Bird populations in the Mojave Desert have collapsed over the past century, and scientists now say they know why: animal bodies cannot cope with the warmer, drier climate caused by global warming.
Discovery, described This week, in the Proceedings of the National Academy of Sciences magazine, it draws on historical records and virtual modeling of high-tech birds to explain how climate change has caused drastic population losses – and how it is likely to cause even deeper losses in the future.
“There is just a huge and growing scientific literature on how we are spoiling biodiversity, and that is a really important contribution to it,” he said. Paul Ehrlich, an evolutionary ecologist at Stanford University who was not involved in the work.
As climate change and habitat destruction from human activity continue around the world, many species have declined or threatened. A recent study by Science, for example, found that there are nearly 3 billion birds in North America today than in 1970. One can blame a complex mix of factors, including pollution and human invasion into their habitats. But linking climate change to the decline of a specific species is often a complicated task.
Steven beissinger, a conservation biologist at UC Berkeley, has managed to find a clear connection in Mojave. He and his colleagues have spent years researching bird communities that already live in an extreme environment: the deserts of California and other parts of the west, where temperatures rise to three digits in the warmer months.
While California’s deserts have long been home to all sorts of plants and animals, these ecosystems exist close to what many of their resident species can handle. Turn the fire up and it can simply push them the other way. Although bad news for the birds that live there, this quality makes it a useful place to study the effects of climate change.
The desert was also a good starting point because researchers had more than a century of data, thanks to UC Berkeley ecologist Joseph Grinnell. Beginning in 1904 and continuing for nearly four decades, Grinnell and his colleagues surveyed wildlife in many California biomes, providing a wealth of historical data on the state’s small mammals and birds.
As part of an effort to resurrect communities Grinnell had documented UC Berkeley postdoctoral researcher Kelly Iknayan revisited 61 remote locations in the Mojave Desert in California and Nevada, spending days at each. She has identified a variety of different species, including the small Costa hummingbird, Gambel’s quail, and large birds of prey such as the golden eagle.
A sharp cane hawk in California.
She and Beissinger found that 39 of the 135 bird species that lived there a century ago were less common and less widespread today. On average, these sites have lost 43% of their species in the last 100 years. These results were Published at PNAS last year.
“It’s quieter than it used to be,” said Beissinger.
The human invasion could not explain the declines because many of the sites surveyed were in protected areas. But the researchers noticed a remarkable decline in rainfall – and it made them wonder if water shortages could be to blame.
Scientists suspected that rising temperatures meant that birds were no longer able to effectively cool their bodies. Birds, like humans, are endothermic: we maintain our internal temperatures by generating our own heat rather than relying on the environment.
The disadvantage is that we need good cooling mechanisms to prevent overheating. Birds and humans use water to help remove this heat, though in different ways; humans sweat as birds gasp and radiate from their skin.
Thanks to climate change, the Mojave Desert has grown about 2 degrees Celsius (about 3.6 degrees Fahrenheit) on average over the past century. This means that birds would have higher “cooling costs” – that is, they would need to consume more water to radiate more heat. Unfortunately for them, climate change has made deserts simultaneously drier – meaning that water has become increasingly difficult to find, as has its need.
To test this idea, lead author Eric Riddell, a postdoctoral scholar at UC Berkeley, put his physics background to work. He built a virtual bird model that allowed his team to simulate the cooling costs of 50 bird species. With the help of UC Berkeley’s specimens Vertebrate Zoology Museum, scientists adapted the models to different bird species, taking into account their body size, shape and even the properties of their feathers, such as reflectivity and isolation.
Riddell placed the virtual birds in the virtual sun and shadow to see how their bodies reacted. As virtual birds warmed, scientists could estimate how much water a bird would need to evaporate to cool.
The scientists found that as the temperature rose, the birds’ cooling costs increased by an average of 18.8%. These rising costs have matched the declining population observed over the past century.
A Cactus Wren in the California desert.
Another pattern has also emerged: increased heat and dryness reach larger birds, especially as they need much more water than smaller birds to cool. It is also harder for larger birds to find enough shade to protect their bodies from the sun. That could explain why the average body size of many bird species has declined, the scientists said. Species that became smaller saved up to 14% in cooling costs and suffered less decline than species that remained the same size.
Birds that ate insects or other animals also fared worse than their herbivorous cousins that eat seeds, the scientists found. Probably because insectivores and carnivores don’t really drink water – they get their hydration straight from the creatures they eat. And the warmer, the more water they need. This holds them in a vicious cycle: overheated birds would have to spend more time and energy hunting in the hot sun, probably increasing their cooling costs.
Seed-eating birds drink water and therefore do better if they have some water around. But these are increasingly difficult to find in a dry climate, the scientists said.
These are just the costs that desert birds have already suffered, Beissinger said. Their situation will almost certainly get worse. In the future, as the world warms, these cooling costs are likely to rise by 50 to 78 percent – and an insignificant 14 percent discount on body shrinkage won’t help much.
Many previous studies have examined animal physiology and made predictions about how species might be affected by climate change, they said. John Wiens, an evolutionary ecologist at the University of Arizona who was not involved in the work. But the new article shows how animal physical limits have already been tested by global warming.
“It’s very different,” Wiens said of the work.
California desert birds are probably not the only ones that will suffer this kind of consequence, Wiens added. Deserts around the world are getting hotter and drier, and their bird inhabitants may be suffering as a result.
Iknayan said the researchers plan to conduct a similar analysis with small mammal populations, which Grinnell also researched.
“Unfortunately, one of the major impacts of climate change is that many places will become even more arid – so those principles can also be applied more broadly,” Wiens said.
Finally, he said, the way to help birds is to address the underlying cause of their situation.
“The best thing to do is to stop climate change – or improve it as much as possible,” he said.