U.S. salamanders face a ‘tough situation’

North America is the world’s hotspot of salamander diversity. The continent is home to all salamander families but one (Hynobiidae, found only in Asia) and nearly half of all salamander species. The U.S. – particularly the eastern U.S. – is a hotspot within this hotspot, hosting more species than any other country.

But biologists are worried an emerging disease may quash this diversity. It’s a fungus called Batrachochytrium salamandrivorans (Bsal) that eats away a salamander’s skin, causing big lesions and, for many, death.

Bsal was first identified in Europe after it almost entirely killed off populations of fire salamanders in Netherlands, Germany and Belgium. Researchers believe it traveled to Europe from Asia via the pet trade, and they think it may just be a matter of time until it gets to the U.S. Once there, biologists say Bsal is likely to have a huge impact on salamander populations. Preliminary research indicates many, if not most, salamander species are susceptible to infection, and those that don’t die outright may act as carriers and help spread the disease to new areas and ecosystems.

But Bsal isn’t the only threat to North American salamanders. Habitat loss, existing diseases, and other human pressures have already led to steep declines, with more than 40 percent of U.S. species at risk of extinction. Scientists worry Bsal could be the nail in the coffin for already-threatened species and are prioritizing their conservation in the rush to develop a plan of action for combatting a possible pandemic.

‘In the right place at the right time’

Living underground and underwater, in the trees and on the forest floor, North American salamanders have evolved into a dizzying array of species that are able to inhabit a wide variety ecosystems. Biologists think their diversity has something to do with the fact that salamanders from most species tend to spend their lives in small home ranges in places where there are a lot of different kinds of habitat. This means that populations were easily separated from each other, and this separation facilitated their evolution into new species over millions of years.

Basically, North American salamander diversity is due to “a mix of opportunity and being in the right place at the right time,” said David Wake, a salamander expert and professor emeritus at the University of California Berkeley. Wake has been researching salamanders for decades and literally wrote the book on Plethodontidae, the world’s biggest salamander family.

According to Wake, Plethodontidae likely evolved in the Appalachian Mountains of the eastern U.S. and owes its success to two bizarre adaptations. First, most plethodontid salamanders are “direct-developing,” which means they don’t go through the aquatic larval phase typical of most amphibians. Instead, these salamanders lay their eggs on land (and often guard them), which hatch out into tiny, terrestrial facsimiles of adults.

Plethodontid salamanders also lack lungs, “breathing” entirely through their skin and by gulping air into their vascularized mouths. Lunglessness has its drawbacks –cutaneous respiration means their skin must always be kept moist to allow the absorption of oxygen, and it imposes a pretty strict size limit – but it’s also allowed plethodontid salamanders to repurpose the muscles and cartilage traditionally used for breathing for something else entirely: tongue-slinging. Unlike other salamanders that need to get right up to their prey to it catch it, plethodontids project their tongues out of the mouths to feed, tongues that can be nearly as long as their bodies. This, Wake says, opened up new habitat frontiers – including in the tropics – and contributed to the evolution of the more than 400 plethodontid species that exist in the world today.

“The specialized feeding system of plethodontids was not constraining but rather efficient in many ecological settings,” Wake said. “When you add direct development and the freeing from reliance on water, the salamanders had access to many microhabitats not used by other salamanders (e.g., trees and bromeliads in trees).”

Salamanders aren’t just diverse in their appearances and habitat preferences. They’ve also evolved a wide array of courtship behaviors – some of them quite complex.

“The courtship rituals and pheromones of plethodontid salamanders make for great study systems for scientists interested in the evolution of these behaviors and chemical compounds,” said University of Tennessee PhD candidate Todd Pierson. “Plus, there’s something magical about standing in the rain and watching dancing salamanders.”

Pierson was inspired to study salamander reproduction after witnessing the courtship dances of the Yonahlossee salamander (Plethodon yonahlossee), a calico plethodontid found in the southern Appalachians. He and his colleagues are currently finishing up a study of two-lined salamanders (Eurycea bislineata) that found that this species has two different kinds of males: a “searching male” that is equipped for finding females, and a “guarding male” that protects the female at nesting sites.

North American salamanders are often the most abundant vertebrates in their ecosystems, with researchers finding three salamanders per square meter in forests of the eastern U.S. Biologists say this abundance likely adds up to big ecosystem impacts.

