The Presence Of Absence: How Do We Know A Species Is Really Extinct?

When the only image available of your species is a painting, that’s a really big clue that the species is extinct

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They were nomadic. Shy. They slipped quietly through the skies over seemingly endless tropical forests, their lives devoted to travelling from one patch of bamboo to another in search of the seeds upon which they depend.

These small, unassuming — almost ghost-like — birds are purple-winged ground-doves, Paraclaravis geoffroyi. Adult males are a handsome slate-blue with rich maroon bands on their wings, whilst brown replaces blue on adult females. They once were relatively common throughout the Atlantic Forests of Brazil, Argentina and Paraguay (Figure 1), with flocks numbering as many as 100, according to historical reports. And yet, they are vanishingly rare now — or more likely, they are vanished. Completely gone from the planet. With only two reported (but undocumented) sightings in the 1980s, nothing at all has been seen nor heard from them since.

Their forest home has diminished rapidly during the last century. Tens of thousands of square kilometres of rainforests were destroyed and replaced with vast plantations and cattle ranches, which interrupted the natural movements of this dove as it followed its ancient flyways between countless patches of flowering and seeding bamboos.

Does this rare dove still live today? Sometimes, as in this case, it’s difficult to know for sure. Almost every week, we read in the news about the rediscovery of one animal species or another that has not been seen for 50 years, 100 years, or even longer, and is thus presumed extinct — until someone photographs it in the wild. For example, approximately 30% of mammal species “claimed or suspected to be extinct” have been rediscovered (i.e.; ref).

Is it possible that the purple-winged ground-dove, now officially listed as Critically Endangered by the IUCN Red List, may be like those long-lost mammals? Tropical rainforests are very dense and enormous, after all, and are populated by an astounding variety of species, at least some of which are still being discovered (for the first time) and described by scientists today. Further, tropical rainforests are poorly known, so a wide-ranging nomad like the purple-winged ground-dove could easily be overlooked for many decades, even by the armies of camera-wielding birders and citizen scientists that increasingly can be found creeping through remote jungles.

To better understand the current extinction crisis and to direct conservation funds and efforts most effectively, it is just as important to identify which species truly are extinct as it is to avoid erroneously reporting extinction of a Critically Endangered species whilst it still lives.

Typically, “lost” species are rediscovered through extensive targeted field surveys, even though such surveys are expensive and difficult to implement, especially for low-density wide-ranging species like the purple-winged ground-dove. Developing a new, less intensive, methodology for assessing whether the purple-winged ground-dove still persists in some remote pocket of the Amazonian rainforest could be valuable for making efficacious use of existing occurrence data for this bird, and possibly serve as a model for designing surveys that target other rare species in the future besides (ref).

Extinct or not extinct: this is the question

This is where Alexander Lees comes in. A Senior Lecturer at Manchester Metropolitan University and a lab associate at the Cornell Lab of Ornithology, he and his collaborators sought to work out a new method for censusing the purple-winged ground-dove to determine whether this bird still lives.

Dr Lees assembled an international team of scientists from Brazil, Argentina and the UK, and together, they started their project by collecting historical records throughout time from a variety of sources including; (1) records listed in the primary academic and grey literature resulting from targeted searches for this species and from general bird surveys, (2) specimen records (including tissue samples), (3) rich media sources (photo, video or audio) archived on citizen science initiatives or social networks, and (4) undocumented sightings from citizen science initiatives.

Dr Lees and his collaborators then collected the same data for another pigeon species that lives in the Atlantic Forest, the violaceous quail-dove, Geotrygon violacea. This dove, which is not endangered and is not closely related to the purple-winged ground-dove, lives alongside the purple-winged ground-dove. It has a similar ecology as well as similar habits of skulking quietly through the forest understorey. For these reasons, the violaceous quail-dove is rarely spotted too.

These data were analyzed using several statistical methods and compared to infer whether the purple-winged ground-dove may be extinct.

First, Dr Lees and his collaborators traced 79 records of purple-winged ground-doves of which 49 were specimens collected between 1820 and 1985 located in 11 museum collections. The last unambiguously documented record of a purple-winged ground-dove in the wild is a female collected in Rio de Janeiro, Brazil on 15 January 1985 (Figure 2A). Another female was found dead on 15 March 1991 on the campus of the University of São Paulo, but this individual this may be an escapee, because some private breeders were known to have their aviaries less than 1km (0.62 mile) from the campus. Further, this date coincides with the collapse of the captive breeding population. Intensive online searches turned up only a very few images of a captive bird on the defunct website, Arkive19.

In contrast, Dr Lees and his collaborators were able to trace 146 records of violaceous quail-doves from the Atlantic Forest region, of which 73 were specimens from 14 collections, and 73 were photographic records of the species, all since 2012 (Figure 2B).

