Boreal toads used to be abundant in Summit County. What happened?
In the past, boreal toads thrived in Summit County’s high-Alpine environment, living into their 20s and moving across mountain passes. However, since the 1990s, a microscopic fungus has been a major factor in the decline in boreal toads in Colorado, threatening one of the Southern Rockies’ important ecological middlemen.
“It’s out of Pandora’s box,” said Valerie McKenzie, a professor at the University of Colorado Boulder who has done research on parasites and wildlife diseases for years.
The leading evidence, she said, is suggesting that this fungus got moved around by human activities. A foreign strain hybridized with a local strain of fungus, which, in turn, created a super pathogen. Wildlife officials estimate there may be as few as 800 wild adult toads left in Colorado.
The concerning decline was first noticed in the 1990s, she said, but at the time scientists didn’t entirely understand why it was happening. In 1993, Colorado Parks and Wildlife designated the toads as endangered, and it wasn’t until 1998 that scientists formally described a fungal pathogen that negatively impacts the skin of frogs and toads, the chytrid fungus Batrachochytrium dendrobatidis. This pathogen has been detected in approximately 700 amphibian species, according to research published in December 2021.
Surviving better at colder temperatures, the fungus is especially dangerous for mountainous creatures and causes chytridiomycosis. Exospores of the fungus use skin cells like a breeding ground amplifying their numbers, exploding those cells and then spreading to more cells.
Because amphibian skin is a mucous membrane surface, a lot of water and electrolyte exchange happens across their skin to keep their bodies in balance physiologically. If their skin is disrupted, they can’t manage their hydration and electrolytes properly, and it can lead to organ failure.
Now, a majority of the toads have been wiped out by the virus. Currently, there are few options for boreal toads to mitigate its impacts. McKenzie said one is evolution, and some small pockets of species are evolving natural defenses against the pathogen. There are also efforts by scientists to think about breeding amphibians that have more resistance genes, but that is, so far, easier said than done.
“The technologies to do that are becoming more available, but understanding the genomes of amphibians is a totally different beast than understanding genomes of humans,” she said. “When we think about gene editing for people, for medical issues, that’s cutting edge right now. If you want to try to take that technology and apply it to amphibians, we’re still really far behind in terms of understanding the basics of the genomes to be able to even get a handle on that yet.”
It’s not just a Colorado problem, either. According to Colorado Parks and Wildlife, amphibians around the globe are experiencing alarming declines. Of the over 3,000 species of amphibian species, 39% are threatened with extinction. This contrasts sharply with the status of threatened birds at 10% or mammals at 16%. In June, teams from the Denver Zoo and Parks and Wildlife trekked into Gunnison National Forest to introduce 570 tadpoles into wetlands that officials hope could eventually host an established population.
McKenzie said that more than 50% of the boreal toad populations have been wiped out. What does that mean for the environmental landscape?
“To some degree, that’s been largely understudied,” McKenzie said. “We don’t know. It happened too quickly for anyone to have the resources or the advanced thinking to think, ‘Well, we should try to understand what’s going to happen without this species there.’”
McKenzie said that frogs and toads — in most ecosystems — are in the middle of the food web, meaning they eat smaller animals like insects and provide food for larger predators like foxes and birds.
“Their declines provide early warning signs to scientists that stressors like habitat loss, climate change, pollution and disease are making ecosystems unhealthy,” a report from Colorado State University reads. “Without amphibians, insect and algae populations multiply, causing cascading effects on other organisms — including humans.”
Without them as an abundant species, this can cause a trophic cascade, or a series of changes in population sizes at different levels when a species within an ecosystem is impacted. In addition to direct impacts, the loss of toads in the mountains could have periphery downsides, as well, such as beavers.
“If we’re removing amphibians from a whole ecosystem, that can lead to larger populations of insects, or you could all see that the predators of those toads are maybe having a harder time finding meals, and those populations are declining,” McKenzie said. “It’s like taking out a pretty important middleman in the food web ecosystem and disrupting those food web chains.”
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