The Kalahari desert, which straddles Namibia, Botswana and South Africa, is already 1.95°C hotter than it was just 60 years ago. Temperature increases of this magnitude normally take place over centuries and millennia, giving life forms the time to evolve and develop coping mechanisms.
But this rate of temperature increase is too fast for natural evolution to happen. And it is occuring around the world. This is part of the reason that the sixth mass extinction of species is taking place now.
Birds in the Kalahari have evolved to survive the already extreme temperatures, but they won’t survive the rapid increase.
Researchers from the universities of Pretoria and Cape Town say that, in the dry Kalahari, the heat will become too much for most bird species by the year 2100.
An article, titled Chronic, Sublethal Effects of High Temperatures Will Cause Severe Declines in Southern African Arid-zone Birds During the 21st Century, was published in the journal Proceedings of the National Academy of Sciences of the United States of America this month.
Looking at 11 different bird species, the researchers warn: “We anticipate that much of the Kalahari’s avian biodiversity will be lost by the end of the century.”
This has got to do with how birds — and other living things — process heat. Where humans are good at regulating their temperature by sweating (a spectacular evolutionary advantage) other species aren’t. Birds can’t sweat, so they have to move into shade and pant to reduce their temperature or die of dehydration.
Panting takes up time that is needed for other activities. The researchers look at the Southern Yellow-billed Hornbill, which has evolved to thrive in dry places. Its breeding season is summer, when rain tends to fall in the Kalahari, turning grey and brown desert into the green hues of life. If the temperature during breeding season hits 35°C, the probability of hornbills successfully breeding drops by 50%. Fewer birds are born and they also tend to be smaller. The research says that these smaller birds are less likely to have “breeding success” as adults.
That 35°C average mark will be reached by 2090, according to the researchers, forcing the species into a downward spiral. There is also no escape because that heat will be across the hornbill’s range.
This hot weather has two effects — sublethal and lethal.
Lethal is when the temperature is so extreme that living things cannot survive; in humans it’s the point at which we cannot sweat and regulate our temperature. The researchers say that these kinds of temperatures are not likely to be reached in Southern Africa.
Sublethal is consistently hot temperatures. Birds can survive these temperatures over short periods of time, but not for days on end. The researchers say that by the 2080s the region will experience 10 to 20 consecutive days a year, which will make the survival of bird species such as the Southern Pied Babblers unlikely.
Losing birds in the Kalahari has a wider effect. Desert areas are critical to overall evolution in avian species, adding diversity to the species. That diversity could be on the way out as a result of a rapidly warming world.