In the shady recesses of unassuming forest patches in eastern Brazil, bird species are taking their final bows on the global evolutionary stage, and winking out.
Bird extinctions are nothing new. Human activity has already wiped out over a thousand species. But the vast majority of these occurred on oceanic islands. Today, although island species remain disproportionately threatened, we are witnessing a historic shift towards the endangerment of continental species of birds. The Alagoas Foliage-Gleaner, last seen in 2011, looks increasingly like the tip of an iceberg.
This new wave of threats, driven primarily by habitat loss, is deeply troubling because South American forests are home to such a concentration of bird diversity, yet our conservation strategies are still a work in progress.
The trouble with the tropics
To appreciate the significance of todays looming extinctions in the tropics, we must travel north to the great deciduous forests of the eastern United States, which are haunted by the ghosts of extinctions past. Here, the opportunity to experience the double raps of Ivory-billed Woodpeckers, sun-obscuring clouds of Passenger Pigeons, raucous flocks of Carolina Parakeets, and the monotone song of the Bachmans Warbler is seemingly forever lost.
The blame for these four infamous extinctions has been laid firmly at the door of historic deforestation.
In the early 20th century, the last remaining old-growth fell to the sawmills, almost without exception. Given the ubiquity of the logging, perhaps the most noteworthy feature of this extinction episode is that it did not involve more species.
The European experience was even more striking. The wholesale clearing of Europes primeval forest apparently did not cause a single bird extinction. The logical conclusion is that it is very difficult to drive continental birds extinct.
Why then are forest birds beginning to go extinct on mainland South America, home of the largest and most intact tropical forests on Earth?
We must face two equally unsettling conclusions. The first is that forest destruction, particularly in Brazils Atlantic rainforest, has reached continental-scale proportions, with almost no nook or cranny spared. And the second is that it may not be nearly as difficult to drive extinct in the tropics as in the temperate zone.
Biologists Stuart Pimm and Robert Askins have argued that the eastern USA witnessed few avian extinctions simply because most of its birds have very large geographic ranges. In South America, the situation is dramatically different.
South America is both the evolutionary cradle and current champion of global bird biodiversity; the authoritative regional list totals 3,368 species around one third of all the words birds. Many of these species have small ranges, restricted to particular countries or even to particular mountains or forest types.
Unique features of the life history of tropical birds led to an overly rosy assessment of their future. Author and academic Bjorn Lomborg, for example, claimed that the lack of extinctions following the destruction of Brazils Atlantic forest showed that the biodiversity crisis is overblown.
But extinctions may lag far behind forest loss, a phenomenon known as the extinction debt which may be paid over hundreds of years.
Tropical birds typically live for longer than their temperate counterparts. Thus, the last pairs of rare species may make their last stand in their fragmented forest redoubts for decades. Indeed, several species have paid this price, and more may already be committed to extinction.
The last known Alagoas Foliage-gleaner photographed in Pernambuco, Brazil in November 2010 Ciro Albano/NE Brazil Birding
Need to develop strategies
The situation in northeast Brazil is particularly dire.
A few dozen Alagoas Antwrens cling to survival in less than six tiny forest patches. The Alagoas Foliage-gleaner, presented to science along with the Antwren for the first time in the 1980s, was known from only two patches. The last known individual was photographed for the final time in November 2011. We can only guess how many more species will be lost from this region where new species are discovered and others are disappearing on a near-annual basis.
But what of Amazonia, the last great tropical forest wilderness and bastion of tropical biodiversity?
Although deforestation rates have fallen since 2004, there are still grounds for concern. Pressure on existing protected areas from dam-building and mining interests is increasing, and the existing reserve network poorly protects the hardest hit regions.
Arable fields in eastern Amazonia, former forest haunts of the endemic Belem Curassow, illustrated in the inset to the right of the similar Bare-faced Curassow. This former species was last documented in the wild decades ago. both images Alexander Charles Lees, curassow specimens Museu Paraense Emlio Goeldi
Furthermore, Amazonia is divided into different biogeographic regions known as areas of endemism that each contain species found nowhere else. Even today, taxonomists continue to recognize new divisions in Amazonian birds, often elevating former subspecies to species status. The Belem Curassow was recently recognized as a species and occurs only in the most deforested part of the Amazon. The last documented record in the wild was over 35 years ago.
Unless a population is discovered in the embattled Gurupi reserve, this species may be the first recorded Amazonian bird extinction. Hot on its heels is the Iquitos Gnatcatcher, known only from a tiny and heavily deforested area of unique stunted forest in Peru. Only six pairs are known, and the bird has proven harder to find every year.
The Iquitos Gnatcatcher hangs by a thread in small patches of stunted forest near Iquitos, Peru. Only six pairs are known. Jos lvarez Alonso, used with permission
Some of these species need immediate and drastic conservation interventions, but their plight seems to be largely ignored by governments and international environmental groups. Restoring forest around these last fragments is crucial for long-term population viability.
However, for some species captive breeding with an eye to future reintroduction may be the only way forward. Such measures have already saved the Spixs Macaw and Alagoas Curassow from global extinction populations of these species exist only in captivity. However, while we have centuries of experience breeding parrots and gamebirds, we know far less about breeding small songbirds.
In fact, most of what we know about managing songbird populations comes from islands, and it is unclear how well this knowledge will translate to the mainland. Island species are adapted to maintain small populations and may be better able to recover from genetic bottlenecks. And, quick fixes such as controlling invasive predators have helped to restore populations. But mainland birds face a different suite of threats, dominated by habitat loss.
Clearly, we must not assume that tropical forest birds will prove as resilient to human activity as their temperate brethren. But though the situation is critical, we also see grounds for optimism.
In Peru, for instance, new endangered species legislation has convened a working group to develop a conservation strategy for the Iquitos Gnatcatcher. In the meantime, a small reserve has been created that protects the few remaining territories. Across the border in Brazil exciting plans are being drawn up to reintroduce the Alagoas Curassow back into the wild.
There is an immediate need to support and expand such actions. The next five to ten years will be critical for many species of South American birds teetering on the brink of extinction.
Wonder Of Science