Laura Brannelly and Michel Ohmer: tadpoles, frogs and drying ponds

In this Insight, Drs. Laura Brannelly and Michel Ohmer – both post-doctoral researchers in the Richards-Zawacki lab at the University of Pittsburgh when the research was conducted, discuss their paper “Effects of hydroperiod on growth, development, survival, and immune defences in a temperate amphibian.”  The research in their paper is a first step in understanding the effects of climate and anthropogenic change on frogs and their immune function. The authors answered the questions about the paper and the research together, but individually for “about the Author”.

What’s your paper about?

Frog in laboratory. Photo by Jakub Zeger
Frog in laboratory. Photo by Jakub Zeger

Our research investigates the effects of pond drying on amphibian larval development, and how stressful developmental conditions can negatively influence the body size, survival and immune function of frogs after metamorphosis.

Human-driven environmental change, urban growth, agriculture, and climate change are causing dramatic shifts in freshwater availability throughout the world.  These changes often have unexpected and overlooked consequences for the wildlife in the surrounding habitats.  Our research aimed to clarify how early pond drying, which is a direct result of changing freshwater availability, can affect the growth, survival, and immune function of metamorphic frogs. 

What are the key messages of your article?

Laura with Mesocosm
Laura with Mesocosm

One of our important findings is that pond drying does indeed have a negative effect not only on the size of frogs at metamorphosis, but also their survival. Furthermore, we found a lasting effect of larval period and pond drying on the immune defences of young frogs, indicating that they might be more susceptible to pathogens in their environment. We even found sex-specific differences in development time, in which male frogs metamorphosed more quickly than females under drying conditions, leaving the females at higher risk of death by desiccation.  These effects might have large consequences for amphibian populations, especially if the frequency of pond drying increases due to climate change or human development.

How is your paper new or different from other work in this area?

Mesocosm Setup. Photo by M. Ohmer
Mesocosm Setup. Photo by M. Ohmer

There are many papers that investigate the effects of pond drying on tadpole growth and development, as well as the size at metamorphosis and survival.  However, our paper is unique in that we also discuss how many facets of the immune system are affected by drying during development. Thus, there can be sublethal effects of climate-driven developmental stressors on amphibians, even when they successfully escape a drying pond.

Does this article raise any new research questions?

From this study we know that pond drying impacts the immune function of frogs, however we don’t know how this might impact their susceptibility to specific diseases.  We are currently investigating how pond drying affects susceptibility to pathogens, such as the fungus Batrachochytrium dendrobatidis, which has devastated amphibian populations worldwide.

About the research

Michel in field
Michel in field

Did you have any problems setting up the experiment or gathering your data?

This was an ambitious project – we set up and monitored 21 mesocosms daily that contained over 800 tadpoles in total, and then cared for and examined immune function in hundreds of metamorphic frogs.  We were really lucky to have so much help from undergraduate students who were interested in amphibians and climate change research, as well as two middle school teachers funded by the NSF Research Experience for Teachers program. These teachers helped the Richards-Zawacki lab develop a science curriculum that explores the impacts of climate change on frogs called Ectotherm ER. All of our assistants and volunteers were a massive help in taking care of the animals; we couldn’t have done it without them!

Were you surprised by anything when working on it?

One result we weren’t expecting was that in our fastest drying treatment, males increased their development rates in order to metamorphose more quickly, while the females did not.  This seems to be a relatively understudied phenomenon that could have dramatic population-level impacts; if males metamorphose more quickly and are able to escape a drying pond while females are unable to, then the sex ratio of the population could shift.  This could result in dramatic impacts on recruitment and population level stability, because fewer females could mean fewer eggs, tadpoles, and young frogs to ensure the population persists into the future.

What are the big questions still to answer?

Overall, we are interested in how climate change, which includes changing water availability and temperature, impacts frog development, including the development of the immune system.  We are particularly interested in how these developmental stressors impact susceptibility to the amphibian chytrid fungus, so that we can better predict how a changing climate may influence disease dynamics into the future.

About The Authors

What are you currently working on?

Laura – I am currently exploring the impacts of amphibian disease on reproduction in declining species.  I’m interested in mechanisms of population persistence in the face of devastating disease, especially the ones that are often overlooked, like reproduction in infected frogs. I’m currently a research fellow at the University of Melbourne in Melbourne, Victoria, Australia. 

Michel – As a postdoc in the Richards-Zawacki lab, I am exploring the impacts of variable rearing conditions on amphibian development, and how this can impact the thermal physiology, immune function, and behaviour of frogs later in life.

What is the best thing about being an ecologist?

Site of egg collection. Photo by L. Brannelly
Site of egg collection. Photo by L. Brannelly

Laura – There is so much to learn, and so much unknown – there are always questions to answer which is very exciting.  Ecological research can be a good mix of field work, live animal research, and molecular based lab work. This keeps things interesting and allows us researchers to answer questions in a holistic way, using multiple techniques.

Michel – My favourite part about being an ecologist is the chance to answer questions that are not only scientifically fascinating but will also have important conservation implications. As an ecophysiologist, I work to understand how the environment impacts an organism’s physiology, and this is increasingly important as the climate changes. I like stepping back from my work and thinking about the big picture; what do we need to know to make predictions about how environmental change will impact organisms, and how will my work help achieve that aim?

What is the worst thing about being an ecologist?

Laura – With scientific research there are a lot of repetitive and tedious tasks to perform (for example, taking care of hundreds of juvenile frogs housed in individual enclosures, and having to feed and clean them daily) that need to be done carefully.  Losing sight of the overarching goals of the research can make those monotonous tasks really challenging, and mistakes more likely. It really helps to be aware and excited about the big questions, end goals, and the importance of research, which helps keep you interested in the science even when the tasks at hand are repetitive.

One piece of advice for someone in your field…

Michel – I think my piece of advice would be to try to focus on the aspects of your research that make you tick – focus on what interests you. And don’t be afraid to own your ideas and be confident in your abilities. It can be easy to lose sight of what excites you in research because of the everyday difficulties you may experience at work and even in life in general; when you feel yourself losing confidence or even losing your way, try to refocus on what is most important to you. And don’t be afraid to ask for help! The support of collaborators and friends can make all the difference in the world.

You can read the paper in full here or the free plain language summary here.

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