“A comparison with another rare pigeon species, the Violaceous Quail-Dove (Geotrygon violacea), reinforces the idea that photo documentation [of the purple-winged ground-dove] ‘ought’ to have been obtained, but [the Violaceous Quail-Dove] may be easier to detect in the field”, Dr Lees said.

Historically, both species were spotted regularly, but only the violaceous quail-dove has been unambiguously documented in the last 35 years. Further, 37,000 bird banding records from Argentina included no records of either target species, whereas out of the 956,360 birds banded in Brazil, 29 were violaceous quail-doves, 21 of which were from the Atlantic Forest. No purple-winged ground-doves were banded.

Statistical analyses showed that the purple-winged ground-dove range gradually contracted to 2 extensive areas of forest: one on the Argentina/Brazil border and the other in southeastern Brazil (Figure 4). Reports of the purple-winged ground-dove declined steadily and, depending on how much confidence you assign to sightings-only records, this species is probably extinct.

But there may yet be a glimmer of hope.

“It does seem crazy that the bird might have been missed by the legions of local birders taking images and posting them to WikiAves, eBird and other citizen science sites but we do show that some areas are still poorly sampled”, Dr Lees pointed out.

And this study hopes to direct avid birders and other citizen scientists to explore those areas in search of this elusive bird.

A glimmer of hope?

“We knew that the ground-dove had been bred in captivity and in interviewing aviculturists [senior co-author, Luís Fábio Silveira] found out that 100s of individuals had been kept — but that this population was lost with legislative changes aimed at ‘saving’ species by stopping amateur breeding efforts”, Dr Lees pointed out.

“Clearly, aviculturists were a part of the problem by taking birds from the wild — against a backdrop of massive habitat loss & fragmentation — but they should have been key to the solution”, Dr Lees continued.

“The loss of the precious captive population was a bitter blow for conservation efforts — this well-intentioned but misguided legislation means we lost the best chance of saving this species from extinction”, lamented senior co-author, Luís Fábio Silveira, a professor and curator of the Ornithological Collections of the Museum of Zoology at the University of São Paulo.

“The best chance of saving the species was lost with the end of the captive population”, Dr Lees agreed sadly.

Dr Lees and his collaborators were determined to pursue every possibility. They interviewed aviculturists and amateur bird keepers about the purple-winged ground-dove, which was a popular avicultural subject until the early 1990s, when the government prohibited keeping and breeding this species in an effort to protect the wild population from illegal trapping.

Dr Lees and his collaborators ‘struck gold’ when they found aviculturist Carlos Keller, who shared the first videos and sound recordings of this species along with photographs and detailed notes about the species’ biology, habits and husbandry. Ultimately, Mr Keller’s contributions were so valuable that he was invited to be a co-author on the paper.

“Finding this previously unknown sound recording gives us a new and powerful tool to search for the purple-winged ground-dove”, said study co-author Carlos Barros de Araújo, a researcher at the Federal University of Paraíba. “Our chances of hearing the species are of course low, but by deploying dozens of autonomous sound recorders near seeding bamboo, and automatically searching for the vocalizations [in] the recordings takes the searching effort to a whole new level.”

“In the 1980s, we knew this was a rare bird, but it still had a very wide distribution, so nobody realized just how close to extinction it was,” said study co-author, Mr Keller. “It was a sophisticated bird, moving quietly like a ghost through the dark shadier parts of the rainforest.”

“Carlos Keller’s treasure trove of data offers us information that may prove critical to finding the species in the wild again”, said Ben Phalan, head of the conservation group at Parque das Aves. “[H]is knowledge of the captive husbandry of the species may prove critical if we can find and capture wild birds and establish a conservation breeding programme.”

But conserving this species will be an empty gesture if its habitat has been completely logged, burned, and transformed into a vast hamburger ranch.

“The decline of this species is a sign that bamboo-rich forests and their many associated species are under threat”, pointed out study co-author Juan Areta, an ecologist at the Institute of Bio and Geosciences of Northwest Argentina. “The best hope for long-term survival of the purple-winged ground-dove is protection of large expanses of rainforest with native bamboos, whose ecological importance often goes unappreciated.”

Source:

Alexander C. Lees, Christian Devenish, Juan Ignacio Areta, Carlos Barros de Araújo, Carlos Keller, Ben Phalan and Luís Fábio Silveira (2021). Assessing the Extinction Probability of the Purple-winged Ground Dove, an Enigmatic Bamboo Specialist, Frontiers in Ecology and Evolution, 29 April 2021 | doi:10.3389/fevo.2021.624959


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