The second drowning of the water rail

The paradise of Aldabra

Rare are the birds that do not fly: the cassowary, ostrich, emu, penguin to name but a few. However, the Aldabra rail is by far the strangest of them all. This species of bird owes its name to the Aldabra Atoll, an area of the Seychelles archipelago lost in the middle of the Indian Ocean. It was here that this species perished about 136,000 years ago, when a rise in sea levels submerged its unique habitat.

Nevertheless, this species is once again inhabiting the Aldabra territory resurrected from the waters. How can this mystery be explained? This phenomenon is one of the rare known cases of iterative evolution. The species was wiped off the face of the Earth, but its cousin, the common Cuvier’s rail, survived. It currently populates the island of Madagascar, where more uncertain and dangerous living conditions have led it to retain its ability to fly.

When the sea levels dropped, rails bravely travelled more than 400 km between Madagascar and Aldabra to recolonise this corner of paradise where no predator is on the lookout. Unthreatened in the island’s peaceful atmosphere, the rails of Aldabra have for the second time lost the wings that became useless in such a small territory.

Indeed, flight offers advantages for the survival of the species when it is necessary to escape cats or to travel great distances searching for food, but flying is tiring and life on the Aldabra Atoll does not require that much effort.

The proof is in the bones

Julian Hume from Tring Natural History Museum in the United Kingdom discovered this phenomenon. The palaeontologist relies on the anatomical study of skeletons found in various local sedimentary layers to reconstruct this surprising evolutionary history. This is enough to steal the show from Charles Darwin’s finches studied in the Galapagos Islands.

A joyful exception

Other species of bird have fallen victim, not of rising waters, but of their cohabitation with humans, who have denied them the opportunity to reappear in the course of evolution. The most famous example is the dodo, which disappeared from Mauritius around 1680. A similar fate befell the great auk, the American migratory pigeon, the ivory-billed woodpecker, the Labrador duck and the prairie hen, to name but a few. In total, it is estimated that about 100 species of bird have become extinct since the early 17th century.

According to the latest inventory of the International Union for Conservation of Nature (IUCN) published this year, 27% of known animal and plant species are threatened with extinction. The 30,178 species identified may not survive the spread of human influence. In particular, 14% of birds are endangered. Furthermore, aquatic animals are even more vulnerable: 30% of sharks and rays, 33% of reef-building corals, and even 40% of amphibians are labelled as “threatened” by the IUCN.

There are, however, some glimmers of hope. Eight species of bird and two freshwater fish species are improving their chances of survival. In particular, a third species of rail has escaped extinction. This bird lived on only one island, too: Guam, in the Pacific Ocean. Like its cousin in Aldabra, the Guam rail lost its ability to fly through evolution. It was extirpated by the brown tree snake, which had been accidentally introduced to the island in the 1940s. After 1987, the species existed only in a captive breeding program. It has recently been reintroduced in the wild in the neighbouring Cocos Island, where it survives but remains “critically endangered”.

The enigma of Pig Island

On the 15th of November, six biologists (accompanied by two journalists) disembarked from the oceanographic vessel Marion-Dufresne for five days of investigation on Pig Island. This island in the French Southern and Atlantic Lands Nature Reserve is part of the Crozet Archipelago, in the southern Indian Ocean somewhere between Madagascar and Antarctica. This archipelago was discovered by explorer Marc Joseph Marion du Fresne, whose second-in-command, Julien Crozet, landed there in 1772.

Nobody has set foot on Pig Island since 1982. The territory is undoubtedly in better shape after the departure of whalers and American seal hunters, which introduced pigs in 1820 and named it Île aux Cochons  (Pig Island).

Today, the archipelago is home to half of the world’s population of king penguins. The population is growing everywhere else except here. According to estimates made from helicopter flights and satellite images, the colony of Morne in Tamaris Island has seen its population collapse from half a million breeding pairs in 1982 to less than 70,000 today. This is the enigma that the expedition is trying to solve.

France committed this matter to the IUCN when the French Southern and Atlantic Lands were inscribed as a UNESCO World Heritage Site on the 5th of July 2019. Observations and sampling were carried out in compliance with draconian biosafety rules to avoid contaminating the island with invasive organisms (which may be more inconspicuous than the pigs that were introduced in the past). For the moment, the mystery remains unsolved.

Birds and men

We urgently need to understand what is causing this decline in some penguin populations in order to protect them better. It is essential to save the rails from rising water levels by finding appropriate habitats for their roaming habit. However, these birds’ fate must also alert us to the dangers of global warming caused by human populations.

The increase in the number of hurricanes threatens cities like New York, and it has already been flooded by a catastrophic storm surge in October 2012. The sea rose three metres, caused by a gigantic atmospheric depression. Eventually, many cities, island states and large swathes of coast are at risk of  disappearing underwater.

The IPCC has studied four types of coastal geographies exposed to the risks linked to sea-level rise: large coastal cities such as Shanghai, New York and Rotterdam; large agricultural deltas such as the Ganges River Delta; urbanised islands in atolls and Arctic communities. Beyond the material damage and displacement of directly exposed populations, the food security of entire regions is also at stake.

The water is already rising

The average sea-level rise has been about 2 mm per year since 1880. In total, the waters rose by 17 cm in the 20th century, and its pace seems to be accelerating: NASA estimated the rise at 8 mm per year between 1992 and 2015. This high figure suggests that the oceans could swell very rapidly by the end of the century, even though the ranges are wide and the forecasts difficult to make. Moreover, there will be wide variation from region to region, and some will inevitably suffer more.

This phenomenon is due to global warming: the ice is gradually melting and the oceans are expanding as their temperature rises. Some densely populated countries are particularly exposed, such as China, India, Bangladesh, Vietnam and Indonesia.

The Asian Development Bank estimates that 42 million Indonesian homes will be flooded by 2050 and 2,000 islands will be submerged by rising sea levels. The submergence of the Indonesian coast is not a distant projection; the catastrophe is under way and its effects are visible: many archipelago inhabitants have already been forced to leave their homes.

A global study published on the 29th of October estimates that, by 2050, 300 million people living on the coast may face flooding at least once a year. Asia appears particularly vulnerable to this humanitarian risk, with 94 million people affected in China, 43 million in Bangladesh, 36 million in India, 31 million in Vietnam, etc.

The United Kingdom will have to find a solution for 3.6 million people, and France will not be spared either: one million people will have to leave their homes within 30 years. At the end of the century, if greenhouse gas emissions remain at their current level, 1.7 million people will have to be rehoused.

There is reason to believe that this study’s figures are underestimated because they do not consider population growth or the trend towards coastal urbanisation. And of course, unless drastic measures are taken, the rise in sea levels will continue after 2100, affecting ever-increasing numbers of people.

What will happen then to the Aldabra rail and its cousin in Guam? Will these flightless birds be able to rely on us?