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Research finds that temperature variation between ponds affects fungal infection outcomes

Amphibian populations, including frogs, toads, salamanders and newts, have been declining globally since the 1980s. Many species have even gone extinct.��

There are several potential causes for this decline, but one contributor is disease. For example, infection by parasitic flatworms can cause frogs to grow extra limbs, making it harder for them to evade predators. Another prominent amphibian disease called chytridiomycosis has been specifically��. It is caused by the fungus Batrachochytrium dendrobatidis, or Bd.

In a study comparing the temperatures of ponds to their level of infection over time, researchers�� ���Ի���Valerie McKenzie, a �Թ��� of Colorado Boulder professor of ecology and evolutionary biology, discovered that ������thrives on hosts within a specific range of temperatures and level of temperature variability, above or below which infections are not as severe. This relationship was found to be driven primarily by differences between ponds rather than seasonal differences.

Valerie McKenzie, a �Թ��� of Colorado Boulder professor of ecology and evolutionary biology, worked with PhD graduate Brendan Hobart and other research colleagues to study how temperature affects amphibians' susceptibility to fungal infections.

Hobart worked on the study as a PhD student at CU Boulder and has since completed his PhD and moved on to a research scientist position at the �Թ��� of Wisconsin. Another CU PhD student, Timothy Korpita, was also involved, along with several people from the��. McKenzie is the principal investigator of the��.

What makes Bd unique?

Fungi grow on substrates, which are surfaces that provide them with the nutrients they need to develop their reproductive structures and release spores. Some of these spores will end up in new substrates, beginning the next generation. Instead of growing on decaying biological material or living plants like many other species of fungi, Bd’s substrate is the skin of a living animal, specifically an amphibian. Additionally, rather than releasing spores that float through the air, Bd propagates using zoospores, which can swim short distances through the water using their whip-like tails.

“They are microscopic,” McKenzie says, “and they will attach themselves to a skin cell, then penetrate and go inside. They use amphibian skin cells as a place to replicate themselves, rupture that skin cell and let out more zoospores that can go on to infect nearby skin cells or go in the water and infect other individuals.”��

Bd’s ability to spread from one pond to another is still something of a mystery, however.��

“We still do not understand all the mechanisms by which it is getting spread,” McKenzie says. “People have made guesses that it could be birds that land in the pond water picking up some of these zoospores in their feathers and then fly off and land in another pond.” Even their ability to infect different hosts is surprising, considering that the zoospores can swim only one or two centimeters, but they are able to chemically target molecules found on amphibian skin to make the most of this short range.

Regardless of how the fungus gets around, its strategy is clearly effective, as it has infected a large number of diverse amphibians. According to McKenzie, there are something like 8,000 species of amphibians, which is only slightly fewer than the number of mammalian species.��

“This one fungal pathogen is causing declines, or is predicted to cause declines, in maybe a third of amphibians. Imagine if COVID, for example, was causing massive die-offs of not only humans, but all kinds of mammals, like squirrels, whales, wolves, cats, dogs. That is sort of what is happening to amphibians with this fungus. It is unprecedented for what one pathogen can do.”

������is dangerous for amphibians because it targets their skin, which they rely on for many purposes, like balancing hydration. According to McKenzie, disruption to the skin can result in secondary organ failure. The disease can be more or less severe for different species, but there are many species that have been seriously affected worldwide. ������is currently most prominent in the Americas—particularly the Central and South American tropics—eastern Australia and east Africa, but may spread to other parts of the world over time.

How temperature influences infections

Previous research into Bd has singled out thermal conditions, meaning the temperature of the habitats that hosts live in, as key drivers of host outcomes. Particularly, the variability of temperatures and the mean (average) temperature are important variables. “Temperature is the ultimate determinant of most or all biological processes,” Hobart says.

“It is especially relevant to ectotherms”—cold-blooded animals do not produce their own heat—"and their pathogens because their body temperature largely fluctuates with the environment,” Hobart says.

��This study is directed toward exploring the relationship between temperature and infections further, particularly by separating changes in temperature into seasonal and among-site components. To do this, the researchers measured temperatures and Bd infections of eastern newt populations across 20 ponds in Wisconsin over the course of two years.

“All of these ponds were within a few miles,” Hobart says. “From a broad scale perspective, they all have the same climate. If you were to look up a weather forecast on an app, it would be the same for all the ponds, but the actual conditions are very different depending on things like how much tree cover there is over the pond, how clear the water is, how much stuff is floating on the surface, all these different biotic and abiotic factors.”��

These differences lead to significant variation in pond-to-pond water temperature, and they are what the study covered rather than gradients in temperature within a given pond.

When the researchers looked at the temperature variability and average temperature, they found that both changed at the same time, or in other words, covaried. According to Hobart, this is because the ponds with the most variable temperature also tended to be the warmest. For this reason, the two variables were combined into a thermal mean and variability index (MVI), which ranged from cool and stable to hot and variable temperatures. When combined with infection data obtained by capturing, swabbing and releasing newts, this index was shown to have a non-linear relationship with infection load (meaning not only whether the fungal disease was present but also how much was on the animals’ skin).

Considering thermal variation both over time and between ponds, infection load was highest at middling MVI values, declining similarly when the index either increased or decreased from there.

“It is this primary hump-shaped relationship,” Hobart says. When the variations over time and space were separated out, the spatial variation resembled the overall relationship very closely, while the temporal variation looked different. “That is what produced this finding that variation from site to site was driving the overall pattern.”

Implications for conservation

Considering how severe the effect of Bd has been on amphibian populations, anything people can do to reduce infections is of interest. The results from this study suggest that changing the temperature of a pond could be an effective way of doing this, but it is not as simple as it sounds.

Like many fungi, Bd does best within a limited range of temperatures, which is about 23–28 degrees Celsius or 73–82 Fahrenheit, according to the researchers. At middling MVI values, the temperature is right for Bd, and there is even some evidence that ������handles temperature variability better than its hosts, giving it an additional advantage.��

However, once the temperature increases out of Bd’s ideal range, the benefits of variability cannot counteract the unfavorable heat, especially because amphibian immune responses often increase in strength at these temperatures. On the other hand, when the temperature is low, ������does not get any advantage from variability and is also outside of its ideal temperature range.

This means that, depending on the starting conditions, the severity of ������infections in a pond might be diminished by either increasing or decreasing the temperature, but in some cases, changing the temperature would only make things worse.��

“It has been suggested,” Hobart says, “that one could cut down trees around a pond to let more light in and make that pond hot. In principle, that seems like a fine idea.” However, “if you did not know where you were on that index, and you cut down a bunch of trees, you could inadvertently increase infection.”��

In other words, if a pond’s temperature is middling, increasing it could help with infections, but if the pond is cooler to begin with, it could bring the thermal MVI into the range where ������thrives.

“There have been a lot of studies looking at the relationship between temperature and this amphibian pathogen,” McKenzie says. For example, there was recently a study that involved�� that frogs can crawl into to heat up and kill off the Bd. “I think what this study shows is that what works for one site may not be applicable for another site, even if that site is relatively close and similar.”

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