Christmas Island pipistrelle – an avoidable extinction

In the middle of the Indian Ocean, around 350 kilometers south of Java, lies remote Christmas Island—an Australian external territory just 135 square kilometers in size. Despite its small size, it once harbored a remarkable diversity of endemic species found nowhere else on Earth. One of them was the Christmas Island pipistrelle—Australia’s smallest known bat.

In the 1890s, when the island was slowly being settled and the first scientific expeditions took place, this tiny bat, according to its first describer Charles William Andrews (1900), was still “common” everywhere. A century later, the Australian zoologist Charles Tidemann likewise confirmed in his 1984 study that the bat was “widespread” not only in the rainforest, but even near human settlements. Up to that point, everything seemed to be in order.

But by the mid-1990s, researchers Lindy Lumsden and Karen Cherry were already reporting the first declines in both population density and range. They realized that the pipistrelle was hardly ever encountered anymore in the island’s northeast, the only settled area. By 1998, the species had vanished there completely.

Shrinking numbers

The downward spiral accelerated: a survey in 2002 showed that the range had shifted noticeably from east to west—accompanied by a 58% population decline compared with 1994. According to a report by Parks Australia North (2004), the species could no longer be detected in around 80% of its original habitat. And between 2004 and 2006, its presence was reduced to just three sites in the west of the island—each with only a few individuals. Long-term data documented a 90% decline between 1994 and 2006, and that decline continued unabated.

In 2005, experts estimated the number of mature animals at fewer than 250. Hope briefly arose when telemetry studies a year later identified seven shared roost trees with about 150 animals. That allowed—optimistically—for a possible remaining population of 500 to 1,000 individuals. But by 2007, four of those trees had been destroyed by storms or erosion, and the species had disappeared from more than 90% of its former range.

The last signal

Between 2006 and 2009, the team of Christmas Island National Park documented the final collapse: monthly acoustic surveys showed a 99.4% population decline in less than three years. In December 2008, only a single foraging area and one known roost remained. That roost had once sheltered up to 50 animals, but by January 2009 only four females with young were left. The entire remaining population was now estimated at fewer than 20 individuals.

In August 2009, one final rescue attempt began. For three weeks, researchers searched for surviving animals with acoustic detectors in order to capture them for a breeding program. But night after night, the devices recorded only a single individual. On August 26, 2009, the last acoustic signal of a Christmas Island pipistrelle was recorded. Since then, the species has remained missing despite intensive searches.

Christmas Island pipistrelle – fact sheet

alternative names	Christmas Island bat, Murray’s pipistrelle, Murray’s pipistrelle bat
scientific names	Pipistrellus murrayi, Pipistrellus tenuis murrayi
original range	Christmas Island (Indian Ocean, Australia)
time of extinction	2009
causes of extinction	unclear, probably animals introduced to the island, habitat loss
IUCN status	extinct

Officially extinct

In May 2012, the Australian government declared the Christmas Island pipistrelle extinct in official statements. In September 2017, the IUCN followed with the same classification. That meant a formerly common bat species had vanished completely in less than two decades—and to this day no one knows for certain why.

The legal status of the species under Australia’s Environment Protection and Biodiversity Conservation Act (EPBC Act) initially remained unchanged. Only on March 3, 2021 was the listing by the Threatened Species Scientific Committee (TSSC) changed from “threatened with extinction” to “extinct”.

That the Christmas Island pipistrelle is extinct is considered beyond doubt. It was the only bat on the island that used echolocation, which meant it could be reliably detected with ultrasonic detectors. That technology allowed continuous monitoring: sporadically at first in the 1990s, then almost continuously from 2005 onward. This produced an unusually detailed record of its decline.

What remains disturbing is not only the rapid speed with which the pipistrelle disappeared. There is also no clear cause. Were the animals eaten by introduced snakes? Did their insect prey decline because of invasive yellow crazy ants? Did rainforest loss caused by phosphate mining play a role? Perhaps all of these together, or perhaps something entirely different. Only one thing is clear: the warning signs were there, but action came too late.

