by Climate change is not the only threat facing California’s birds. During the 20th century, urban sprawl and agricultural development have dramatically altered the state’s landscape, forcing many native species to adapt to new and unfamiliar habitats.
In a new study, biologists at the University of California, Berkeley, use current and historical bird surveys to show how land-use change has exacerbated — and in some cases mitigated — the impacts of climate change on bird populations in Los Angeles and the Central Valley. .
The study found that urbanization and much hotter and drier conditions in LA have reduced more than one-third of bird species in the region over the past century. Meanwhile, agricultural development and a warmer and slightly wetter climate had a more mixed impact on biodiversity.
“It is quite common in studies of the impact of climate change on biodiversity to only model the effects of climate and not consider the effects of land use change,” said senior study author Steven Beissinger, professor of environmental science, policy and management at UC. Berkeley and a researcher at the campus’s Museum of Vertebrate Zoology (MVZ). “But we are finding that the individual responses of different bird species to these threats are likely to drive unpredictable changes that complicate predictions of extinction risk.”
The study, published this week in the journal Scientific Progresspresents the latest results from UC Berkeley’s Grinnell Resurvey Project, an effort to revisit and document birds and small mammals at sites surveyed a century ago by UC Berkeley professor Joseph Grinnell.
In the current study, the researchers resurveyed birds at 71 sites in LA and the Central Valley. They then used their findings – along with current and historical data on land use, average temperature and rainfall – to analyze how changes in climate and landscape have contributed to changes in bird populations.
In LA, they found that 40% of bird species were present at fewer sites today than 100 years ago, while only 10% were present at more sites. Meanwhile, in the Central Valley, the percentage of species that declined (23%) was only slightly higher than the percentage that increased (16%). In many cases, the impacts of each threat were mitigated against responses to climate change and land use by bird species, where one threat led to an increase in a species while another caused the same species to decline.
The decline in bird species in LA over the past century is similar to the dramatic collapse of bird populations documented by the research team in national parks in the Mojave Desert over the past 100 years, which has been linked to heat stress from climate change.
“The Central Valley, in general, had less change — there were winners and losers,” Beissinger said. “Whereas in LA, we mostly saw losers.”
Ages and double whammies
Grinnell was a teenager when he began documenting birds in the late 1890s near his childhood home in Pasadena, California. He later applied his detailed approach to surveying as a professor of zoology at UC Berkeley and the first director of the MVZ.
“In those days, they didn’t have fancy binoculars. They didn’t have recordings of bird calls. So, they had to go in and learn the birds through the resources available. Often, they were from specimens in museums .that was through popular guides or handbooks,” Beissinger said. “Grinnell was ahead of his time in taking field notes, and he was very draconian in requiring all his students to take those notes.”
Grinnell’s mechanical field notes allowed Beissinger and his team to construct a historical baseline of California’s bird life at the beginning of the 20th century. The notes are so detailed that the researchers are able to recreate the birds they encounter every day and account for the ways in which new technologies, such as better binoculars and field guides, have made it easier for contemporary bird biologists to detect. This analysis allowed the team to directly compare current bird surveys with historical surveys.
To disentangle the different and sometimes opposing impacts of land-use change and climate change, the researchers analyzed historical maps of urban development and agriculture to determine how the landscape at each study site had changed during the 20th century. They also found historical average temperatures and rainfall at each site.
In LA, they found that species such as Anna’s hummingbird and the American crow were able to adapt to warmer, drier conditions and urban development, experiencing what the researchers call a population “windfall.” Both changes had a negative impact on other species, such as the western meadowlark and the lark sparrow, and instead suffered a “double whammy”.
Species with mixed impacts include the black phoebe, great gray warbler, house wren and blue-grey warbler.
“Our results strongly reflect the fact that we are experiencing climate change and land use at the same time, creating happy conditions for some species, while others are declining due to the same changes,” said Beissinger. “Sometimes, species could be pushed and pulled in different directions by changes in climate and land use.”
Also, bird species in the Central Valley experienced a combination of windstorms, double whammies, and mixed impacts, but the proportion of species that experienced windstorms was much higher in the Central Valley than in LA and almost avoided the percentage that were double whammies.
“There are some species that have managed to survive the agricultural changes, and some species that have even colonized and increased due to those changes. But they tend to be species that are more common and widespread, and the most sensitive species are the ones that started to disappear when agriculture replaced the natural grasslands,” Beissinger said. “In urban areas, there are fewer species that can find what they need and avoid the dangers of the city.”
Additional co-authors of the paper include Sarah A. MacLean of the University of La Verne and Kelly J. Iknayan and Perry de Valpine of UC Berkeley. This work was supported by grants from the National Science Foundation (DEB 1457742, DEB 1911334 and DEB 1601523), the National Geographic Society (9972-16), a UC Berkeley Chancellor’s Fellowship and a Miller Institute Research Professorship.
