Which animals are threatened with extinction? These 8 species have fewer than 100 individuals

Species extinction is rarely an abrupt event. In many cases, the decline stretches over decades until only small, isolated remnant populations remain. Some of these species now survive with fewer than 100 individuals worldwide.

Such extremely small populations are especially vulnerable to chance events, genetic impoverishment, and ongoing habitat loss.

8 animal species whose populations, according to current knowledge, are below 100 individuals:

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Cherry-throated tanager: fewer than 30 birds

The cherry-throated tanager (Nemosia rourei) was scientifically described in 1870. After that, it disappeared from the view of research for decades, was briefly rediscovered in 1941, and then was again considered lost until its rediscovery in 1998.

Today, this passerine species is restricted to two small areas in the state of Espírito Santo in southeastern Brazil, especially in the regions of Castelo and Santa Teresa.

Since 2021, international conservation efforts involving the ZGAP, several zoos, and local partners have focused on the immediate protection of the remaining populations. These measures include monitoring individual nests, protection from predators, and the creation of a protected area covering around 3,400 hectares. They led to a slight increase in the number of known individuals, from about 15 birds in 2021 to around 23 today, but that still amounts to fewer than 30 individuals overall (as of 2026).

One central problem remains the limited knowledge of the species’ biology. Aspects such as feeding ecology, breeding behavior, and habitat use are currently being studied because they form the basis for possible future conservation strategies. There is still no captive breeding program; the cherry-throated tanager is not kept in any institution worldwide.

The causes of its decline are not fully understood. The most likely factors are the ongoing loss and fragmentation of the Atlantic rainforest as well as possible additional pressure from predation.

Devils Hole pupfish: around 38 fish

The Devils Hole pupfish (Cyprinodon diabolis), only 2 to 3.4 centimeters long, lives exclusively in the so-called Devils Hole, a small limestone pool measuring about 5 by 3.5 meters in the US state of Nevada. This pool lies around 15 meters below the surface and is fed by a warm spring in which temperatures of 32 to 38 °C prevail year-round.

The entire global population is confined to this single habitat. Accordingly, its numbers fluctuate strongly and usually remain in the range of fewer than 100 individuals. In some years, only a few dozen fish were counted. After a slight increase in recent years, 2025 brought a massive decline due to earthquakes, leaving only around 38 individuals.

The causes of these fluctuations are varied, but they always have the same origin: extreme dependence on a single, very small habitat. In the past, groundwater withdrawals had already caused water levels to fall. Although the area is now strictly protected and closed to the public, it remains vulnerable to disturbance, for example through vandalism or changes in water quality.

In addition, there are natural events that can have immediate consequences for such a tiny population. Earthquakes, sudden water movements, or temperature changes can destroy eggs, larvae, or food resources. Climate change is also already having an effect: rising water temperatures shorten the time windows in which eggs can develop successfully.

Northern sportive lemur: fewer than 70 animals

The northern sportive lemur (Lepilemur septentrionalis) is now restricted to a small forest area in northern Madagascar, the Montagne des Français. There, the last known population survives with only about 50 to 70 individuals.

Originally, the species was distributed across large parts of northern Madagascar. As early as 2007, the population was estimated at about 100 animals. Just a few years later, only individual animals could still be detected in former range areas; today, the species is considered gone outside its remaining core area. Overall, the population has declined by more than 80% in recent decades.

Its habitat consists of dry and semi-humid forests at low to medium elevations. These forests are now heavily fragmented and largely destroyed by deforestation. In particular, illegal logging for charcoal production has destroyed large parts of the original habitat and isolated the remaining forest patches from one another.

Besides habitat loss, hunting is also a threat. In the course of charcoal production, the animals are often caught opportunistically. However, targeted use within the increasingly urban lemur meat trade probably plays only a subordinate role for this species. The increasing fragmentation of the forests also makes genetic exchange between the remaining individuals more difficult. Today, only a single isolated population remains, and its habitat continues to shrink. Despite conservation measures, the population is still declining.

Seychelles sheath-tailed bat: fewer than 100 animals

The current population of the Seychelles sheath-tailed bat (Coleura seychellensis) comprises fewer than 100 adult individuals and is restricted to a few small colonies on the islands of Mahé and Silhouette in the island nation of Seychelles.

