Mauritius' pink pigeon faces extinction threat from inbreeding

The pink pigeon might be held up as a conservation success story, but the bird is still in trouble.

With as few as 10 individuals left in the wild on not one but two occasions, the pink pigeon has been seen as a remarkable turnaround, with about 500 individuals living in the wild and in zoos. As the last native pigeon from the island of Mauritius, it preserves a unique portion of the world's biodiversity.

However, a new study published in Conservation Biology has suggested any celebrations might be premature. The pink pigeon is rapidly losing its genetic diversity, and without help could become extinct in the next century.

Dr Matt Clark, an expert in genetics and sequencing at the Museum, says, 'There were once tens of thousands of pink pigeons across Mauritius, but this was reduced to as few as 10 in the late 20^th century.

'Such a low population size has caused genetic bottlenecking with many related birds in the population. Our data suggests that if we continue to without new interventions, the species will go into decline and eventually become extinct.

'Mixing wild and captive populations, which preserve different pools of the pink pigeon's genetic diversity, can help to rescue the species and make it much more viable in the long term.'

What is the pink pigeon, and what happened to it on Mauritius?

The island of Mauritius is in the southern Indian Ocean, near to the islands of Reunion and Madagascar. Until the 1500s, Mauritius was largely untouched by humans, but this changed with the arrival of Europeans.

The arrival of humans brought unprecedented changes as the island's first predators - cats, mongoose, macaques and rats – were introduced and land was cleared for plantations and settlements. This took its toll on the island's native species, many of which rapidly went extinct.

Among these was the dodo, itself a member of the pigeon family. The large and flightless bird was no match for sailors hunting for fresh meat and habitat destruction, and is believed to have become extinct within a century of its discovery.

While its relatives vanished, the pink pigeon hung on. Part of this is attributed to its flesh being toxic, causing human and animal hunters to avoid it in favour of more palatable prey.

Though it outlasted the dodo, populations of the pigeon have generally been on the decline ever since. In addition to habitat loss, another key reason for this is Trichomonas gallinae, a parasite which is thought to kill over half of all pink pigeon chicks born on the island. 

'T. gallinae is an protozoan parasite introduced to the island by humans, which infects the throat and affects the ability of birds to swallow,' Matt explains. 'The pink pigeon's lack of genetic diversity probably makes it much easier for the parasite to adapt to their immune system than a more diverse population with more types of natural immunity.'

With the population dropping into the tens during the 1970s, the Durrell Wildlife Conservation Trust captured a group of 12 individuals to form a breeding colony, whose offspring were later released back onto the island.

However, feral cats wiped out over half the wild population in the 1990s, leading the International Union for the Conservation of Nature (IUCN) to classify the species as Critically Endangered. A conservation program spearheaded by the Mauritian Wildlife Foundation, including captive breeding and reintroductions, has now brought numbers up to around 400 individuals, many of which have been released back onto the island.

While its numbers may be much healthier, the genetics of the species remain a source of concern. By coming so close to extinction on two occasions, the pink pigeon has experienced multiple population bottlenecks and significant amounts of inbreeding.

When species inbreed, over time the genes of individuals become more similar to each other so that offspring are more likely to inherit copies of the same gene from both parents - a state known as homozygosity. This causes a reduction in genetic diversity, and so in general it is better to for a species to maintain greater genetic diversity, or heterozygosity.   

'A healthy pink pigeon genome is one with lots of heterozygosity,' Matt explains. 'At the moment, there are whole sections of chromosomes inherited from each parent that are genetically identical over tens of millions of bases, which mean that there are no alternative copies if a particular gene isn't working.

'Combined with our knowledge of these birds' pedigree, this has allowed us to better understand the decline of this species.'

Without help, the species' genetics will push it to the edge of extinction for the third time.

Will the pink pigeon go extinct, and how can we avoid it?

The pink pigeon is predicted to go extinct in the next 50-100 years without intervention. Increasing the numbers of birds through breeding, without focusing on improving the genetic diversity, was also predicted to lead to extinction within a century.

Though five subpopulations of the pink pigeon are established across Mauritius, the scientists found there was very little genetic difference between them. While their genetic diversity had increased when new birds were introduced from the Durrell breeding program in the 1990s, the wild populations had returned to inbreeding by 2005.

Introducing zoo-bred birds will help introduce genetic diversity into the wild populations, and is the most promising way to save the species.

To save the pink pigeon, the scientists suggest that knowledge of the birds' genotypes can be used to mate the least related and most diverse individuals together. Their offspring can then be reintroduced to Mauritius, where they can stabilise the genetics of the wild populations.

But these introductions can't just be one-offs. Reintroductions need to occur continuously to bring down the high genetic load of the species.

While the scientists don't advocate that the species is upgraded from its current Vulnerable classification to a more severe conservation status, they suggest that genetic information is consistently included in the IUCN's Green List. This is a recently introduced counterpart to the more famous Red List, and assesses the potential of a species to recover from being at risk of extinction.

'This type of approach could be more widely used by the IUCN to understand how to manage species out of being Vulnerable,' Matt says. 'At present, the pink pigeon isn't regarded as having as big an extinction threat because its population is significant and so it doesn’t look in immediate danger.

'We argue that it is will go extinct unless a genetic rescue takes place, and this is something the Green List should account for.'