The healthy herds hypothesis (HHH) suggests that predators decrease parasitism in their prey, as they tend to prey on infected individuals or susceptible populations. However, further studies have found varying impacts from predators on their prey’s parasitism. The failure of empirical testing to support the HHH is likely because this hypothesis assumes a single predator and prey species in a system where other factors remain constant, which does not reflect the complexity of species interactions in an ecosystem. The hypothesis also does not account for the important variability among species of prey. This variability can include changes in prey physiology or behavior in response to risk of predation. Predator preference for prey species of a certain body mass or other specific characteristic can also contribute to variance among species of prey. Predation can lead to variance in a prey population’s age, body mass, density, and sex structure, which in turn may affect parasitism.
CEID’s Robert Richards, John Drake, and Vanessa Ezenwa worked with a research team to conduct a predator exclosure experiment to measure how the absence of predators would affect gastrointestinal parasite abundance in different species of prey between seasons, and before and after major environmental disturbances.
This experiment was conducted in Baker County, Georgia at the Jones Center, a 12,000 hectare property covered with longleaf pine trees and wiregrass that is utilized for natural resources research and management. The Center has four predator exclosure plots, which are surrounded by wire fences with electric lines, to prevent predators such as foxes, coyotes, skunks, opossums, raccoons, and armadillos from entering. Four control plots with no fence were used to measure for comparison. Small mammals, which these predator species would typically prey upon, were still able to pass through the wire fence. The prey species of interest in the experiment were primarily cotton rats and cotton mice, since these small mammals are susceptible hosts for many gastrointestinal parasites.
Results showed that predator exclosure (predators were unable to reach prey within the fenced plots) affected the abundance of parasites in cotton rats and mice differently in certain seasons. For cotton rats in the predator exclosures, abundance of the Spirurid parasite was significantly greater in the winter. For cotton mice in the predator exclosures, the Strongyle 1 parasite abundance was significantly decreased in the winter and summer, but not during the spring. The parasite abundance of cotton mice in the exclosure plot increased for the Strongyle 2 parasite in the fall and winter, but not in the spring or summer. Seasonality may also be important due to changes that occur in the predator-parasite-prey interactions or changes in vegetation cover and food availability. Predators that were not excluded by the fencing, such as snakes and birds may have also played a role.
A prescribed fire- a controlled fire purposefully set as a forest management strategy- also took place at the time of the study, allowing the research team to observe the impact of a disturbance event. In the year when a prescribed burn did not occur, a typical healthy herds effect was observed, with significantly greater parasite abundance detected in the prey from the predator exclosure plots. In the 12 months following the fire, the data did not show any difference between the control and exclosure treatments that were relevant to cotton mouse predation.
Overall, this experiment shows that seasonality, ecosystem disturbances, and species identity influences the role of predators on parasites in their prey. Further research can investigate additional drivers that may impact the predator-parasite-prey interactions.
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By: Brenna Daly