A bat weighing barely five grams

The British zoologist Charles William Andrews spent several months on Christmas Island in 1897/98 documenting its natural history before the expected phosphate mining began. In the process, he discovered a previously unknown bat species, which he scientifically described in 1900 as Pipistrellus murrayi. Andrews emphasized that this species was distinctly smaller than both the Japanese pipistrelle (P. abramus) and the common pipistrelle known from Europe (P. pipistrellus). The Christmas Island pipistrelle was indeed tiny: its head-and-body length was only 3.4 to 4 centimeters, and its tail was around 3 centimeters long. It weighed just 2.6 to 4.6 grams.

The crepuscular and nocturnal bat fed on flying insects such as beetles, moths, flying ants, or wasps, which it caught in flight. Its echolocation calls were in a high frequency range and allowed it to locate even tiny prey accurately. It often hunted along forest edges, above narrow tracks, or over clearings where insects gathered in greater numbers.

All known roosts of the species were in primary rainforest. Unlike many other bat species, the pipistrelle did not use caves, buildings, or rock crevices. Instead, it relied on tree hollows, cracks in trunks, dense foliage, or peeling bark. Dead trees were used especially often, which had the disadvantage that decomposition made them unstable and prone to collapse—a risk for a species closely tied to such structures.

The pipistrelles formed small colonies, usually of only a few dozen individuals, sharing a roost. During telemetry studies in the 2000s, several groups could be documented in different roost trees. In some cases, up to 50 animals used the same tree. Females probably gave birth in early summer.

As small as it was, the pipistrelle played an important role in Christmas Island’s ecosystem. As an insect hunter, it helped keep populations of flying insects in balance. That made it part of a sensitive ecological web that was quickly and irreversibly damaged by human interference in the rainforest.

Why did the Christmas Island pipistrelle go extinct?

The extinction of the Christmas Island pipistrelle is one of the most puzzling losses of recent times. As late as the 1980s, the tiny bat species was widespread on the remote island in the Indian Ocean—scarcely two decades later it was gone. To this day there is no definitive explanation.

Experts discussed numerous possible causes: introduced predators such as snakes, giant centipedes, and rats; feral cats; the loss of roost trees through storms and clearing; as well as invasive species such as the yellow crazy ant, which altered entire forest stands. Extreme weather events and fires were also considered.

In a comprehensive 2007 report, Australian mammal expert Lindy Lumsden and her team concluded that above all introduced animals, all of which were increasing in number, likely disturbed the pipistrelle at its roosts or hunted it directly: the common wolf snake, the giant centipede, the giant African snail (Achatina fulica), feral domestic cats, black rats, and the introduced nankeen kestrel. Which of these species exactly caused the decline is uncertain, but the researchers leaned toward the common wolf snake, whose spread correlated in time with the bat’s decline.

In a consultation document on the listing of the Christmas Island pipistrelle under the EPBC Act, the authors mention, in addition to various introduced animal species, habitat loss through clearing, severe storms, and invasive yellow crazy ants as possible causes of extinction.

Some hypotheses could also be ruled out. Lumsden (2008), for example, wrote in The Australasian Bat Society Newsletter that a lack of food cannot explain the extinction. The Christmas Island pipistrelle fed on flying insects, and these were still abundant. Diseases were also investigated, but could not be demonstrated.

Which of these threats ultimately tipped the balance remains uncertain, but much points to an interaction of several factors that drove the little bat to the brink of annihilation within just a few years.

Prime suspect: the common wolf snake

Among all threats, the introduced common wolf snake (Lycodon capucinus) is regarded as the most likely trigger of the disappearance of the Christmas Island pipistrelle. The snake was first recorded on the island in 1987. Within a few years, it spread massively, reaching densities of up to 500 individuals per hectare in some places. The species was most often documented near human settlements and along the edges of primary rainforest. It is unclear, however, whether it was confined to forest edges or also spread into closed rainforest.

Strikingly, the current distribution of the common wolf snake almost exactly overlaps with the bat’s habitat in the 1990s. Since the first declines in the pipistrelle population were also noticed at that time, the spread of the snake and the population decline would correlate temporally.

The common wolf snake is known to hunt both small mammals such as rodents and lizards in the lower vegetation layers of the forest. The Christmas Island pipistrelle was considerably smaller and lighter than many other vertebrate species known to serve as prey for the snake.

