- Deer are not to blame for rising rates of Lyme disease.
- Where there are lots of coyotes, risk of Lyme is high. Where there are lots of foxes, risk is low.
- Healthy predator populations and stable food webs are essential for human health.
Deer frequently take the blame for spreading Lyme disease. But a new study suggests that foxes are a more critical link in a complex web of interactions that have contributed to rising rates of the insidious infectious disease, while deer have nothing to do it with it.
Foxes don't spread Lyme disease directly. Instead, they cull populations of small mammals, which are responsible for the bulk of infectious ticks. Where foxes are thriving, the risk of disease drops. But when fox numbers fall – often because coyotes move in, small mammal populations surge and Lyme disease flourishes.
The study emphasizes how important stable food webs and strong predator populations are for human health.
"Small changes in predation can lead to large changes in Lyme disease risk," said ecologist Taal Levi, who completed the study while at the University of California, Santa Cruz, and is now at the Cary Institute for Environmental Studies in Millbrook, NY. "This argues for top predator conservation, or at least that there are unintended consequences for people of losing them."
For decades, cases of Lyme disease have been increasing dramatically in North America, with a more than 1,000-percent rise in disease rates in some places over just the last decade. Lyme disease is caused by a type of bacteria, which is transmitted to people by ticks in the nymph stage. And because ticks rely on deer to reproduce, conventional wisdom has long held that revived deer populations are responsible for a surge in the disease.
Yet, culling deer in various places has done little to control Lyme. At the same time, studies have shown that up to 90 percent of infected ticks pick up the Lyme bacteria from small mammals, including mice, chipmunks and shrews.
To figure out what's really driving the Lyme explosion, Levi and colleagues used a combination of computer modeling and real-world regional examples to look at the relationships between ticks, their small mammal hosts and predators that eat those little mammals. Results showed no link between numbers of deer and density of ticks.
Far more important were numbers of coyotes and red foxes, the researchers report today in the journal Proceedings of the National Academy of Sciences.
Over the last few decades, there have been major increases in coyote populations in the northeast and Midwest – with numbers of coyotes up 2,200 percent in Minnesota since 1982, for example, and 1,600 percent in Pennsylvania since 1990. Coyotes kill foxes. Foxes also refuse to breed in a coyote's home range.
As a result of the coyote boom, fox declines have been dramatic. In Minnesota, the number of foxes harvested by hunters dropped 95 percent from 1991 to 2008. In Wisconsin, numbers dropped 80 percent in about the same time period.
In turn, a drop in foxes has allowed numbers of small mammals to increase, while also reducing the turnover in mammal populations -- essentially giving the Lyme bacterium free reign to propagate. The shift has had direct effects on human health.
"Where there are lots of coyotes, you don't get foxes, and where you don't get foxes, you have a lot of Lyme disease," said Richard Ostfeld, a disease ecologist at the Cary Institute of Ecosystem Studies, who was not involved with the research.
Introducing wolves to the Northeast and Midwest would control both coyotes and Lyme disease, along with other rodent-hosted infectious diseases, Ostfeld said whimsically, though that is unlikely to happen. Still, the new research points out how important it is to value the role of predators in healthy ecosystems.
"I hope this is the final nail in the coffin of the notion that it's just so simple that if you manage deer, you manage Lyme disease," Ostfeld said. "The positive message is that certain predators protect our health. And if we can manage the environment so we have these healthy predator populations, we will have less Lyme disease and potentially other diseases."