At just a number of inches in length, with a lemon-colored belly, the foothill yellow-legged frog could appear unusual. But its range once prolonged from central Oregon to Baja California. In 2023, it was listed under the federal Endangered Species Act. Its rapidly declining range is due partly to a fungal pathogen called Bd, which has decimated amphibians worldwide.
A team of researchers, including UC Santa Barbara's Andrea Adams, has conducted essentially the most comprehensive study of the history of disease dynamics in yellow-legged frogs within the foothills. The team's data — obtained from each wild frogs and specimens in museum collections — enabled them to trace infection patterns over a big geographic range. In a study published in , researchers revealed that drought, rising temperatures and increased conversion of land for agriculture look like the most important aspects for Bd infection on this species.
The goal of the researchers was to gather as much data as possible in each space and time. They surveyed streams and rivers in California and Oregon, where they swept wild yellow-legged frogs for the presence of Bd. He also modeled them within the Fluorescent Light Museum's collections dating back to the Eighteen Nineties.
The team leveraged a big network of individuals and institutions to gather this wealth of samples. “Many foothill yellow-legged frog field researchers had data they weren't actively analyzing,” said co-author Adams of UCSB's Earth Research Institute. “And that's how we were in a position to bring all that data together and get it right into a usable form that we will paint a much bigger picture of what's been, and is being done, with Bd on this species. will be used to do.”
The researchers swabbed the skin of every frog to find out if the animal had been infected. To test for Bd, they used a PCR test, much like some tests for COVID. By searching for Bd DNA from hundreds of samples, the researchers were in a position to discover the speed and severity of infection. Co-lead writer Ryan Peck ran this information through statistical models, including climatic, geographic, biological and land-use variables. This enabled the team to trace disease patterns over a big geographic range over nearly 120 years.
The team discovered that BD disease patterns correlated with historical frog declines. The pathogen began spreading from the southern coast of California within the Nineteen Forties, moving northward and eventually infecting nearly all the region. Drought, rising temperatures and increased land use for agriculture look like the essential aspects contributing to infection.
Bd is a fungus that spreads through spores in water, but this spread can occur in a different way amongst yellow-legged frogs in numerous regions and climates, the researchers found. In some locations, drought increased infection, while in others, it didn’t, possibly resulting from the presence or absence of other species that may carry Bd and share the identical water, e.g. that of the American Bullfrog, a species introduced from eastern North America.
“If you combine the fact that bullfrogs are increasing the number of eggs that these frogs are exposed to, and then they're trapped together in these little ponds, that explains why the drought is important. . A really large number of spores are getting sick and dying,” said lead writer Anat Belasin, a postdoctoral fellow at UT Austin and research associate with the Smithsonian Conservation Biology Institute. Moreover, foothill yellow-legged frogs live exclusively in rivers and streams, not in ponds and lakes. So when these waterways shrink into isolated ponds, species are already under pressure.
Development of Bd within the foothill yellow-legged frog also differed from its course in other western amphibians. In many other species, the disease spreads from urban centers reasonably than on this clear south-to-north trend. Moreover, the disease appeared later within the foothill yellow-legged frog than other species in its range. “These findings open up more questions about what was preventing transmission and what allowed it to occur later,” Bellason said.
Frogs transition from herbivores to carnivores as adults, which implies they link different nutrient cycles in a food web. Their position at the middle of the food chain also affects ecosystems.
“When you remove frogs from an ecosystem, what you get is less control over insects, the things that frogs eat,” Bellason said. “There's also less food for things that frogs eat, like snakes, birds and small mammals. It really throws things off and makes the ecosystem less stable and less functional.”
Land conversion for agriculture was one other major factor influencing the spread of BD. “There are areas that have wet soil that would be with suitable habitat for these frogs,” Bellasin said. “In areas where more of this land has been converted to agriculture, we see a higher risk of frogs becoming infected with the fungus.”
In addition to disease hotspots, the team also identified several cold spots — areas where the pathogen is present but less effective. Having many cold spots in numerous areas is a very good sign, as it could mean that many areas have conditions suitable for keeping disease rates low, even with climate change in temperature and drought. The samples increase.
The authors are curious what might explain this clustering, especially when cold spots appear in unexpected places: for instance, with similar habitat, land use and climate effects as hotspots. . This suggests that there could also be some genetic basis for the differences, whether on the pathogen or host side. Adams is currently researching the feasibility of reintroducing yellow-legged frogs to Southern California.
The results of this paper shed considerable light on the dynamics of where Bd occurs, what drives its spread and the way the pathogen and frogs may interact in the long run. “We took a big snapshot of the disease relationship of this species over time,” Adams said. “Early studies gave researchers a glimpse of disease patterns in small geographic regions,” Adams said, “but now we have a much larger dataset.” which further confirms, and expands upon, a lot of these patterns.”
This research was supported partly by the Cedar Tree Foundation, the National Science Foundation, the Schmidt Family Foundation and the US Geological Survey.