“It is difficult to quantify how important salamanders are for ecosystems, but the incredible biomass reached by some species suggests that it must be considerable,” said David Beamer, a biologist at Nash Community College in North Carolina who has been studying salamanders for the past 15 years.

Studies have found that by eating so many insects, salamanders help regulate forest nutrient cycling and carbon emissions, and have other important functions as predators.

“In at least some situations such as in vernal pools they function as keystone predators,” Beamer said. Keystone predators are top predators that play a pivotal role in maintaining the biodiversity of an ecosystem by keeping the populations of other species in check. Salamanders are themselves an important source of prey for other animals.

But salamanders and their ecosystem functions are increasingly under threat, and many species have already undergone precipitous declines. The IUCN lists 41 U.S. salamander species as Vulnerable, Endangered or Critically Endangered; 35 of these are plethodontids. The causes are nearly as diverse as salamanders themselves (and some remain unclear), but they cluster under a few age-old headings: habitat loss, invasive species, and introduced disease.

Nowhere to live, nothing to eat

Habitat loss and degradation is generally regarded as the biggest threat to amphibians globally. Most land cover change in the U.S. took place before 1910, but degradation, fragmentation and other types of alteration continue to affect wildlife habitat.

Surveys conducted in the 1990s found salamander densities were five times lower in areas of the Appalachian Mountains that were logged than in mature, native forest. The researchers estimated that timber harvesting in national forests in western North Carolina led to the loss of some 14 million salamanders every year.

“It also is chronically reducing regional populations by more than a quarter of a billion salamanders (9%) below that which could be sustained if mature forests were not cut,” they write in their study.

Beamer says salamanders are “relatively resilient” and can tolerate a fair amount of habitat loss since they don’t need large areas to persist.

“The same features of salamanders biology that have resulted in the large diversity of species probably helps lessen the impact of habitat degradation relative to other animal species,” he said.

However, he added that he has witnessed local declines due to habitat destruction. He points specifically to the plethodontid dusky salamanders (genus Desmognathus), many of which live in the U.S. Coastal Plain, a flat, swampy skirt of land that extends along the east coast of the U.S. and down into Mexico.

“The decline in these cases predates my observations,” Beamer said, “but by revisiting localities where collections were made historically it has become clear that many of these populations (probably even the majority) have declined.”

The habitat of dusky salamanders includes Coastal Plain longleaf pine forest. Once one of the most extensive forest ecosystems in North America, less than 5 percent of longleaf pine forest remains today. The Coastal Plain as a whole is considered a biodiversity hotspot, which means it contains more than 1,500 endemic plant species and has lost more than 70 percent of its habitat. It’s also home to more frog species than anywhere else in the U.S. and Canada, and it boasts six of the planet’s nine salamander families.

One species of dusky salamander that has undergone a particularly steep decline is the southern dusky (D. auriculatus). Once found in much of Florida, Georgia, and part of Alabama, the species has vanished from more than 99 percent of its range and today exists only in a handful of locations. Researchers are still trying to figure out just what caused its decline, but they think one clue might be that the only known remaining populations are found in protected, long-undisturbed forest.

Other Coastal Plain salamanders threatened by habitat loss include the striped newt (Notophthalmus perstriatus), frosted flatwoods salamander (Ambystoma cingulatum), and the reticulated flatwoods salamander (A. bishopi). All of these species are native to longleaf pine forest and all depend on islands of protected forest to survive. The frosted and reticulated flatwoods salamanders are listed as Vulnerable by the IUCN, while the striped newt is listed as Near Threatened and was recently denied federal protection.

In addition to outright habitat loss, Beamer says “well-intentioned habitat management” is also harming salamanders. Take, for example, the flatwoods salamanders that live in among longleaf pines, one of the most fire-dependent tree species in North America. Instead of hibernating during the winter, flatwoods salamanders hibernate underground during the summer because that’s wildfire season in their habitat. Or at least it used to be. Today, forest managers set controlled burns in an effort to mimic natural conditions and help restore longleaf pine forest – but they do it in the winter instead.

Flatwoods salamanders breed in ephemeral ponds that dry up in summer and fill up again in the winter. Studies have found that fires set in the winter tend to be less effective and can’t burn pond basins that they normally would be able to in the summer. Without fire to burn back vegetation and provide nutrients, researchers think aquatic salamander larvae may not grow as quickly as they need to in order to metamorphose into adults before their ponds dry up again.

