April 25, 2015, by Tim Radford
The exotic rhinoceros hornbill is one the species examined for genomic evidence about falls in bird numbers.
Image: AbZahri AbAzizis via Wikimedia Commons
Evidence from the Ice Ages helps show how vulnerable bird populations are to change driven by human-induced global warming. LONDON, 25 April, 2015 − Climate change can seriously alter the numbers and the prospects for survival of the planet’s living things, according to researchers in Sweden and China. The scientists’ findings are the result of taking a long, cool look at the big picture – rather than the still-sketchy evidence from climate change now – of what happened to bird populations during the Ice Ages. Krystyna Nadachowska-Brzyska and Hans Ellegren, of the Evolutionary Biology Centre at Uppsala University, and collaborators at the Beijing Institute of Genomics used a sophisticated new technique to calculate the rise and fall of population sizes of 38 species of bird during the last several million years − a period punctuated by the advance of vast sheets of ice and shorter warm interglacial periods.
The results answer questions about how species fared during periods of natural change, in an era when human numbers were tiny and human technology insignificant. But they also highlight the vulnerability of already-endangered bird populations during a period of change driven by global warming as a consequence of greenhouse gas emissions into the planet’s atmosphere from the widespread use of fossil fuels. The researchers exploited techniques made possible only in the last decade or so to recreate the past. They compared genomic mutations among unrelated individuals in each species, and employed a new mathematical technique that goes by the name of “pairwise sequentially Markovian coalescent”, or PSMC, to tease out the history encoded in those comparisons.
“Climate events significantly affect the effective breeding sizes of bird populations”
The reasoning goes like this: mutations in DNA occur at a more or less predictable rate through the generations of a species, so DNA can be considered both as an indicator of relationships and as a kind of clock. Using such a clock, based on DNA inherited only through the maternal line, scientists long ago calculated a potential date for the origins of Homo sapiens. In 2011, a team of scientists reasoned that the same clock, subjected to some mathematical interrogation, might answer questions about how many individuals might have existed in a population at a particular time. Their first result was to confirm from the variation – or lack of variation – in the genome samples that some distant catastrophe caused a sharp drop in human population numbers 60,000 years ago or more, after which human numbers expanded again. Now the same techniques have been applied to genomic evidence from barn owls, budgerigars, bald eagles, Dalmatian pelicans, domestic pigeons and Pekin ducks, as well as less familiar creatures such as the rhinoceros hornbill of south-east Asia, and the kea, a ground-dwelling alpine parrot from the South Island of New Zealand. “The majority of all species exhibit cyclical swings in numbers, and these swings often coincide with the periods of ice ages,” Dr Ellegren says. “The last ice age (110,000-12,000 years ago) had a particularly heavy impact on birds. Many species suffered their most dramatic falls in numbers then.” What the scientists were looking for was something called “historically effective population size” in their choice of species − broadly, the number of survivors that could interbreed and rear the next generation. In the case of, for example, the downy woodpecker of North America, this number was at a low point of 150,000 two million years ago, then rose to 1.2 million before falling, around 100,000 years ago, to about 200,000.
They found severe declines in 22 of the 38 species over a period that coincided with the last ice age. Two species of eagle and the common ostrich saw their numbers reduced from tens of thousands to mere thousands. But even much more numerous species, such as pigeon and budgerigar, experienced significant change. Scientists have already begun to measure change in the natural world in response to average temperature rises by shifts either in timing or latitude, or even altitude. The Uppsala research offers a kind of baseline of historical change, and could help conservation bodies now concerned with saving species already considered at risk of extinction because of habitat destruction, hunting and human-induced climate change. “The results from our study document that such climate events significantly affect the effective breeding sizes of bird populations,” the authors conclude. – Climate News Network
Tim Radford, a founding editor of Climate News Network, worked for The Guardian for 32 years, for most of that time as science editor. He has been covering climate change since 1988.