefinitely into the future (in generational terms), not factoring in the extrinsic conditions it faces in the real world. A single individual of a sexually reproducing taxon clearly lacks such potential (unless parthenogenetic or a self-fertile hermaphrodite), and thus is no better chance to survive >0 number of generations into the future than a wholly extinct one because it is impossible for both to do so (despite one of them having an extant individual). There is clearly therefore not just a non-zero lower bound for the size of a population of a sexually reproducing taxon in order for it to be able to persist indefinitely, generation after generation, into the future. Other criteria must be met that collectively reach the necessary conditions for intrinsic viability (e.g. minimum population size, reproductive connectivity, genetic diversity). This necessary (but not sufficient) condition for the long-term persistence and indeed recovery of a taxon has not been lost on scientists, with rediscovery sometimes being explicitly or implicitly couched in terms of the population rather than individuals (e.g. Keith & Burgman 2004; Humphreys et al. 2019; Toledo et al. 2023). Yet at least 12 papers (representing 11 taxa) published in 2022 reported only a single individual (e.g. Sumanapala et al. 2022; Turner & Bradley 2022; Wood & Walsh 2022), and a further 12 papers (representing 13 taxa) reported collecting every individual they found for preservation/study (e.g. Huang et al. 2022; Jałoszyński 2022; Vilela et al. 2022). Meaning that for 24 taxa there was no more than a single known extant individual at the time of publication (see Appendix). There is therefore no evidence presented that these taxa have been meaningfully rediscovered as they have not been shown to be intrinsically viable.

On the other hand, vegetative reproduction and self-compatibility in plants, and self-fertile hermaphrodism and parthenogenesis in animals, means that such taxa remain intrinsically viable even when down to a single individual. Yet despite this intrinsic viability, the practical risk that they or their offspring will die for any number of reasons before successfully reproducing is unacceptably high. Having genuinely declined because of one or more threatening processes (or stochastic events), these remain as extrinsic factors (when not abated, such as via ex situ conservation or by heavily managing the population) that will almost certainly prevent the theoretically minimum number of individuals required for intrinsic viability (this is the theoretical lower bound) from actually being viable over time in the real world. To this end, the concept of a Minimum Viable Population (MVP) has been very important (Nunney & Campbell 1993), which seeks to quantify the "needed" number of individuals such that the population will have an acceptably high chance of surviving a certain number of years in the future, and is therefore probabilistic in nature (this is the probabilistic lower bound). Unfortunately, precisely quantifying the MVP for a taxon is extremely difficult because it differs from taxon to taxon (Reed et al. 2003), and through time as each taxon’s genetic/demographic/connectivity/threat profile changes. While various models and assumptions have been/are/can be used to arrive at "the" MVP for a population, and it is important to acknowledge this diversity. But the general point here is that there is both a theoretical lower bound (i.e. intrinsic viability) and a probabilistic lower bound (i.e. the MVP), no matter how disputed the latter may be, and so we can simply bracket off the specific/disputed details.

Proto-viability

The probabilistic lower bound by its very nature cannot be lower than the theoretical lower bound, and given the nature of the myriad factors that negatively affect population size through time the probabilistic lower bound is invariably significantly higher than the theoretical lower bound. Conservationists in practice seek to conserve significantly more individuals than the theoretical lower bound as a buffer against extinction, and optimally will create further insurance populations if feasible. But the difficulties inherent with quantifying either lower bound presents an epistemic problem that affects the publication process and thus presents a practical problem. The demand to arrive at a full blown MVP value for a population (since the probabilistic lower bound is more important in practice), and then show that the population size actually meets that mark, would be extremely onerous on those seeking to publish peer-reviewed papers that report global rediscoveries. Regular publication of papers helps to disseminate knowledge and aid communication between researchers, which has benefits over fewer papers of a larger size, and helps to catalyse conservation actions/study into individual populations. Withholding these papers altogether would do active damage to the field of conservation biology. Therefore, the reporting of a global rediscovery should be reserved for cases where it has at least been shown that the taxon is comprised of a sizeable (sexually diverse, if applicable) population with active breeding and recruitment of young into the adult population, which we might term proto-viability. There are potentially many ways of defining what constitutes a "sizeable" population, but as long as it is sensible it will work well enough for the purpose. This then places less burden on those involved in the highly laudable exercise of trying to rediscovery viable populations of taxa thought to be (functionally) extinct, while still rightly holding them accountable to a satisfactory level by demanding that they meet a standard of evidence that a particular taxon has been meaningfully rediscovered.