Initial doubts about whether the snakes were capable of climbing high trees to hunt bats under bark or in tree hollows were quickly dispelled: in 2007, an infrared camera aimed at the trunk of a pipistrelle roost tree recorded a common wolf snake climbing. It was therefore clear: sleeping pipistrelles were also at risk.

From the species profile of the Christmas Island pipistrelle (SPRAT) in the Species Profile and Threats Database of the Australian Environment Ministry, it emerges that flightless juveniles left behind in the roosts at night while the adults went out to feed were especially vulnerable. In narrow tree hollows with only one exit, entire groups of adult animals could also be trapped and preyed upon. Australian biologist John Woinarski points out in A Bat’s End (2018) that it would not have taken many snakes to cause substantial mortality among the bats. Targeted hunting at communal maternity roosts, where females often formed colonies of up to 50 animals, while males usually roosted alone, would mean that a large number of bat pups could be eaten in a single night.

Biologists also described the pipistrelles as “naive toward snakes”—in their evolutionary history they had never encountered tree-dwelling predators and had developed no strategies to evade attacks (SPRAT).

Woinarski (2018) and other specialists likewise consider it most likely that the introduced common wolf snake was the principal driver of the pipistrelle’s disappearance. Lindy Lumsden (2008) emphasized the double threat of predation and disturbance at roost sites.

Not everyone shares this assessment: D. J. James, probably a staff member of Parks Australia North, listed in 2005 several arguments against the common wolf snake being the main cause of the decline. He pointed to the snakes’ rather sluggish movement, their “sit-and-wait” hunting strategy, and the lack of bat remains in stomach-content analyses. In addition, it remained unclear whether they inhabited primary forest to the same extent as forest edges and areas near settlements (SPRAT).

Other invasive predators

The black rat (Rattus rattus) is one of the most consequential invasive species worldwide. It was introduced to Christmas Island with the first settlement around 1890 and is now found everywhere there. Rats are excellent climbers that use trees and palms as habitat—exactly where the Christmas Island pipistrelle had its roosts. It is therefore plausible that rats not only plundered nests, but also preyed on bats in tree hollows or under bark. The fact that rats have repeatedly been implicated in the decline or extinction of small bats in other parts of the world supports this assumption (SPRAT).

The domestic cat (Felis catus) was likewise introduced at the end of the 19th century and established a feral population. Cats are globally notorious for massively destabilizing island ecosystems—millions of birds, reptiles, and small mammals fall victim to them every year. For the Christmas Island pipistrelle there is no direct evidence from stomach-content analyses, but given its low roosting and hunting height, it could potentially have become easy prey (SPRAT). Even a small amount of additional predation pressure from cats could have been devastating for a small, shrinking population.

The nankeen kestrel (Falco cenchroides), originally from mainland Australia, was first recorded on Christmas Island in the 1950s. At first present only in small numbers in the northeast, it spread strongly in the 1980s. Today it uses mainly secondary rainforest as well as forest edges, clearings, and open areas for hunting, while largely avoiding dense primary forest (SPRAT).

A study published by Charles Tidemann in 1985 showed that the Christmas Island pipistrelle regularly went insect hunting in the late afternoon in 1984. Since bats in many regions are usually nocturnal where birds of prey are present, this earlier foraging behavior may indicate the former absence of diurnal raptors. Later investigations by Lumsden and Cherry, however, showed that the species no longer hunted during the day—presumably an adaptation to the increasing predation pressure from the nankeen kestrel. The bats shifted their hunting into twilight, when the risk from day-active raptors is lower (SPRAT).

Even so, the kestrel is not considered the main cause of extinction. It occurred both in areas where the pipistrelle had already disappeared and in those where it was still present. It probably acted more as an additional stress factor that accelerated the decline (Schulz & Lumsden, 2004).

The two endemic raptors—the Christmas goshawk (Accipiter fasciatus natalis) and the Christmas Island boobook (Ninox natalis)—may also have occasionally taken pipistrelles. But because they had long coexisted with the bat on the island without seriously threatening its numbers, there is no evidence that they played a larger role in recent times through intensified hunting or altered prey behavior (Schulz & Lumsden, 2004).

The giant centipede Scolopendra subspinipes also arrived on Christmas Island with the first settlers. Charles William Andrews reported in 1900 that the animals were introduced with palm-leaf roofs. A short time later, in On the fauna of Christmas Island (1909), he noted that giant centipedes had multiplied greatly—and since then they have been widespread across the entire island.

