Searle Lab

Aquatic disease ecology at Purdue University


Research Overview

Research in the Searle lab investigates the community ecology of infectious diseases. In natural systems, disease dynamics can be altered in a variety of ways as communities lose species (e.g. from habitat loss, pollution, etc.), gain species (e.g. from species invasions), or experience changes to population sizes and intraspecific diversity. These factors can influence the frequency and severity of disease outbreaks. To understand these processes, we combine field observations with experimental manipulations in the field, mesocosms and laboratory. Our primary hosts systems are freshwater zooplankton (Daphnia) and amphibians (with a focus on chytridiomycosis). Through these studies, we aim to understand how changes to biodiversity are altering disease dynamics.

Intraspecific host diversity

Intraspecific diversity of hosts can affect disease risk and epidemic size in multiple ways. For example, the within-host environment can shape pathogens through selection or environmental effects, which can alter the outcome of future infections. As a population of pathogens passes through one host genotype, the conditions within that host may alter the pathogen through environmental effects or rapid evolution. We have shown that pathogens can respond to the within-host environment by plastically changing their phenotype. In just a single generation, a novel host genotype is sufficient to cause phenotypic changes to a parasite that influence future infections. This has implications for both short and long-term effects of host diversity on pathogens.

host diversity

Host species diversity

The relationship between host species diversity and disease risk is complex. We have been studying this using the emerging fungal pathogen, Batrachochytrium dendrobatidis (Bd), which has caused population declines and extinctions of amphibians around the world. We have found that amphibian species vary in their rate of survival and infection loads when exposed to Bd. These patterns can also be influenced by host species diversity. We have demonstrated a negative relationship between biodiversity and disease risk in Bd (Fig. 2) indicating that preserving biodiversity may help reduce infection.

dilution effect

Invasive species

Invasive species have the potential to impact many aspects of their invaded communities, including disease dynamics. Some invasive species bring pathogens with them, which may spill into native communities. Alternatively, invasive species may also become infected with native parasites in their new ranges. When this occurs, invasive species may alter disease dynamics for native hosts. We have been investigating the impacts of Daphnia lumholtzi, a species invasive in North America, on disease dynamics of native Daphnia species. Native pathogens have been found infecting D. lumholtzi and we are studying how this will influence invasion success.


We are investigating how predators can directly and indirectly affect pathogens and their hosts. For example, we have found that Daphnia can directly consume Bd, reducing rates and severity of infection in tadpoles. These effects are context-dependent, indicating that Daphnia may reduce infection only in specific environments. We are also investigating how predators can affect susceptibility to infectious disease through trait-mediated effects. For example, predators can alter the morphology and behavior of their prey, influencing their risk of infection.