By assuming that a taxon must be proto-viable even if we only find one or two individuals, we risk trivialising the concept of global rediscovery and inflating the perceived rate of global rediscovery relative to the rate of global extinction. Which would then make it appear as if the current biodiversity crisis isn’t as bad as it is. As well as potentially offering false hope, which if actualised can hurt the reputation of conservation biology, de-incentivising public donations, resources and co-operation. To remedy this situation, I propose a unique interim conservation category, quasi-extant, to label taxa that have been discovered to technically be extant after being considered (probably) extinct but have not yet been shown to be proto-viable. It is also applicable to taxa that have been continuously sighted/reported, but whose numbers are now suspected to be so low as to call into question whether they are viable any more. Once a taxon has been shown to remain proto-viable after initially disappearing it can then be meaningfully reported as rediscovered and assigned to the appropriate conservation category, such as ‘Critically Endangered’ or ‘Endangered’. Or if the taxon proves not to be proto-viable, it should then be listed as functionally extinct, at which point human intervention and technology may or may not be able to assist in the recovery of its proto-viability status (Folch et al. 2009; Albani Rocchetti et al. 2022).

The error of assuming other individuals must exist

The implicit reasoning behind such a high rate of reporting of global rediscovery in the absence of known proto-viability (or greater) is plausibly explained by the belief that it is extremely unlikely that the individuals discovered actually represent the entire global population, given (1) the existence of suitable habitat, (2) the fallibility of targeted searches, (3) the very short window during which populations survive in a non-viable state, and (4) when young are recorded it implies relatively recent reproduction by other individuals that likely still exist. Each point can be addressed to show that it does not have any serious force.

Suitable habitat (1) is a necessary, not a sufficient, condition for the existence of a proto-viable population. Habitat destruction/degradation is a leading cause of extinctions, but many other threats (e.g., introduced species, hunting/poaching, illegal collecting) leave the broader habitat largely intact while driving individual taxa extinct. Which can be exacerbated by concomitant emerging threats such as inbreeding depression due to loss of reproductive connectivity. Therefore, suitable habitat only potentially indicates the existence of a viable population and cannot be used to infer the actual existence of such a population.

The fallibility of targeted searches (2) is perhaps best illustrated by the phenomenon of rediscovery. Successive failure to find a taxon that we later record, and hence can retrospectively know that we missed it previously. But the generic potential to miss one or more individuals, which might not even make the population proto-viable anyway, is evenly matched on the other side by the clear potential efficacy of taxon specific targeted surveys to find individuals from extremely small populations or even lone individuals (e.g., Chelonoidis abingdonii, C. phantasticus, Nesopta elliptica, Stenostomum tomentosum). Such considerations need to be applied on a case-by-case basis for one to potentially eclipse the other.

The window will be very short (3) in terms of relative duration compared with the long existence of the taxon, but the actual duration can be centuries for extremely long-lived organisms like giant tortoises and trees, and years for many if not most other species. During which they can still consist of a small number of fertile individuals, and therefore reproduction may still take place, so the mere observation of recent reproduction in a population (4) cannot be used to infer that it is still proto-viable. While the potential to find these last individuals is increased by the chance of luck, in addition to targeted surveys that may involve many people. Moreover, since we are in the midst of a biodiversity crisis, there will be many thousands of taxa that dip below a proto-viable level, so that even if any single population goes extinct rapidly in a non-viable state, there are so many such populations that the chance of finding individuals from non-viable populations in general is a virtual certainty.

Conclusion

Global rediscovery is not as simple as showing that a single individual remains extant. The ecology of a population is an important but often overlooked factor when reporting rediscoveries, which places inherent limits on how few individuals can manage to constitute a viable population. While the threats that have historically affected the taxon, and contributed to its decline and disappearance, often remain. While others can emerge as compound effects of other threats, such as inbreeding depression and population fragmentation. For putatively extinct taxa, and those considered functionally extinct, the discovery of one or more extant individuals should not move the needle much in terms of the conservation status of the population, and should instead be treated as quasi-extant until such time as they are shown to be proto-viable. At which point, they can be meaningfully reported as globally rediscovered, although this is very much the beginning of research and conservation, and certainly not the end.

Appendix: Reported global rediscoveries published in 2022.

References

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