Humans behind bird species’ loss

Further to the results of a study published towards the end of last year that estimated the extinction of 279 bird species and subspecies – principally from islands in the Pacific – had occurred during the last 500 years, more recent research that studied fossil records as well as evidence from mathematical modelling, has found that bird loss in the Pacific region is closer to 1000 species. The results of this study were published in the March issue of Proceedings of the National Academy of Sciences, and they confirm extinctions coincided with humans colonising the region approximately 4000 years ago. The research showed the subsequent disturbance of fragile ecosystems from a combination of deforestation, hunting, and the introduction of invasive species such as cats, rats, and pigs – together with the diseases they carried – drove the decline.

Co-authors of the report, Tim Blackburn, director of the Zoological Society of London’s Institute of Zoology, and Richard Duncan, professor in conservation ecology at the University of Canberra’s Institute for Applied Ecology, led the research that widened the earlier study’s scale and extent of the extinctions by incorporating the use of bird fossils to calculate the results. These were collected from 41 remote islands in the eastern Pacific that were among the last to see human habitation. The collected data was used to create a mathematical model that estimated each island’s extinction rates, and showed the islands were once home to a total of 618 populations of 193 nonpasserine landbirds. This comprised 371 populations present at the time of European contact, and 247 populations known only as fossils.

Researchers adapted a Bayesian mark-recapture method to achieve their estimate. This method is more commonly used to catch, mark and release animals, some of which are then recaptured at a later date in order to estimate the overall population size from the proportion of animals caught on both occasions. A similar principal can be used when studying fossils and live specimens.

“It’s been known for a long time that many of the species that were on those islands have gone extinct. We thought it would be worth reinvestigating the data with the use of new methods developed in the last few years. The aim was to come up with better estimates for the magnitude of the extinction that had occurred on those islands,” explained Prof Blackburn.

“The live birds present act as the marked individuals. Those present in the fossil record are in the pool for recapture. If every species observed in the living sample is also in the fossil record, you know the fossils are a reasonable representation of what was there. If many of the species you know from life are not found fossilised, you know there are many missing from the fossil record.”

The study showed the extinction rate varied, both between the larger and smaller islands and those with higher rainfall, suggesting weather conditions and topography were important factors in the birds’ survival. Species endemic to a single island were 24 times more likely to go extinct than more widespread species, while the extinction of flightless birds was 33 times greater due to over-hunting by humans for food. Species loss was found to occur more rapidly on smaller drier islands, possibly because land was easier to clear, leaving less habitat for the birds.

“The extinctions could potentially have happened very quickly, particularly on small islands where there were a group of birds encountering a kind of predator they simply had no defences against. It’s been described as less like hunting and more like gathering when people arrived on these islands, because the evidence suggests you could just walk up to these birds and hit them over the head.

“It’s thought that many small-bodied species went extinct due to predation by the Pacific rat, which the Polynesians brought with them. Many medium-sized, and arboreal species were left because the Pacific rat is largely a terrestrial predator,” Blackburn said.

An additional 40 species of bird vanished after the Europeans arrived, with even more now threatened with extinction from exotic introduced predators. Blackburn said only a concerted conservation effort is keeping a check on the further decline of vulnerable species like the hihi, (Notiomystis cincta) which is native to New Zealand but had been reduced to a single remnant population on a small island off the coast of North Island.

“The hihi is found nowhere else in the world but although it’s now been translocated to a few other islands, the species is under threat.

“The spread of people across the Pacific resulted in the loss of a huge swathe of biodiversity and we are left with very few clues as to how these species looked, and how they behaved, or what adaptations and life histories they exhibited. This was a tragic loss of biodiversity, but it’s indicative of the kind of impact people can have on the planet,” he said.


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