Giant centipedes are considered extremely aggressive, nocturnal predators. In addition to insects and other invertebrates, they also hunt small vertebrates: lizards, frogs, mice, young birds—and even young snakes. Because the animals are active both on the ground and in trees, they could easily have entered the roosts of the Christmas Island pipistrelles to disturb them or injure them with bites.

A bite is especially dangerous for small vertebrates: with its poison claws, the giant centipede injects neurotoxins and cytotoxins that not only paralyze but can also destroy tissue. For animals weighing only a few grams, a single attack would likely be fatal. According to the SPRAT species profile, the most vulnerable were the flightless young left alone in the roosts at night. Whether the giant centipede actually preyed on pipistrelles to any significant degree has not been scientifically demonstrated. What is clear, however, is that it represented a potential danger.

Habitat destruction and roost loss

Besides invasive species, systematic phosphate mining, which began in 1899, is also suspected of having played a role in the decline of the Christmas Island pipistrelle. Mining altered the island’s ecosystem profoundly: to create extraction areas, transport routes, and settlements, large parts of the primary forest were cleared—precisely the habitat the Christmas Island pipistrelle needed for its roosts. This early phase of intensive clearing probably already led to initial declines (Schulz & Lumsden, 2004). The bats’ food base may also have been affected by erosion and the decline of insect habitats.

Clearing of primary forest ended in 1987—before the species’ population collapse in the 1990s. Habitat loss alone therefore does not explain the extinction. Nevertheless, every further change or fragmentation hit the weakened population. This included the removal of secondary forest on former mining and settlement sites, which probably offered important foraging habitat (SPRAT). In addition, mining brought invasive species such as rats, cats, dogs, and the yellow crazy ant to the island on supply ships and with workers—threats that later contributed to the bat’s disappearance.

A special role was played by the so-called “drill lines,” hundreds of cut corridors laid across the island for mining in the 1960s. Pipistrelles foraged especially often along these open structures. With regrowth and storm damage, however, these hunting habitats disappeared by the mid-1990s—a factor that may have favored the decline without being able to explain the extinction on its own (Schulz & Lumsden, 2004).

Natural events also changed living conditions. For example, a cyclone in March 1988 damaged large parts of the primary forest. Such tropical storms could have enormous effects on bat populations. Direct consequences were not documented, but the loss of tree hollows and bark roosts likely further reduced the already scarce roosting opportunities.

The loss of suitable roosts—especially maternity roosts—may have been a key factor in the disappearance of the Christmas Island pipistrelle. Maternity roosts were located predominantly under loose bark of dead trees of the species Tristiropsis acutangula (SPRAT). Yet these structures were extremely unstable: Lumsden and Schulz (2009) report that of seven maternity roost trees documented in 2005, more than half toppled over or became unusable because of falling bark within just four months.

In the species’ final refuge, the so-called “Dales” in the west of the island, only a few such trees remained—presumably the remnants of a single natural event (SPRAT). Why females preferred to give birth in these precarious bark roosts rather than in more stable tree hollows is unclear. Possibly the bark crevices offered better escape routes from predators, whereas tree hollows often have only one exit and increase the danger of being trapped. Dependence on such a short-lived structure made the species highly vulnerable: the loss of only a few suitable trees could have had severe consequences for the small remaining population.

Although around three quarters of the island remained covered in rainforest until the very end—much of it in what is now the national park—the early loss of primary forest and the ongoing disturbance from mining activities were probably decisive in weakening the Christmas Island pipistrelle. Because survival depended not only on habitat area, but on habitat quality and continuity. The Arthur Rylah Institute put it aptly in 2007: “Large-scale habitat changes caused by mining activities have potentially contributed substantially to the species’ endangerment.”

Persecution and disturbance by the yellow crazy ant

According to the IUCN, the yellow crazy ant (Anoplolepis gracilipes) is among the 100 worst invasive species worldwide. Accidentally introduced to Christmas Island between 1915 and 1934, it changed its behavior there from the mid- to late 1990s onward. It no longer formed single colonies with individual queens, but supercolonies with multiple queens and millions of workers—behavior that made the yellow crazy ant, as an introduced species, even more aggressive and ecologically destructive.

