We are currently experiencing an unprecedented loss of biodiversity. Species are disappearing at an alarming rate, and many more are on the brink of extinction. Among these, a peculiar category has emerged: ‘lost’ species. These species exist in a state of uncertainty, neither confirmed extinct nor recently observed.
The Extinction Crisis: A Numbers Game
Quantifying species extinction is a challenging task. The exact number of species declared extinct each year is difficult to pinpoint. This is partly because it is almost impossible to monitor all known species, but also because we don’t even know how many species currently exist on Earth.. Despite these uncertainties, estimates of current extinction rates are between one to ten thousand times greater than the background extinction rate – the natural rate at which species go extinct.
To put this in perspective, the background extinction rate is approximately 0.1 extinctions per million species per year1, which translates to just under one species per year. In contrast, current estimates suggest that between 10,000 and 100,000 species may be lost annually2. The scale of this biodiversity crisis becomes even more apparent when we consider that between 16% and 50% of all species since 1500 are either threatened or already extinct3.
It’s important to note that these figures might actually underestimate the true extent of global biodiversity loss. There are significant knowledge gaps in understudied taxa and regions, meaning that many species may be disappearing before we even discover them.
The Challenge of Declaring Extinction
Determining whether a species is truly extinct is far more complex than it might seem at first glance. It’s rare, and often impossible, to know with certainty that the last individual of a species has died. To make such a determination, we would first need to know that the species exists, and second, we would need to be actively monitoring its population.
The absence of sightings or evidence doesn’t necessarily mean a species is extinct; it might still be alive in remote or unexplored areas. Prematurely declaring a species extinct can have negative consequences. This can undermine conservation efforts as “extinct” species being rediscovered could lead to the public thinking the problem isn’t as bad as it is. Further, it could lead to important funding being removed from species or regions desperately needing it.
To address this challenge, the International Union for Conservation of Nature (IUCN) has developed rigorous guidelines and models to determine extinction to a very high degree of accuracy. While this approach ensures scientific integrity, it also results in an increasing number of species that haven’t been observed for many years or even decades, yet cannot be officially declared extinct. This is where the concept of ‘lost’ species comes into play.
The Enigma of ‘Lost’ Species
Lost species occupy a unique space in the realm of conservation biology. These are species that have not been reliably observed for at least a decade or more. Some researchers use an even more conservative definition, considering species as lost if they haven’t been seen in 50 years or more.
Using the 50-year criterion, there are an estimated 562 lost terrestrial vertebrates. This includes 257 reptiles, 137 amphibians, 130 mammals, and 38 birds. Strikingly, over 90% of these lost species occur in tropical regions, highlighting the urgent need for conservation efforts in these biodiverse areas4.
If we broaden the definition to include species not reliably observed in ten years or more, the number of lost species increases dramatically to over 1,6004. Recent estimates for birds, which are among the best-studied taxa, suggest there could be as many as 144 species considered lost5.
The implications of these numbers are sobering. A significant proportion of lost species are likely at high risk of extinction, and some may already be extinct without our knowledge. Of the terrestrial vertebrates not seen in 50 years or more, 13% are considered possibly extinct by the IUCN.
The Importance of Continued Search and Conservation
The concept of lost species underscores the critical importance of ongoing biodiversity research and conservation efforts. We cannot work to protect what we don’t know. These elusive species represent both a glimmer of hope and a stark warning. They offer the tantalizing possibility of rediscovery, as has happened with several species in recent years. At the same time, they serve as a poignant reminder of the fragility of life on Earth and the devastating impact of human activities on natural habitats.
Their story challenges us to intensify our conservation efforts, to protect habitats, and to never cease in our quest to understand and preserve the rich tapestry of life on Earth. Each lost species represents a unique evolutionary history and ecological role, and their potential rediscovery could provide valuable insights into resilience, adaptation, and survival in a changing world.
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References and further reading
- De Vos, J.M., Joppa, L.N., Gittleman, J.L., Stephens, P.R. and Pimm, S.L. (2015) Estimating the normal background rate of species extinction. Conservation Biology, 29: 452-462. https://doi.org/10.1111/cobi.12380
- https://wwf.panda.org/discover/our_focus/biodiversity/biodiversity/
- Isbell F, Balvanera P, Mori AS, et al (2023) Expert perspectives on global biodiversity loss and its drivers and impacts on people. Frontiers in Ecology and the Environment 21:94–103. https://doi.org/10.1002/fee.2536
- Martin TE, Bennett GC, Fairbairn A, Mooers AO (2022) ‘Lost’ taxa and their conservation implications. Animal Conservation acv.12788. https://doi.org/10.1111/acv.12788
- Rutt CL, Miller ET, Berryman AJ, et al (2024) Global gaps in citizen-science data reveal the world’s “lost” birds. Frontiers in Ecology and the Environment n/a:e2778. https://doi.org/10.1002/fee.2778