UNDERSTANDING THE CAUSES AND CONSEQUENCES OF PARASITISM IN AQUATIC SYSTEMS
Photo by Loren Merrill
The unifying theme of my research is understanding the causes and consequences of parasitism and infectious disease in ecosystems
Questions that drive my research include:
Why are some hosts susceptible to parasites and not others?
What processes promote or inhibit the spread of infection?
Why do epidemics start and what constrains their size?
How do parasites affect host fitness?
Are parasites a significant source of mortality for populations?
When and how do the suppressive effects of disease scale up to affect communities and ecosystems?
I ask these questions primarily in freshwater systems, but have also expanded my research to include tropical forests and North American birds. I am increasingly addressing my questions within the context of environmental change, considering how global climate change and biodiversity loss affect disease patterns and processes. Below is a sample of my ongoing research areas, with links to relevant publications.
The basis and extent of variation in susceptibility
An uninfected female Daphnia dentifera. Photo by Tara Stewart Merrill also appears on the volume 99 cover of the journal Ecology.
Mechanistically quantifying susceptibility in Daphnia. Figure from Stewart Merrill et al. 2019 Integrative and Comparative Biology.
An organism's susceptibility determines the extent to which it can be infected by parasites and pathogens. But what is the functional basis of susceptibility and how can we best measure it? Using the freshwater zooplankter Daphnia as a model organism, I have combinednatural history with my own empirical methods and theoretical models to describe and measure the traits underlying host susceptibility. The ultimate goal for this work is to identify functional susceptibility traits that are complex enough to capture meaningful variation in host defense, while remaining general enough to be linked with ecological patterns and processes. Some new exciting research on this front is using RNA sequencing to understand the molecular mechanisms underlying Daphnia immune defense.
A general Markov model for quantifying host defenses and within-host parasite dynamics. Figure from Stewart Merrill et al. AmNat in press.
One of six study lakes used to connect Daphnia susceptibility to fungal epidemics (in Stewart Merrill et al. 2021 Ecology).
A new frontier in ecology is to identify the consequences of intraspecific variation for ecological interactions. I have worked toward this goal in freshwater host-parasite systems, asking: does individual-level variation in host defense scale up to drive parasite transmission at the population-level? My work focuses on the predictability of fungal epidemics by combining organismal processes (those that shape host-parasite dynamics within the host) with ecological patterns (those that shape transmission between hosts). While experiments and field studies form the foundation for this research, I also test theory using mathematical models.
The rough-skinned new, Taricha granulosa, one of five study amphibians for quantifying within-species variation in competence. Photo by Loren Merrill.
Consequences of within-species variation (owing to genes or the environment) for diversity-disease relationships. Figure from Stewart Merrill and Johnson 2020 Parasitology.
Biodiversity loss is thought to exert a strong influence on the spread of infectious disease. Because free-living species vary in their parasite competence (ability to support and transmit infection), changes in the composition of ecological communities can produce concomitant changes in parasite transmission. Despite the importance of biodiversity-disease theory, we lack a clear grasp of the factors that determine competence, as well as the extent of its variation. The mechanisms that underlie competence are rarely quantified systematically, and we have not yet ascertained how plastic competence is or how much variation exists within a particular species. Using a freshwater disease system (parasitic trematodes that infect snails and amphibians in ponds), I am developing conceptual, mathematical, and empirical frameworks to advance our understanding of competence. With refined metrics of competence in hand, I am exploring how diversity loss affects disease risk through changes to average competence in a community, as well as through alterations in aquatic food webs structure. Collaborators: Andrew Dean (mathematics), Andrew Fenton (theoretical ecology), Brendan Hobart (community ecology), Pieter Johnson(community ecology), Travis McDevitt-Galles(community ecology),Wynne Moss (conservation biology)
Effects of parasites on aquatic systems - from interactions to ecosystems
Differential predation and parasitism of Ceriodaphnia morphotypes. Figure from Stewart et al. 2018 Journal of Parasitology.
Sampling in the Panama Canal for research on Ceriodaphnia parasitism. Photo by Tara Stewart Merrill.
Host-parasite interactions have been neglected in limnology, despite their importance for the survival and reproduction of free-living species. Much remains to be learned regarding how parasites fit into our understanding of important limnological processes, like seasonal succession and trophic cascades. As a fellow with the Smithsonian Tropical Research in Panama, I revisited a classic system of predator-mediated coexistence (Ceriodaphnia cornuta) and explored parasitism as an additional top-down force balancing morphotype competition. More recently, I have been leading a working group using twenty years of parasite infection data combined with long-term food web, chemical, physical, and nutrient data (North Temperate Lakes LTER), to explore the effects of parasites on aquatic ecosystems. My continuing goal is to bring parasites into our understanding of limnology, so that we have a more complete understanding of the diversity and extent of top-down forces in freshwater systems. Collaborators: Carla Cáceres (aquatic ecology), Kyle Christianson (physical limnology), Brendan Hobart (community ecology), Pieter Johnson (community ecology), Kelly Loria (limnology), Travis McDevitt-Galles (community ecology), Wynne Moss (conservation biology), Mark Torchin(parasite ecology)
Resolving the relationship between tropical trees and lianas
Lianas and trees in the Panamanian forest surrounding Barro Colorado Island. Photo by Tara Stewart Merrill.
An experimental roadmap for determining the true nature of liana-tree interactions. Figure from Stewart and Schnitzer 2017 Biotropica.
Lianas are vines that use tree structure to access light in the canopy. The substantial negative effects of lianas on trees are well-known, but the reciprocal response (the effects of trees on lianas) has not been studied. This raises the question: are lianas competitors or parasites of trees? Answering this question is critical for understanding the long-term dynamics of tree-liana interactions, especially as lianas continue to spread and increase in their abundance in many parts of the world. In collaboration with Stefan Schnitzer at Marquette University, I have developed empirical frameworks and initiated long-term studies to resolve the true nature of liana-tree relationships.