Adding insult to injury are invasive species like feral pigs that root around and destroy salamander habitat, eat salamanders and their eggs, and spread diseases from one ecosystem to another. While they’ve been around for centuries, research indicates pigs are a much bigger threat now for species whose ranges have shrunk due to habitat loss. One study published in Nature in 2018 found that feral pigs share habitat with up to nearly 88 percent of imperiled species in the contiguous U.S., a number the authors predict will only grow over the next decade. This list includes both the frosted and reticulated flatwoods salamanders, as well as threatened plethodontids like the Barton Springs salamander (Eurycea sosorum), the Austin blind salamander (Eurycea waterlooensis), the Jollyville Plateau salamander (Eurycea tonkawae), and many others.

And unlike industrial logging, pigs don’t avoid protected areas.

“Annual costs of controlling invasive species occupying habitats of U.S. endangered species have been estimated to total $29–38 million (1997 $U.S.),” they write, “and without this continued level of support, 60% of these species could be subject to declines or extinction, even if their habitats are nominally protected.”

There may be another factor in salamander decline, according to David Wake: global drops in insect abundance.

The news broke via a 2017 study in PLOS ONE that found the biomass of flying insects at sites in Germany had declined 76 percent over a 27-year period. But several studies had been published before then with similar findings. More recently, a 2018 study published in PNAS found arthropod biomass in Puerto Rico experienced a 10- to 60-fold plummet since the 1970s.

Other studies have detected declines of animals that eat insects, such as anoles in Puerto Rico, and birds in Germany and North America. While salamanders haven’t been directly studied when it comes to this phenomenon, Wake calls it “a grave threat that has received too little attention,” adding that it has the potential “for great impact on salamander populations.”

A coming plague

With many salamander species already threatened, researchers are very worried about what may happen if Bsal arrives in North America. Most species in the U.S. are plethodontids, which research suggests are particularly susceptible to the disease. Newts from the family Salamandridae are also considered highly susceptible to infection.

Biologists are rushing to test as many species as they can, with a few dozen under their belt so far. While they haven’t yet published their results, they said that most species they’ve tested can become infected. They’re giving testing priority to threatened salamanders so they know where to aim conservation action in the event of an outbreak, with plans to remove species from the wild if it looks like extinction is on the horizon.

Researchers aren’t just testing salamanders – they’re checking to see if North American frogs are also susceptible to infection. And it seems like they are, with several species capable of becoming carriers.

A complicating factor is the presence of another disease. A related fungus called Batrachochytrium dendrobatidis (Bd) has already spread around the world and affected some 200 species of amphibians, mostly frogs, driving more than a few of them to extinction. Bd, which favors warm environments, is present in many southern U.S. ecosystems, and research indicates it worsens the impact of Bsal.

“Overall, I think there is good justification to be worried about the fate of salamanders,” Pierson said.

Biologists say the best thing to do would be to make sure it never gets to the U.S. by imposing a strict trade ban on all amphibians coming into the country. In 2015, researchers, including Wake, released an article urging for an “immediate ban on live salamander imports” until effective decline prevention and management protocols were in place. The U.S. government responded in January 2016 with just such a ban, enacting an interim rule under the Lacey Act that lists 201 salamander species as “injurious” and restricts their trade.

But since then, research has shown that frogs can also act as carriers – including species closely related to those that are popularly sold as pets.

“Now that we know frogs are potential vectors, the strongest action would be to ban all salamander and frog imports,” Tiffany Yap a staff scientist at the Center for Biological Diversity who specializes in amphibian disease, told Mongabay in October.

For Beamer, a ban is a good first step. But he worries there’s a good chance Bsal has already been introduced by the millions of salamanders imported into the U.S. over the past 50 years. He also says Bsal likely won’t be the last disease threat for salamanders, and underlines the importance of research.

“In the global society that we live in I suspect that we will have many more issues with pathogens reaching novel hosts and environments and the potential for widespread problems is great,” Beamer said. “This is a tough situation with no easy answers.

“We definitely need to support research to investigate emerging pathogens – that will be the best way to have a fighting chance of containment.”

 

 

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