Carl Zimmer examines the complex and pathogen-laden world of ticks:
Foxes were originally very abundant in the eastern United States, where they feasted on small mammals like white-footed mice. But the past few decades have not been good to them. “Fox harvests in the Northeast have declined substantially,” says Levi.
A number of studies suggest that coyotes have been responsible for the decline. Originally, foxes coexisted with wolves in the eastern and midwestern United States. Once wolves were eradicated, coyotes expanded from the Midwest to take their place. Coyotes kill foxes or scare them out of range.
Levi and his colleagues built a mathematical model of how these changes can affect rates of Lyme disease. When foxes disappear, the model suggests, numbers of small mammals like white-footed mice boom, feeding a growing population of ticks and their pathogens. For evidence, Levi points to historical records from sites across the Midwest and eastern United States. In some places, Lyme disease rates have gone up even though the deer population has not. But the rates in those places match up nicely with a decline in fox numbers.