These supercolonies spread rapidly, with the result that the ants numerically dominated and consumed both the forest floor and the canopy. According to James (2005), at the height of their spread, colonies that could cover several hundred hectares occupied around 28% of Christmas Island’s total rainforest area.

The consequences were severe: using formic acid, the insects killed not only countless invertebrates, but also larger animals such as newborn rats, cats, or dogs, as well as the Lister’s gecko (Lepidodactylus listeri), now extinct in the wild, and coconut crabs (Birgus latro). Their best-known impact is their contribution to the mass death of Christmas Island red crabs (Gecarcoidea natalis), of which more than 20 million died. The invasive ants probably also contributed to the extinction of the Christmas Island shrew (Crocidura trichura) in the 1990s; even chicks of threatened seabirds and land birds fall victim to them.

The role played by yellow crazy ants in the extinction of the Christmas Island pipistrelle is less clear. Direct attacks are documented—in 1998, for example, a bat caught in a harp trap died after an ant attack (Schulz & Lumsden, 2004)—but they probably acted more indirectly: through stress, blindness, or injuries caused by contact with formic acid, through disturbance of roosts, or through a decline in available insect prey.

Particularly problematic was the fact that all known pipistrelle roosts lay directly along ant trails—for example under peeling bark or in tree crevices that the ants passed on their way into the canopy (SPRAT). This meant that yellow crazy ants had easy access to the bats’ roosts. Tree hollows used as roosts may also have been taken over by ants (SPRAT).

In order to restore balance to Christmas Island’s ecosystem and get rid of the yellow crazy ants, large-scale Fipronil bait was spread by airplane in 2009. The contact poison reduced the ants by around 98% and destroyed entire supercolonies. In a report by the Christmas Island Expert Working Group (2009), possible side effects of the poison were speculated about, such as poisoning or prey scarcity, but because its use only began in 2009—the same year the last pipistrelle vanished—Fipronil likely played no role in the extinction.

Current assessments hold that yellow crazy ants were not the principal trigger of the decline, because the pipistrelle’s downturn had already begun before the ants proliferated. In addition, the species’ main range at the time was in the west of the island—precisely where most supercolonies were located. If the ants had been the decisive trigger, the timing would suggest that the Christmas Island pipistrelle should already have vanished in the late 1990s or early 2000s (SPRAT). It is more likely that yellow crazy ants represented an additional burden.

Disease and climatic changes

A crisis caused by disease is considered a rather unlikely explanation for the disappearance of the Christmas Island pipistrelle. Lumsden and her team (2007) emphasize that the animals appeared healthy even during the final years of population decline: in an examination carried out in 2005, all 52 captured bats showed high body weight, no external signs of illness, and most females were breeding. Biological samples were also largely unremarkable—only the white blood cell count was below the average for comparable species, without any identifiable cause. Even the last individual observed in 2009 showed no signs of illness or weakness.

Even so, a disease factor cannot be ruled out entirely. Schulz and Lumsden (2004) point out that the island’s two endemic rat species were also probably wiped out by a pathogen introduced by the black rat. A similar scenario could theoretically also have affected the pipistrelles.

Besides invasive species and habitat loss, climatic influences may also have represented an additional burden. Drought periods, which became more frequent on Christmas Island in recent decades, probably reduced the insect supply and altered the thermal properties of roosts (Schulz & Lumsden, 2004). Forest fires during dry periods—for example in 1994 and 1997—also certainly threatened the population. They not only destroyed immediate refuges in primary forest, but probably also changed the composition of insect populations on which the pipistrelles depended (Schulz & Lumsden, 2004). Such climatic extremes alone are unlikely to explain the pipistrelle’s sudden disappearance, but they may have acted as amplifying stress factors.

Which cause of extinction is most likely?

The causes behind the disappearance of the Christmas Island pipistrelle will probably never be clarified with complete certainty. Too many factors came together—each one like another drop in an already overflowing bucket.

It was probably not a single cause, but a lethal combination: early on, phosphate mining altered large parts of the forest, cyclones destroyed roost trees, invasive ants disturbed the roosts—and then the common wolf snake arrived. Everything suggests that it in particular delivered the decisive blow: a novel climbing predator against which the pipistrelle had developed no defensive strategies whatsoever. John Woinarski wrote aptly in 2018: “It may not have taken many snakes to wipe out an entire species.”

