Christmas Island pipistrelle

Description

It was a small bat weighing around 3–4.5 g (0.11–0.16 oz). It had dark brown fur, with the tips of its hairs yellowish. Its forearm was 30–33 mm (1.2–1.3 in) long. It was the smallest described species of bat in Australia. Its ears were triangular and rounded at the tips. Its uropatagium had a distinct calcar. Its tail protruded very slightly (2 mm (0.079 in)) past the uropatagium. The length of its head and body was 35–40 mm (1.4–1.6 in) long; its tail was 30–31 mm (1.2–1.2 in) long; its ear was 9–11 mm (0.35–0.43 in); its hind foot was 6–8 mm (0.24–0.31 in) long.

Biology

The Christmas Island pipistrelle was nocturnal and roosted during the day in old-growth forest. Its documented roost sites included under peeling tree bark, within tree hollows, under dead palm fronds, and between a strangler fig and a tree trunk. It would roost solitarily or in small colonies of up to 47 individuals. Roost fidelity varied, with some individuals switching roosts daily while others used the same roost for a week or more consecutively. Roost switching often occurred within the same area, with the distance between roosts ranging from 14–186 m (46–610 ft).

At night, it foraged for its insect prey along forest edges, using echolocation to detect and catch flying moths and beetles. It foraged both above and below the forest canopy.

Little is known about its reproduction. It is hypothesized that, similar to other species in its family, females formed maternity colonies, though none were ever documented. They were seasonal breeders, with females likely becoming pregnant in September. Females were capable of storing sperm, meaning that there could have been a delay between copulation. After a gestation period of three months, they gave birth in late December. The young, called pups, were flightless and likely only 1 g (0.035 oz) at birth, and were left behind at the roost at night while the mother foraged. It had a low fecundity, with females producing a single pup per year. Its lifespan was unknown, but was estimated at seven years for individuals who survived to fledge. Closely related species of bat live for about fifteen years in captivity.

Range and habitat

The Christmas Island pipistrelle was endemic to Christmas Island, a small island 135 km² (52 mi²) in the Indian Ocean that is a territory of Australia. It relied on mature primary forests for roosting and foraged in both primary and secondary forests. As late as 1985, the Christmas Island pipstrelle was described as "common" and distributed throughout the island, including in Flying Fish Cove, which is the main settlement of Christmas Island.

Decline and extinction

Factors

The Christmas Island pipistrelle declined dramatically from 1990 onwards. The cause of its decline is unknown, though several factors could have contributed. The rainforests on Christmas Island were extensively clearcut to facilitate phosphate mining. This would have negatively impacted both their roosting and foraging habitat. Clearcutting stopped in 1987, so it is suspected that other factors had a more serious impact on the terminal decline of its population.

Invasive species may have also played a role: between 1915 and 1934, the yellow crazy ant was accidentally introduced to Christmas Island, where it eventually started forming supercolonies with multiple queens. A supercolony's size could be several hundred hectares, and the yellow crazy ants' range covered a quarter of the total rainforest area at their peak infestation before widespread poisoning efforts. While the ant nests were on the ground, they climbed trees to forage, where they could have interacted with roosting bats. The exact effects of the ants on the Christmas Island pipistrelle are unknown, but the ants are known to prey on mammals in other parts of their range, and researchers did document a fatal ant attack on a Christmas Island pipistrelle once. As well as directly killing the bats, the ants may have been a significant source of stress that negatively impacted their survival and reproduction. The ants' intense depredation of other wildlife may have also contributed to a scarcity in prey for the bats.

The common wolf snake, also an invasive species, likely had a negative impact as well. The timing of its introduction to Christmas Island immediately precedes the decline of the pipistrelle. As the common wolf snake expanded its range in Christmas Island, the pipistrelle stopped being detected in those same areas the snake inhabited. The wolf snake is not an adept tree climber, though, and mostly preyed on lizards. Given that the pipistrelle's young were clustered in roosts and their reproduction rate was so low, even occasional depredation of a maternity roost could have had an outsize impact on the population. Other invasives that could have become its predators are feral cats, the black rat, and giant centipede.

It has also been speculated that an unidentified disease or poisoning from the insecticide Fipronil used to control yellow crazy ant supercolonies could have contributed to the decline. Fipronil was used from 2001 to 2004, corresponding to a drastic reduction in pipistrelle detections from 2004 onwards.

An ecological cascade could have been chiefly responsible: Authors of a 2009 expert working group report speculated the following chain of events might have been the primary driver of the population collapse:

1. Accidental introduction of yellow crazy ants, giant centipedes, and scale insects
2. Pipistrelle populations decline in the 1980s and 1990s due to direct or indirect effects of yellow crazy ants
3. Drought or human water use lower the island's water table, stressing out rainforest trees, and increasing scale insect populations
4. Yellow crazy ants begin farming scale insects, allowing the proliferation of sooty mold on rainforest trees, and further stressing them out
5. Ant populations explode and they form supercolonies, scale insect numbers grow rapidly
6. The ants reduce the Christmas Island red crab population, a keystone species. The ants become significant predators of roosting pipstrelles
7. Red crabs no longer meaningfully suppress litter levels on the forest floor, allowing the giant centipede to become abundant and also become a significant predator of roosting pipistrelles
8. Efforts to control the ants with Fipronil reduces available insect prey of the pipistrelles

Unsuccessful conservation attempts

In response to its declining numbers, several conservation measures were undertaken: Researchers identified roost trees and installed protective barriers around them to deter predators; artificial roosts were erected; and yellow crazy ant colonies were controlled within the bats' remaining range. Efforts to protect roosts were complicated by the bats' habits of frequently switching trees. The seven artificial roosts, installed near existing roost trees, never showed any signs that the bats used them.

In 2006, its status was changed to "critically endangered" under the Australian Environment Protection and Biodiversity Conservation Act 1999 and some scientists and conservation organizations called for the establishment of a captive breeding program. Acoustic surveys showed a 99.4% decline in detection of the bat from January 2006 to January 2009. A reassessment of the number of individuals remaining in January 2009 suggested there may have been as few as 20 individuals left. The last remaining communal roost contained only four individuals. Three years before, there had been 54 individuals in this colony and there were several other known, similar-sized colonies. Despite predictions that the species would likely be extinct within six months, the Australian government initiated a captivity program for northern pipistrelles and forest pipistrelles to attempt to understand how to manage closely related pipistrelles in human care.

In July 2009, the Australian government announced that it would attempt to rescue the bat by bringing the last remaining individuals into captivity, with assistance of volunteer bat researchers from the Australasian Bat Society.

In early August 2009, the Australian government gave permission to capture the bats to establish a captive breeding program. However, after four weeks of surveying located only a single bat through its echolocation. Researchers were unable to catch it and the last echolocation call of this bat was recorded on 26 August 2009, when it went silent. On 8 September 2009, the Australian government announced that attempts to capture the bats had failed. No Christmas Island pipistrelles have been seen or heard since, and it is likely the species is now extinct. It is believed to be the first mammal extinction in Australia in 50 years.

A 2014 analysis that modeled wildlife management strategies stated that the captive breeding program should have immediately commenced in 2006 after it was suggested, but the political and social consequences of shifting course were difficult for decision-makers to bear. The program would have cost AU$500,000 annually but was predicted to have a high chance of success. Other scientists pointed to the discrepancy between collecting information and acting on it—the decline of the Christmas Island pipistrelle was charted for decades, but decisions to act to conserve the species were put off until they were too late. In the future, the authors of a 2012 post-mortem noted the need to make critical conservation decisions while there is still opportunity to act, also highlighting the failure to establish a breeding program in 2006.