Mentors: Dr. Robbie Richards and Dr. Alex Strauss Abstract: Food web members can dramatically impact host-parasite dynamics through a wide variety of mechanisms. The “healthy herds hypothesis” posits that predators can substantially decrease parasitism in their prey by directly consuming infected individuals. However, experimental tests of this idea remain rare. Moreover, the few experiments that have been attempted have yielded inconsistent results, with predators sometimes decreasing disease, increasing disease, or having no effect on disease in their prey. Reconciling these divergent outcomes is increasingly important as prey species serve as reservoirs for many diseases of concern for spillover into human populations. In this project you will have the opportunity to conduct a large-scale manipulative experiment to test the healthy herds hypothesis. We will use a replicated mesocosm (i.e., artificial pond) system of mosquitofish predators, fungal parasites and their shared host/prey, the water flea, Daphnia dentifera. Finally, we will measure several key traits (e.g., preference of fish for infected vs. healthy hosts) and ask whether they explain why predators increase, decrease, or have no effect on disease in the experiment. Is the project computational, empirical, or both? Mostly empirical, but with opportunities to learn computational skills such as model fitting and parameterization.
Jenavier Tejada, a student at Denison University, worked in the lab of Dr. Alex Strauss
Abstract The dilution effect seeks to explain disease transmission in environments with multiple species. Essentially, the dilution effect predicts an increase in diversity will lead to a decrease in disease transmission. In zooplankton communities, the resistant diluter, Ceriodaphnia dubia can lessen disease in the host Daphnia dentifera caused by the parasite Metschnikowia bicuspidata. However, dilution is only effective when diluters and hosts co-exist; because when they compete, competitive exclusion can occur. Fitness of both D. dentifera, and C. dubia depend on temperature. Specifically, C.dubia benefits in warmer temperatures and D. denifera in cooler temperatures. Therefore, in environments where temperatures fluctuate, this may lead to co-existence, greater abundances of the diluter, and less disease transmission. We are testing whether the dilution effect reduces infection prevalence when a diluter is present, and how dilution effects differ at a constant 20˚C versus a fluctuating temperature around the same mean. We designed a multi-generational mesocosm experiment with communities that contained the host and parasite, and communities that contained the hosts, parasites, and diluters at both constant and fluctuating temperatures. We hypothesize that the changing environmental conditions caused by fluctuating temperature will lead to more diluters, causing a greater dilution effect via co-existence of the host and diluter. This project will help us learn more about the possible effects of climate change – especially variable temperature – on disease dynamics in communities with multiple species.
Emily Landolt, a student in St. Norbert College, worked in the lab of Dr. Alex Strauss
Abstract Freshwater zooplankton, such as Daphnia dentifera, are helpful model organisms for studying infectious disease dynamics and are ecologically important because of their role in food webs. They are consumers of primary producers like algae and are prey for other organisms such as fish. However, field sampling of freshwater zooplankton can be costly in terms of time and effort. The use of eDNA to estimate species abundances rather than an assessment of presence/absence in aquatic ecosystems is an exciting concept because it allows for more efficient and potentially more accurate field sampling. eDNA sampling methods for this study system can also be important for monitoring populations and epidemics in the field to ensure ecosystem health. This methodology has been studied in fish but is not yet well understood for invertebrates. We explored this idea by generating artificial mesocosm populations of Daphnia. Sampling for species abundance was done in two ways: using a traditional observational method of taking a subsample and counting the number of Daphnia, and a novel molecular method to quantify the amount of eDNA using a qPCR assay. Our results are a correlation between the observed species abundance and the molecular quantification of eDNA. Preliminary results show insignificant relationships indicating complexity that needs to be explored. Possible factors that affected the amount of eDNA in the water include water chemistry (i.e. pH), age structure, time since introduction, and water replenishment from evaporation/sampling. In the future, more research will be needed to explore the factors of eDNA persistence and how it can be used to better approximate species abundances of aquatic invertebrates like Daphnia.
Hannah O’Grady, a student at Mount Holyoke College, worked in the lab of Dr. Alex Strauss.
Abstract An important part of understanding how diseases spread and impact a community is understanding the tradeoffs that occur when a parasite generalizes. While sampling ponds in Whitehall Forest we discovered a potentially novel microsporidian that was able to infect at least five zooplankton in the Cladocera superorder at relatively high infection prevalence. We designed an experiment to investigate the potential costs of its generalism. We exposed isoclonal lines of two different species of Daphnia, each from two different lakes, to spores gathered from the dominant host in each lake to test whether the microsporidian was 1) more successful at infecting Daphnia of one species over the other or 2) whether it was more successful at infecting Daphnia from the same lake or 3) the same species from which the parasite spores were gathered. We also counted spore yield (a metric of parasite fitness) from the six infected species gathered from the field to test for differences across species, lakes or time. None of the Daphnia exposed to the spores in the lab became infected, leading us to hypothesize that there is an intermediate host for this parasite. Spore yields from field-collected hosts did differ significantly among host species, with higher spore yields in D. laevis (mean=446,415.55±412,977.53) and Diaphanosoma (mean=253,888.83±78,085.34) than in D. ambigua (mean=40729.12±41,396.68), D. parvula(mean=46250.06±49,432.78), and Simocephalus (mean=17083.17±13,025.14). There were no significant differences in spore yield across lakes or days. More research will be needed to find the intermediate host for the microsporidian as well as to determine its exact genus and species.