The species lives at low elevations and uses caves as daytime roosts. Worldwide, only four active roosts are known. Even the largest known colony comprises only about 16 to 46 bats, while smaller roosts consist partly of only a few individuals. The total population is declining.

Historically, the species was far more widespread and was considered common in the 19th century. Its decline began with the large-scale conversion of original forests into plantations. In the process, not only suitable habitats disappeared, but also the insects vital to the species as a food source.

Today, the Seychelles sheath-tailed bat is threatened by several factors at once. These include the ongoing loss and degradation of hunting habitats, disturbance of cave roosts by human activities, and invasive species. Introduced plants alter the vegetation and can overgrow cave entrances, while introduced predators such as cats, rats, or barn owls exert additional pressure. The use of pesticides can also contribute indirectly to the decline by reducing food availability.

Santa Catarina’s guinea pig: around 42 animals

The Santa Catarina’s guinea pig (Cavia intermedia) has lived for around 8,000 years exclusively on an island of only about 10 hectares in the Moleques do Sul archipelago off the coast of the Brazilian state of Santa Catarina. Although the island belongs to the protected area Parque Estadual da Serra do Tabuleiro, enforcement of the protection measures on site is limited. Despite its isolated location, it can be entered, and occasional hunting therefore cannot be ruled out entirely.

A long-term study of population dynamics estimated the Santa Catarina’s guinea pig population at an average of about 42 individuals. Everything indicates that the species has maintained a similarly small population size over long periods.

The greatest danger comes not from direct human use, but from the rodents’ extreme isolation. Because the entire population is restricted to a very small area and operates close to its ecological carrying capacity, growth is hardly possible. Although there is another small island in the archipelago that could theoretically be suitable for establishing a reserve population, no long-term viable wild population would be possible there either.

There is a real risk that a single chance event—such as a fire or a disease—could completely wipe out the species within a few decades.

Longnose stubfoot toad: fewer than 50 animals

The longnose stubfoot toad (Atelopus longirostris) was long considered extinct. After the species had last been recorded in 1989, it remained missing for more than two decades despite intensive searches. Only in 2016 were four individuals rediscovered in two small forest fragments in northwestern Ecuador.

Today, the longnose stubfoot toad is known exclusively from these two forest patches, which are only about one kilometer apart but completely isolated. Its entire current range covers only around four square kilometers. The remaining population is estimated at fewer than 50 adult individuals and is considered the only one still in existence.

Historically, the species was much more widespread in the western Andes of Ecuador and occurred in tropical montane forests along running waters. The drastic decline probably began already in the 1980s. A combination of habitat loss, climatic changes, and the amphibian disease chytridiomycosis, which has severely decimated numerous species of the genus Atelopus, is considered the likely cause. The Chiriquí harlequin frog (A. chiriquiensis), which went extinct in 1996, illustrates how quickly entire species can disappear under this pressure.

The population remaining today is acutely threatened by the ongoing destruction of its habitat. Agriculture, logging, and especially mining projects lead to further fragmentation and pollution of the waters. A planned copper mining project lies directly in one of the two known localities and could seriously endanger the species.

Because of the extremely small population, the tiny range, and the continuing threats, a further decline is expected. Even under favorable conditions, the species could disappear within a short time.

Sulu hornbill: fewer than 50 animals

The Sulu hornbill (Anthracoceros montani), about 70 centimeters long, is endemic to the Sulu Archipelago in the south of the Philippines and today occurs only on the island of Tawi-Tawi. Only in November 2025 was this rare hornbill species sighted there again.

In the past, the Sulu hornbill was distributed across several islands of the archipelago, including Jolo and Sanga-Sanga. The remaining population is estimated at fewer than 50 adult individuals, possibly even fewer than 20 breeding pairs. This makes the species one of the most threatened birds in the world.

The Sulu hornbill inhabits primary dipterocarp forests, mostly on mountain slopes. It feeds mainly on fruit and depends on large, old trees that serve as nesting sites. Ongoing logging has drastically reduced these forests and split them into small, separated remnants.

