Louise S. Nørgaard: Energetics, population density and pathogen performance

Louise S. Nørgaard is currently working as a Research Fellow at the centre of Geometric Biology at Monash University, Melbourne.

In this Insight, she talks about her recent publication in Functional Ecology, ‘Energetic scaling across different host densities and its consequences for pathogen proliferation’.

About the paper

What’s your paper about?

Daphnia life cycle by Savanna Ganucheau @Srganuch (https://srganuch.carbonmade.com/)
Daphnia infection life cycle by Savanna Ganucheau @Srganuch (https://srganuch.carbonmade.com/)

Using the freshwater Daphnia magna and its gram-positive bacteria Pasteuria ramosa, we explore how energy availability within a host impacts pathogen performance. Specifically, we explore how rates of energy intake and expenditure covaries within individuals reared at varied population densities. By doing so both in infected and uninfected hosts, we then uncovered how discretionary energy (the amount of excess energy available for other biological processes, such as immune response) and density-mediated changes in host size both contribute to a better performance in pathogens infecting the more energy rich hosts reared at low density. Finally, this work also show how disease dynamics operate at two contrasting scales where the pathogen benefits by infecting energy-rich hosts at low density, but there are fewer opportunities for secondary transmission when hosts are sparse. The Daphnia-Pasteuria model system is particularly suitable for such experiments due to density-dependence in both host and pathogen traits, as well as the ease of culturing and maintaining large sample sizes.

What is the background behind your paper?

This study was motivated by some of my previous research where we explored how population dynamics such as invasion and colonisation processes (and thereby also population density), impact on the performance of a pathogen. Moving one step further, we then wanted to explore the energetic consequences of population density on host and pathogen performance, as well as developing a better understanding of the energetic interactions between a host and its pathogens.

What are the key messages of your article?

After reading this paper, it is our hope that a greater understanding is achieved of how the flow of energy at various population densities has major implications for both host and pathogen growth. The key messages are two-fold. First, higher amount of discretionary energy available in low population density provides an energetic advantage for both a host and its pathogen; hosts grow larger and the pathogen benefits from correlated changes in host body size as well as a direct connection between energy scope and spore load. Next, our work emphasises how patch quality for a pathogen operates at two contrasting scales whereby within-host proliferation of a pathogen is optimised in energy rich, low density host populations, and opportunities for between-host transmission is likely maximised in dense populations with higher availability of susceptible hosts.

Daphnia incubated in a temperature controlled chamber
Daphnia incubated in a temperature controlled chamber

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

This paper moves one step further than most studies, by exploring the impact of population density on rates of both energy intake and expenditure, and thereby unravelling the interplay between population density and discretionary energy within an organism. Additionally, by exploring this relationship in infected hosts, we uncover how both a host and its pathogen benefit from high energy availability in low density populations, and simultaneously reinforce how patch quality for a pathogen operates at two contrasting scales.

Louise S. Nørgaard

How did you get involved in ecology?

I have always been intrigued by nature and have always been interested in understanding biological processes. Biology was always my favourite topic, and after high school it was clear to me that I wanted to become a biologist. Up until now, I have been working in a number of different fields (and using multiple approaches) within biological sciences, but my major passion indeed is within invasion biology, evolution and epidemiology.

What’s your current position?

I am currently working as a research fellow at the Centre for Geometric Biology at Monash University where I was expecting to explore the consequences of body size on responses to changes in temperature. However, as we all know, COVID-19 changed our lives, and as a consequence of the long lock down, I diverged into exploring geometric scaling of life-history traits in mosquito disease vectors.

What do you do in your spare time?

In my spare time I am a keen rock climber and mountain bike rider, and I take any opportunity to enjoy the beautiful outdoors.

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