(from the March/April 2004 IES Newsletter)

Many predators require large tracts of undisturbed habitat, making them sensitive to human-induced landscape fragmentation. Their numbers tend to decline as the landscape becomes developed. Ecologists have long documented that predators can have profound effects on prey animal abundance, with small mammals, such as rodents, flourishing in their absence. In a Frontiers in Ecology and the Environment paper (February, 2004), Institute ecologist Dr. Richard S. Ostfeld and colleague Dr. Robert D. Holt (University of Florida) posit a new role that predators may be playing— the suppression of zoonotic diseases.

Defined as diseases transmitted between vertebrate animals and humans, zoonotic diseases include bubonic plague, Lyme disease, salmonella, and rabies. Disease-carrying animals, called reservoirs, infect humans through several pathways: when they are eaten by humans, when they bite humans, or when arthropods that have fed on them, such as mosquitoes or ticks, then feed on a human host. Dr. Ostfeld notes that, "Over 60% of infectious diseases impacting humans are zoonotic in origin and zoonoses are on the rise globally, accounting for over 75% of emerging diseases."

Until recently, zoonotic diseases have not been treated as part of ecological systems. In response to the prevalence of zoonoses, the multidisciplinary field of disease ecology has emerged. It involves the study of any ecological system that includes pathogens and incorporates the complexity of multiple interactions.

Mammals are the most common reservoirs for zoonotic diseases, with rodents leading the pack. The plague (Yersinia pestis), Lyme disease (Borrelia burgdorferi), Hantavirus pulmonary syndrome and Rocky Mountain spotted fever (Rickettsia rickettsii) all owe their spread to the presence of rodents. From an ecological perspective, rodents occupy the middle rung of the food chain. Primarily herbivores, with diets rich in plant matter, they are a food source for vertebrate predators such as fishers, foxes, and owls. Through the act of feeding themselves, predators reduce rodent abundance.

"We know that predators affect prey numbers. If fox are eating mice, there will be fewer mice when fox are present. If mice are a zoonotic disease reservoir, and the human infection escalates with reservoir abundance, habitats with fox present would have a lower incidence of disease," comments Dr. Ostfeld. Knowing that predators consume rodents, and rodents are reservoirs for zoonotic diseases, can we infer that predator presence reduces the risk of human disease transmission? The answer depends on both the disease and the predator in question.

For a predator to reduce human health risks two basic criteria need to be met: human disease transmission must be tied to rodent density and the predator must reduce rodent numbers. While human infection rates often are correlated with reservoir density, the authors outline some notable exceptions. These include pathogens that are limited by factors other than rodent hosts, transmission that is dependent on frequency and not density, and situations where abundant rodent hosts actually prevent vectors from biting humans.

Not all predators are created equal. "Predators that continuously suppress prey are more likely to play a role in also suppressing zoonotic disease- not all predators do this," Dr. Ostfeld explains. Specialist predators, such as weasels, can cause dramatic prey fluctuations called "boom and bust" cycles. Predation activity is tightly linked to a particular prey species. When prey is abundant, predators obtain optimal health and breed vigorously. The next generation of predators is then confronted with a decimated prey population. Predator survivorship decreases and prey numbers slowly escalate again. During prey peaks, rodent-borne diseases can thrive.

Drs. Ostfeld and Holt speculate that generalist predators, like foxes, and highly mobile predators, like hawks, are more likely to protect human health. Dietary flexibility lets these animals switch between prey species; when the population of a given prey animal gets too low, they move on to another one. Predator numbers never plummet in response to the decrease of a particular prey species. The end result is a constant reduction in rodent abundance, a stark contrast to the fluctuations elicited by specialist predators.

Drs. Ostfeld and Holt assert the need for more ecological studies investigating zoonotic diseases from an ecological perspective. If predator presence is tied to a decrease in the number of infected reservoir species, it will present a strong argument for paying closer attention to how human-induced environmental change impacts predator survivorship.

Lyme disease is a zoonotic disease where human risk is correlated with rodent host density. White-footed mice are the primary host for the vector species, blacklegged ticks. As white-footed mouse populations increase so does human infection risk. "Understanding what regulates Lyme disease, predicting its spread, and ultimately reducing its transmission to humans, depends on our ability to look at the disease from an ecological perspective. Knowing the players is of limited usefulness unless we also know how they interact with one another. We know predators have a role and are working to more clearly define it," Dr. Ostfeld explains.

By monitoring predator abundance on fragments of land in New York, New Jersey, and Connecticut, his research team is investigating how fragmentation affects predator presence. By examining correlations among predator presence, small mammal abundance, and infected reservoir species they will reveal the role that mammalian predators play in suppressing reservoir species. This information then will be weighed against other factors regulating rodent populations, such as food availability. Dr. Ostfeld comments, "Our goal is expand our knowledge of how ecological communities work while helping to reduce the incidence of disease transmission."

Past research has shown that the percentage of Lyme-infected ticks is higher in small forest fragments (less than 3 acres). Forest fragments of this size also are less likely to house viable populations of mammalian predators. When they drop out of the environment we may be losing an important ally in the fight against Lyme disease and other rodent-borne zoonotic diseases.