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, we combinenatural history with empirical methods and mechanistic 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 invertebrate immune defense.
As part of our collaboration with the Apalachicola Bay System Initiative, we are also exploring the traits underlying oyster susceptibility to Dermo disease and how oyster susceptibility is shaped by environmental conditions. This research will allow us to generate predictive models for pathogen transmission and disease impacts into the future.
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. We work 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? This work focuses on the predictability of 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, we 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), we are developing conceptual, mathematical, and empirical frameworks to advance our understanding of competence. With refined metrics of competence in hand, we ask how diversity loss affects disease risk through changes to average competence in a community, as well as through alterations in aquatic food web 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 aquatic ecology, 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 ecological processes, like seasonal succession and trophic cascades. As a fellow with the Smithsonian Tropical Research in Panama, Tara revisited a classic system of predator-mediated coexistence (Ceriodaphnia cornuta) and explored parasitism as an additional top-down force balancing morphotype competition. More recently, Tara has 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 freshwater ecosystems. And as a new member of the Apalachicola Bay System Initiative, Tara plans to collaborate on the construction of a bay food web that includes parasites. 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)