In Insights we discover the story behind and beyond a recent publication in Functional Ecology. What inspired the authors to do the research, how did the project develop and what implications might their results have on the scientific community and on society?
This week, Sasha Greenspan, at the James Cook University at Townsville, Queensland, Australia, discusses her recent paper on the interaction between Spotted Treefrogs and a fungal pathogen and the role of temperature thereon (read the free plain language summary here).
Briefly explain your field of expertise, and why you’re so fascinated by your research area?
My research focus is the fungal amphibian disease chytridiomycosis, which poses a threat to frog populations globally. I come from a family of public health professionals, so I developed a keen interest in emerging pathogens over the dinner table conversations of my childhood. As a young adult, my interests began to expand to wildlife ecology and conservation, and I quickly focused in on amphibians and reptiles, a group that is incredibly prehistoric but at the same time faces so many contemporary threats. Studying amphibian disease allows me to pursue a combination of these research themes.
Can you give a brief summary of the paper?
Frogs, being cold-blooded, experience fluctuations in their body temperatures that correspond with environmental conditions but these fluctuations are often overlooked in studies of frog disease dynamics. We looked closely at frogs infected with a cold-tolerant skin fungus and asked how temperature fluctuations influence infected frogs as well as the fungus on its own. We were most interested in the effects of short exposures to temperatures that only slightly exceed the fungus’ optimal temperature range. Frogs are able to reach these warmer body temperatures for short periods of time in Australian rainforest habitats where this fungus has caused severe frog population declines. Infected frogs exposed to these brief, mild “heat pulses” were more likely to survive the fungal infection than infected frogs exposed to constant cool temperatures. When grown in culture (independent of the host), the fungus grew slower under short heat pulses than under cool temperatures, indicating that heat pulses directly interfered with the growth of the fungus. Our results suggest that short exposures to warm temperatures (such as basking in direct sunlight) could help infected frogs survive this pathogen in the wild.
How did your ideas for this research evolve during the project? Did you run into difficulties, and if so, how did you overcome them?
The biggest challenge was to determine how to experiment with realistic temperature treatments in a way that would allow our results to be interpretable. In nature, heat pulses vary in length and extremity from day to day but if we mimicked this type of ultra-variable temperature regime in our experiment, we would not know which attributes of that treatment influenced the frog responses. That’s why we created our rectangular-wave treatments that repeated over multiple days. Once we know more about the effects of simple, fluctuating-temperature regimes on frogs and this fungus, we can test for effects of even more realistic temperature regimes.
Did your research expose any new knowledge gaps?
There are many genetic lineages of Bd so it is important to stress that other lineages could have different thermal tolerances. Also, we still don’t fully understand the effects of temperature fluctuations on the frog immune system.
Because of the research set-up, you were not able to determine if heat pulses enhanced immunity of the host frogs to the pathogen or if it was just the effect of the negative impact of heat pulses on the growth of the pathogen. Why is it so important to stress this?
Disease systems are complicated by the fact that they involve the ecology of the host and the ecology of the pathogen, which can both be very complex. We can’t fully understand a disease system unless we know the role that both components play. There are many aspects of frog skin that we can’t control in a 96-well plate so that’s why we decided to test for similar patterns in live frog hosts.
At the end of your paper, you suggest some management interventions that could reduce infection rates of Spotted Treefrogs. One of them is canopy openings. Is it too far-fetched for me to conclude that you are actually also calling for a type of forest management where there is a place for natural dynamics (i.e. gaps by fallen trees)?
Great point. Natural forest dynamics probably play an important role in this disease system. Here’s an extreme example. But natural disturbances probably influence disease dynamics at less extreme scales as well. In addition to natural canopy openings, our research suggests a role for targeted, low-resource human intervention that would selectively open or thin canopies as an approach to species preservation.
Following up on the above question, your second recommendation is a bit more artificial and involves non-natural heat sources. Could you discuss these measures in more detail, including its practicalities?
Artificial heat sources such as small heating units would probably present more logistical challenges than selective removal of tree branches but an advantage is that artificial thermal refuges could be viewed as a less controversial management strategy to managers who may be concerned about altering vegetation. Interestingly, a similar strategy has been proposed for helping bats cope with white nose syndrome, another emerging fungal disease.
What would be your message to conservationists or policy makers based on your results?
We should always be willing to think outside the box when it comes to conservation and management, especially when interventions are simple and inexpensive. Canopy thinning and artificial heat sources should be tried and evaluated in rain forest areas where infected species are threatening to spread the fungus to naïve species or where species are threatened with extinction from chytridiomycosis.
And one last question: when you’re not at work studying amphibians and reptiles, what do you like to do?
When I am not at work, you are most likely to find me eating a lot of food, cuddling with my dogs, or watching Billy Elliot.