A wide-ranging study of gains and losses of populations of bird species across Mexico in the 20th century shows that shifts in temperature, due to global climate change, are the primary environmental influence on bird species' distributions.
"Of all drivers examined...only temperature change had significant impacts on avifaunal turnover; neither precipitation change nor human impacts on landscapes had significant effects," wrote the authors of the study, which appeared recently in the peer-reviewed journal Science Advances.
Using analytical techniques from the field of biodiversity informatics, researchers compared current distributions with distributions in the mid–20th century for 115 bird species that are found only in Mexico. They then compared those bird community changes to patterns of change in climate and land use.
Yucatan Jay, Mexico (Photo: Colin Manville)
Lead author A. Townsend Peterson, university distinguished professor of ecology & evolutionary biology at the University of Kansas and senior curator at KU's Natural History Museum and Biodiversity Institute, explained: "We assessed which places appear to have lost which species. We picked out places that were well characterized with data. Once we did a dissection of what was lost where, we analysed what could be driving those changes. We got summaries of how much temperature, rainfall and land use had changed across the country, and that was a very interesting result because avifaunal changes were associated with temperature change only."
Peterson and his colleagues also considered 890,000 records of bird occurrences collected since 2000 by citizen scientists and deposited in the aVerAves database.
The researchers related the changes in bird populations to land-use patterns using the Global Human Influence Index, a dataset that includes population density and other measures of people's influence on natural areas through land use, building, roads, night-time lights and other factors. They also explored information captured by hundreds of weather stations documenting climate change across Mexico in the latter half of the 20th century, focusing on temperature and rainfall.
According to Peterson, the researchers suspected temperature fluctuations caused by global climate change would be a key driver determining bird community change in the temperate zone, but they were surprised that it was the prevailing factor in a tropical area like Mexico.
He continued: "Birds are making sure they have space and time and resources to carry out their life cycle — the real name of the game is reproduction. In the temperate zone, birds are waiting until it's just warm enough to start the nesting cycle. They're timing breeding so they'll have young in nests right when they have maximum food available. But in the tropical world, it's always a pleasant temperature for birds, so I'd imagined precipitation might have had more of an effect."
Overall, among bird species endemic to Mexico, numbers of losses greatly outweighed gains, the study showed. The research found "relatively few" gains of species, mainly in the central–northern Chihuahuan Desert and in northwestern Baja California along the border with the U.S. state of California.
Peterson said the findings should provide useful information to Mexican environmental policymakers. "When you design protected areas, like national parks or biospheres for biodiversity conservation, that design needs to take into account how climates are changing," he said. "They've done major planning, more than in the U.S., for present-day distributions of species. But climate change must be taken into account because those distributions are changing."