Woinarski (2018) also formulated a second hypothesis that places stronger emphasis on the interaction of several factors: habitat loss through mining, the 1988 cyclone with the loss of many roost trees, the yellow crazy ant blocking access to roosts, as well as giant centipedes and black rats as additional predators. In this view, it was the combined effect of many smaller threats that overwhelmed the species.

There was also a general problem: once the population had shrunk drastically, extinction risk increased exponentially. Small populations suffer more quickly from inbreeding depression, are more vulnerable to chance events, and lose their ability to compensate for losses.

In a later analysis (2019), Woinarski tried to assign percentage responsibility:

• 55% common wolf snake
• 30% giant centipede and yellow crazy ant combined
• 10% 1988 cyclone
• 5% clearing and other habitat loss

Phosphate mining was therefore probably not a direct “cause of death,” but rather the stage on which other threats could unfold their full effect: it reduced habitat, brought invasive species to the island, and weakened the fragile balance of the ecosystem.

Rescue attempts—early warnings, no response

As early as 2002 and 2003, experts were aware that the Christmas Island pipistrelle would be extinct by 2009 at the latest without protective measures—warnings were passed on, but remained ineffective (Heathcote, 2018). In 2006, Lumsden, Schulz, and colleagues warned in the Australasian Bat Society Newsletter:

“Since 1994, a population decline of 90% has been recorded. This suggests that if the current decline continues, this species will be extinct by 2008.”

Lumsden, Schulz, James, et al., 2006.

In the very same year, the government’s Threatened Species Scientific Committee classified the Christmas Island pipistrelle as “threatened with extinction” and recommended a captive breeding program to prevent the species from actually dying out (Low & Booth, 2023).

The delayed plan

In 2007, Lumsden and her team renewed their demand for a captive breeding program. But concrete action came only much later: in January 2009, when the population had shrunk to only about 20 animals, Lumsden and Schulz warned that without immediate intervention, extinction within six months was likely. The Australasian Bat Society also called for the immediate capture and captive breeding of the last surviving animals.

Instead of acting immediately, however, the Australian government set up an expert group and tested capture and husbandry methods on another bat species in the Northern Territory. Looking back in 2009, Lumsden wrote in the Australasian Bat Society Newsletter:

“It is not easy to report on the decline of a species over 15 years, to warn for years that it would be extinct by 2009—and then to be right.”

Lumsden, 2009.

The last attempt

The expert panel’s report was available on July 1, 2009, and the Environment Ministry finally announced a rescue operation. The accompanying press release stated:

“The Australian Government will invest AUD 1.5 million to begin rescuing Christmas Island’s ecosystem. This includes a mission to capture the last remaining pipistrelles (…) and maintain them in a breeding program. (…) Preparations are underway to attempt (…) in three weeks’ time, from 22 to 27 July, to capture the last pipistrelles (…).”

Australasian Bat Society, July 1, 2009.

But when the ABS team was only allowed to travel to the island in August under difficult conditions, it was already too late. Only a single animal could still be detected. It flew regularly along certain paths, but despite every effort it could not be captured. The last recorded call dates from August 26; bat detectors installed later remained silent. Lumsden (2009) wrote:

“I really think the species became extinct on 26 August 2009, when the last remaining individual suffered the same fate as the rest of the population. It is rare to know the day on which a species disappears from the wild.”

Lumsden, 2009.

Political failure

After the extinction of the Christmas Island bat, the Environment Ministry faced massive criticism for its late response. In his 2018 book A Bat’s End, John Woinarski described the dramatic sequence of events as a textbook example of institutional failure.

Experts such as Martin Schulz and Lindy Lumsden regard the delayed response of the authorities and the failure to implement immediate conservation measures as the decisive factor in the species’ final disappearance. Woinarski and colleagues (2017) also analyzed in an article the causes of extinction of three Australian species—the Christmas Island pipistrelle, the Bramble Cay melomys (†2009), and the Christmas Island forest skink (†2014). Their central finding was: all three extinctions were foreseeable and probably could have been prevented.

