The Ecology of Lyme-Disease Risk: Complex interactions between seemingly unconnected phenomena determine risk of exposure to this expanding disease Richard Ostfeld This article originally appeared in the July-August 1997 issue of American Scientist.
We found that the location of peak abundance of larval ticks shifts dramatically from year to year, coinciding with the presence or absence of acorns in oak stands the prior fall. Oak forests were the sites of larval-tick outbreaks in the summers following acorn years, whereas maple forests were most heavily infested in summers following acorn failure. Subsequently, together with Clive Jones, Jerry Wolff and several technicians, I added almost 1 million acorns to three 2.25-hectare forest plots during the autumn of a year of very poor natural-acorn production. As a control, we monitored three matched plots to which no acorns were added. The following summer we observed that densities of larval ticks were approximately 10 times higher in acorn-supplemented plots than in control plots. This result supports our proposed link between acorn availability, space use by white-tailed deer and location of larval-tick outbreaks. Deer are not the only wildlife species that rely strongly on acorns as a food resource. Long-term studies of population dynamics of white-footed mice, conducted by Jerry Wolff in Virginia, by Steven Vessey at Bowling Green State University in Ohio, by Joseph Merritt at the Powdermill Biological Station in Pennsylvania, and by ourselves in New York, reveal that the abundance of mice in a particular area is closely tied to acorn production. In the fall of a mast year, mice both consume and store acorns in large numbers. Consequently, during the winters following mast production, mice tend to experience higher-than-normal survival rates. Winter breeding by white-footed mice almost never takes place when acorns are unavailable. In contrast, mice breed rampantly during postmast winters and begin the normal spring-breeding season at high density and in good physiological condition. Mouse populations then reach peak density by the middle of the summer following a mast year. By influencing the use of space by deer and the population dynamics of mice, acorn production is tied to peaks in abundance of both larval ticks and mice, the most reservoir-competent host, in oak forests during the summer following a mast year.
LoGiudice, Kathleen, et al. “The ecology of infectious disease: effects of host diversity and community composition on Lyme disease risk.” Proceedings of the National Academy of Sciences 100.2 (2003): 567-571.
The extent to which the biodiversity and community composition of ecosystems affect their functions is an issue that grows ever more compelling as human impacts on ecosystems increase. We present evidence that supports a novel function of vertebrate biodiversity, the buffering of human risk of exposure to Lyme-disease-bearing ticks. We tested the Dilution Effect model, which predicts that high species diversity in the community of tick hosts reduces vector infection prevalence by diluting the effects of the most competent disease reservoir, the ubiquitous white-footed mouse (Peromyscus leucopus). As habitats are degraded by fragmentation or other anthropogenic forces, some members of the host community disappear. Thus, species-poor communities tend to have mice, but few other hosts, whereas species-rich communities have mice, plus many other potential hosts. We demonstrate that the most common nonmouse hosts are relatively poor reservoirs for the Lyme spirochete and should reduce the prevalence of the disease by feeding, but rarely infecting, ticks. By accounting for nearly every host species’ contribution to the number of larval ticks fed and infected, we show that as new host species are added to a depauperate community, the nymphal infection prevalence, a key risk factor, declines. We identify important “dilution hosts” (e.g., squirrels), characterized by high tick burdens, low reservoir competence, and high population density, as well as “rescue hosts” (e.g., shrews), which are capable of maintaining high disease risk when mouse density is low. Our study suggests that the preservation of vertebrate biodiversity and community composition can reduce the incidence of Lyme disease.