On Jolo and Sanga-Sanga, the original forests have now almost completely disappeared, which means the species is probably already extinct there. On Tawi-Tawi as well, the remaining forest areas continue to shrink. In addition, the birds are hunted or killed opportunistically, for example for food.

Today, the entire population is restricted to a few isolated forest areas. Despite local conservation measures—such as patrols against illegal logging and the protection of individual nests—the species is still considered to be at imminent risk of extinction.

Saint Lucia racer: fewer than 50 animals

The Saint Lucia racer (Erythrolamprus ornatus) is considered the rarest snake in the world. The species occurs exclusively on the small island of Maria Major, which covers only about 0.09 square kilometers and lies a few hundred meters off the coast of Saint Lucia in the Caribbean. The entire population probably comprises fewer than 50 adult individuals.

The species once also occurred on the main island of Saint Lucia and was considered comparatively common there. However, with the introduction of invasive species, especially the small Indian mongoose in the 19th century, the population collapsed. Since then, the Saint Lucia racer has been considered extinct on the main island. Only in 1973 was a small remnant population on Maria Major discovered.

Today, the Saint Lucia racer is entirely restricted to this single tiny islet. Strict conservation measures—including access restrictions and the consistent control of invasive species—currently secure the survival of the last population.

The greatest threat continues to come from introduced predators. If invasive species such as mongooses, rats, or cats were to reach Maria Major, the entire population could be wiped out within a short time. In addition, because of its tiny habitat, the snake is vulnerable to chance events such as droughts, storms, or fires.

In the long term, the population is considered not viable as long as it remains confined to a single island. For that reason, efforts are being made to create additional safe habitats and establish new populations. So far, however, the Saint Lucia racer remains entirely dependent on the protection of this one place.

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These examples show how close many species already are to extinction. Fewer than 100 individuals means that every loss can be decisive. At the same time, they show that targeted conservation measures can still have an effect even at this critical stage.

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Sources

• BirdLife International. 2018. Nemosia rourei. The IUCN Red List of Threatened Species 2018: e.T22722293A130617765. https://dx.doi.org/10.2305/IUCN.UK.2018-2.RLTS.T22722293A130617765.en
• BirdLife International (2020). Anthracoceros montani. The IUCN Red List of Threatened Species 2020: e.T22682447A178062684. https://dx.doi.org/10.2305/IUCN.UK.2020-3.RLTS.T22682447A178062684.en
• Bristol, R. & Bouillard, N. (2025). Coleura seychellensis. The IUCN Red List of Threatened Species 2025: e.T5112A271898609. https://dx.doi.org/10.2305/IUCN.UK.2025-1.RLTS.T5112A271898609.en
• IUCN SSC Amphibian Specialist Group. 2022. Atelopus longirostris. The IUCN Red List of Threatened Species 2022: e.T54522A98641996. https://dx.doi.org/10.2305/IUCN.UK.2022-1.RLTS.T54522A98641996.en
• Daltry, J. C. (2016). Erythrolamprus ornatus (errata version published in 2017). The IUCN Red List of Threatened Species 2016: e.T12080A115104404. https://dx.doi.org/10.2305/IUCN.UK.2016-3.RLTS.T12080A71739705.en
• Louis Jr, E. E., Bailey, C. A., Frasier, C. L., et al. (2020). Lepilemur septentrionalis. The IUCN Red List of Threatened Species 2020: e.T11622A115567059. https://dx.doi.org/10.2305/IUCN.UK.2020-2.RLTS.T11622A115567059.en
• NatureServe (2014). Cyprinodon diabolis. The IUCN Red List of Threatened Species 2014: e.T6149A15362335. https://dx.doi.org/10.2305/IUCN.UK.2014-3.RLTS.T6149A15362335.en
• Roach, N. (2025). Cavia intermedia (amended version of 2016 assessment). The IUCN Red List of Threatened Species 2025: e.T136520A284725227. https://dx.doi.org/10.2305/IUCN.UK.2025-2.RLTS.T136520A284725227.en
• Salvador, C. H., & Fernandez, F. A. S. (2008). Population dynamics and conservation status of the insular cavy Cavia intermedia (Rodentia: Caviidae). Journal of Mammalogy, 89(3), 721–729.
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