Climate change and human intrusion converge to imperil birds.
With wild eyes and a dishevelled hairdo, Africa's white-crested hornbill (Tropicranus albocristatus) has a distinctly disgruntled look. Whatever is doing the disgruntling, though, has so far not been seen as life threatening. The bird is numerous and secure enough to have been declared of "least concern" by the conservation group BirdLife International.
But that may be about to change. In a new study, ecologist Walter Jetz of the University of California, San Diego, and his colleagues have predicted the fate of 8,750 non-marine bird species in a world in which continuing climate change and new patterns of land use by humans transform Earth's landscape.
Over the next century, they found, the white-crested hornbill and almost 1,000 other species may lose more than half of their favourite feeding grounds, pushing them well beyond the "least concern" stage. More than 50 of those species could potentially be pushed to extinction. That's higher than the current predictions made by the IUCN, the World Conservation Union;of the 80 species facing extinction at the upper end of Jetz' estimate, only 41 are currently listed as "threatened" by the IUCN.
The results, reported this week in PloS Biology1, add to the deluge of reports predicting the impact of climate change on wildlife. But this study is unique in its consideration of both climate change and changing human land use. While everyone leaps onto the climate-change bandwagon, Jetz hopes the report will serve as a reminder that the age-old problems of deforestation and land conversion are still critical.
Changes to ranges
The findings are based on models created by the Millennium Ecosystem Assessment (MEA) project, a United Nations-commissioned report on upcoming ecological changes that included scenarios for land-use change. Jetz and his colleagues trawled through bird guides and handbooks to find range maps for 8,750 species of birds, which was as many as they could get. They then overlaid these ranges on the MEA's vegetation maps, and measured the change in vegetation over each bird's range.
Those losses were dramatic. In the MEA's relatively benign 'Adapting Mosaic' scenario, in which environmental problems are handled proactively, about a quarter of the Earth's land was transformed by 2100: 16% due to climate change and 9% due to land-use changes. In another scenario, one in which little proactive environmental planning took place, habitat loss in tropical regions was approximately twice as high. Overall, Jetz projected the number of threatened bird species to increase by 19-30% by 2050, and 29-52% by 2100.
A clear trend
There is a significant chance that this is an overestimate of the effects of changing vegetation. Previously published work2 has suggested that many animals may shift their ranges in pursuit of their favourite plants and climes. But without satisfying models of this process, the researchers chose to assume that the birds would not change their geographical range in response to the altered ecosystem around them.
Nevertheless, Jetz says the trend is clear: cultivation of natural lands by humans will continue to threaten tropical regions where birds are highly diverse, but already tend to occupy narrow geographical ranges. "There is a very clear signal here that shows that land-use change is projected to continue to increase in a dramatic way," says Jetz, "and it happens to occur in regions where biodiversity is particularly vulnerable."
The study is an excellent attempt to integrate land use and climate change, says Stanford University ecologist Terry Root. But given the inherent uncertainty of long-term models, Root urges caution before reading too much into the hard numbers. "We need to take what they've said very seriously, if not completely literally," she says. "I think what this is saying is that we are in trouble, and I think we are."
- Jetz W., Wilcove D. S. & Dobson A. P. PLoS Biol., 5 . e157 (2007).
- Araújo M. B. & Rahbek C. Science, 313 . 1396 - 1397 (2007).