Searle Lab

Aquatic disease ecology at Purdue University


Research Overview

Research in the Searle lab investigates the ecology of infectious diseases in freshwater systems. We are particularly interested in how changes to the abiotic and biotic environment affect disease in freshwater organisms. To understand these processes, we use a combination of field surveys, experiments (lab and mesocosm) and modeling. Our primary hosts systems are freshwater zooplankton (Daphnia) and amphibians (with a focus on chytridiomycosis). These complementary systems allow us to investigate a wide range of ecological questions and mechanisms. Within this framework, our research foci change through time, but several are outlined below.

Intraspecific host diversity

Intraspecific diversity of hosts can affect disease risk and epidemic size in multiple ways. For example, populations that are exposed to a novel parasite may be able to rapidly evolve resistance or tolerance to infection. Whether or not a population is able to evolve can depend on the genetic diversity of the initial population. Using a modeling approach, we have shown that higher genetic diversity prior to the introduction of Bd substantially reduced rates of host extinction. However, this persistence came at a cost; as genetic diversity increased fewer populations cleared infection, population sizes were depressed, and neutral genetic diversity was lost (Christie and Searle 2018, Evol App). In a different study, we investigated how the within-host environment can shape parasites through environmental effects, which can alter the outcome of future infections. We have shown that parasites can respond to the within-host environment by plastically changing their phenotype (Searle et al. 2015, Parasitology).


Invasive species

We have been investigating how disease outbreaks can influence species invasions and, in turn, how species invasions might influence infectious disease. For example, invasive species can become infected with native parasites in their new ranges. When this occurs, invasive species may alter disease dynamics for native hosts (Searle et al. 2016, Am Nat). For example, research conducted by three undergraduates in the Searle lab showed that the invasive species is most successful at establishing in communities where the native species experiences disease and high resources (Searle et al. 2018; Oecologia). When resources were abundant, the native species experienced increased population densities which caused larger epidemics when the parasite was present. These epidemics caused a subsequent reduction in native host density and allowed the invasive species to establish relatively dense populations.

Searle et al 2018

Abiotic changes to the environment

One of the main abiotic stressors we have been studying is freshwater salinization. In many temperate regions such as the Midwestern USA, freshwater ecosystems are being salinized through anthropogenic activities including the use of road salts. Road salts are used to de-ice roadways during winter months, but this practice results in salts being carried into streams, wetlands, and lakes through runoff and leaching. Increased salinity can have direct effects on organisms or indirect effects mediated through parasites. We have shown that ecologically relevant increases in salinity can suppress population densities of zooplankton (Searle et al. 2016, Hydrobiologia). Additionally, elevated salinity can actually decrease infection prevalence in freshwater organisms which can cause less-than-additive effects of salinity and disease on zooplankton populations. In an experimental study, we found that the presence of a parasite only reduced host densities in low-salt conditions, presumably due to reduced infection prevalence as salinity increased (Merrick and Searle 2019; Freshwater Biol). Thus, the environmental impact of freshwater salinization can manifest through both direct and indirect effects on target organisms.