Mojave Desert bird diversity fell 43 percent in a century, a study found

Bird communities in the Mojave Desert have become much less diverse over the past century, according to a study by researchers at the University of California, Berkeley, described in this National Park Service article. The team resurveyed locations first surveyed as much as a century ago, including more than a dozen sites in Joshua Tree National Park, and found an average 43 percent decline in bird species diversity. Reduced precipitation, a consequence of climate change, seems to be behind it.

An American kestrel. NPS photo.

Old data, new fieldwork

The study built on survey data from a project that Joseph Grinnell, the first director of UC Berkeley's Museum of Vertebrate Zoology, began early in the 20th century. Using his field notes, the team found and resurveyed 55 of the same Mojave locations and added 6 more, all on federal land protected from direct human impacts such as farming, grazing, housing, mining and alternative energy development. The new fieldwork used standard bird transect methods adapted to match the historic data, and statistical occupancy models compared the two periods while allowing for differences in how easily species are detected.

What changed

Grinnell's team had recorded 135 species of breeding birds at the 61 sites. In the resurvey, 39 species were less likely to turn up at any given site. Other studies have found birds expanding north as the climate changes, but the only species found at more sites were the common raven, which benefits from new food sources that people create, and four non-native species. Diversity fell significantly at 55 of the sites, and on average the modern surveys found about 18 fewer species per site, the 43 percent decline.

Ravens and Eurasian collared doves, a non-native species, are more common today. NPS photos.

Birds that were already rare a century ago, such as mountain quail and Lawrence's goldfinch, are rarer still. Raptors, including the prairie falcon, American kestrel and sharp-shinned hawk, have all but disappeared from many sites. A related study found that the physiological effects of climate stress are especially harsh on larger birds and on birds that eat animals, such as raptors and insect-eaters. The long-running citizen-science Breeding Bird Survey agrees: volunteers' observations show a 46 percent decline in desert bird "indicator species" since the survey began in 1968.

A second study in Joshua Tree

In 2016-17 biologists from the San Diego Natural History Museum studied birds at six locations in Joshua Tree National Park, timing their visits to match those of Grinnell-era researchers in 1945 to 1953. They found that water had dwindled or vanished at all six sites; only Stubbe Spring and Cottonwood Spring still had some surface water. Two sites had extensive fire damage, and a significant amount of pinyon-juniper and scrub oak habitat had been lost to long-term drought.

The park had lost many mountain and chaparral birds, with notable drops in mountain quail, oak titmouse, blue-gray gnatcatcher, gray vireo and Western screech owl, while common ravens were seen more often and the California towhee appeared to have extended its range into the park. In future, fire and reduced precipitation are likely to threaten species such as the California scrub jay and spotted towhee.

Mountain quail are in serious decline at Joshua Tree because of lost surface drinking water and preferred food plants. NPS photo.

Why the decline

The researchers also resurveyed non-desert ecosystems Grinnell studied, and bird diversity held steady there. Deserts, the article says, have dried faster than other regions as the climate has changed, and the Mojave now gets 20 percent less rain and snow than in the early 20th century. Shortages of water, rather than the direct effects of higher temperatures, appear to be making the desert less habitable.

Why it matters

Birds pollinate, scavenge, hunt and spread seeds, so less diverse bird communities may disrupt how plant and animal communities work. The article cautions that open desert may look ideal for renewable energy, but solar and wind farms change desert vegetation and could reduce habitat and food for birds, and communities are eyeing every drop of water as reservoirs shrink, including the water that feeds the springs birds rely on.

Sources

• National Park Service, “Climate Change and Declines in Mojave Desert Bird Diversity” (https://www.nps.gov/articles/000/climate-change-and-declines-in-mojave-desert-bird-diversity.htm), describing work by Kelly Iknayan and Steven Beissinger (UC Berkeley; Proceedings of the National Academy of Sciences, 2018) and by Lori Hargrove and Philip Unitt (San Diego Natural History Museum); photographs credited to the National Park Service; a study-site map built on a commercial base map is left out; rewritten in hubnx's own words.