The causes lay less in the biology of the species themselves than in structural and political failures—legislation without a clear obligation to prevent extinction, unclear responsibilities and lengthy administrative processes, insufficient funding, research and monitoring, as well as missing public outreach and emergency mechanisms.

Although the decline of the species was clearly apparent, no timely emergency measures were taken. For Woinarski and his coauthors, the fate of the pipistrelle is therefore an exemplary case of how institutional inaction and inadequate conservation structures can directly lead to a species’ extinction.

Woinarski (2018) accuses both the Environment Ministry and the responsible ministers of actively contributing to extinction through bad decisions or omissions—without ever taking responsibility. Particularly serious is the fact that after the disappearance of the Christmas Island pipistrelle, neither an official investigation was initiated nor legal reforms were launched to prevent a similar scenario in the future (Heathcote, 2018).

Christmas Island pipistrelle: distinct species or subspecies?

In the 1970s, the taxonomic classification of the Christmas Island pipistrelle was still disputed. In 1973, the American bat researcher Karl Friedrich Koopman argued in Systematics of Indo-Australian pipistrelles that the Christmas Island pipistrelle was not a distinct species but merely a subspecies of the widespread least pipistrelle (Pipistrellus tenuis). This species is endemic to South and Southeast Asia, with Java hosting the closest known population to Christmas Island. Koopman regarded the Christmas Island animals as conspecific, interpreting differences in coloration and size as geographic variants.

At that time, however, knowledge of the diversity and peculiarities of isolated Pipistrellus island populations in the Indo-Australian region was still very incomplete. A closer examination of morphology then led to a reassessment: in his revision of Australo-Papuan pipistrelles, Australian biologist Darrel John Kitchener classified the Christmas Island pipistrelle in 1986 as a distinct species. Shortly afterward, John Edwards Hill and D. L. Harrison (1987) confirmed this status. They based their conclusion in part on differences in the structure of the penis bone (Baculum)—a feature often decisive in bat taxonomy.

This classification is supported by a comprehensive morphometric analysis entitled Taxonomic status of the Christmas Island Pipistrelle by the American zoologist Kristofer Helgen (2009). On behalf of the Australian government, he compared skull, wing, and body measurements of Christmas Island pipistrelle specimens with Asian relatives. The result was clear: although closely related, Pipistrellus murrayi differs clearly from the least pipistrelle and similar species in numerous traits.

Since no fresh tissue samples were available, no direct DNA sequencing could be carried out. But the combination of morphological findings with molecular phylogenetic analyses of related species confirms its distinctness. Koopman’s hypothesis of a mere subspecies is now considered obsolete.

Almost all endemic mammals exterminated

“As might be expected on an oceanic island, the mammals are both few in number and small in size. The collections brought back by the officers of H.M.S. Flying Fish and Mr. Lister comprised two species of rat (Mus macleari and M. nativitatis), a large flying fox (Pteropus natalis), and a shrew (Crocidura fuliginosa, var. trichura); a small insectivorous bat was seen, but no specimens could be captured.”

Andrews, 1900.

With this early description, Charles Andrews aptly summarized the special situation of Christmas Island: as is typical for many remote islands, the number of mammal species was low—and they had adapted in their evolution to the isolated habitat. Originally, only five endemic mammal species lived here: two rat species, a shrew, a flying fox, and the Christmas Island pipistrelle.

Today, almost nothing of that remains. The two endemic rats—the Maclear rat and the bulldog rat—died out around 1903, probably because of diseases brought by the introduced black rat. The Christmas Island shrew had been considered lost since the beginning of the 20th century, was surprisingly rediscovered in 1985, but then vanished for good—today it too is considered extinct.

With the loss of the Christmas Island pipistrelle in 2009, the fourth endemic mammal species disappeared. That leaves only one species: the Christmas Island flying fox (Pteropus melanotus natalis). But it too is severely threatened, because phosphate mining, habitat loss, and invasive species have drastically reduced its population, leading the Australian government to list it officially as “threatened with extinction” since 2014.

The little island of Christmas Island in the middle of the Indian Ocean was thus almost completely stripped of its original mammal fauna within a single century. The fate of these animals shows just how fragile island ecosystems are—and how quickly species can disappear when different human-caused threats interact and conservation measures come too late.

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Sources

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