The team's findings, published in the Proceedings of the National Academy of Sciences, reveal that although deer populations have stabilized, Lyme disease has increased across the northeastern and midwestern United States over the past three decades. The increase coincides with shrinking populations of the red fox, which feeds on small mammals, such as white-footed mice, short-tailed shrews, and Eastern chipmunks, all of which transmit Lyme disease bacteria to ticks.
The loss of red foxes can result in an increase in the abundance of the smaller animals that serve as hosts for bacteria-carrying ticks. Red foxes may have once kept those populations under control.
Due to the widespread eradication of large carnivores like wolves and cougars, top predators in many terrestrial ecosystems are now medium-sized carnivores such as coyotes. These medium-sized carnivores can indirectly increase the abundance and diversity of low trophic-level species, such as rodents and songbirds, by suppressing populations of smaller carnivores such as foxes. Strong interactions among predators that lead to cascading effects on prey have been documented for over 60 systems worldwide.
"We found that where there once was an abundance of red foxes there is now an abundance of coyotes," said Levi, a researcher at the Carey Institute for Ecosystems Studies in the hotspot for Lyme disease, Duchess County, north of New York City. There he works as a postdoctoral research fellow with Lyme disease expert Rick Ostfeld, who literally wrote the book on Lyme disease, Lyme Disease Ecology of a Complex System.
Lyme disease was first reported in Old Lyme, Conn. in 1975. Ticks pick up the bacteria when they bite infected mice and later infect other animals including humans. Levi said tick nymphs, about the size of a sesame seed, carry the bacteria and are so small that many people who contract Lyme disease never knew they were bitten.
As top predators are extirpated in some parts of the world, it is important to understand its the consequences to ecosystems, in general, and for the abundance of low trophic-level species, in particular. Such restructuring of predator communities, may have unintended consequences for human disease.
Dwindling numbers of red foxes, the authors suggest, might be attributed to growing populations of coyotes, now top predators in the Northeast where wolves and mountain lions are extinct.
"A new top predator has entered the northeast and has strong impact on the ecosystem," said researcher, Taal Levi Ph.D. graduate in environmental studies. "Coyotes can and will kill foxes and more significantly," Levi said, "foxes often don't build dens when coyotes are around."
Levi and his UCSC co-authors used an extensive data-set from five states as well as mathematical models to determine why Lyme disease continues to rise despite stabilized numbers of deer, long known to act as reproductive hosts for adult ticks that carry Lyme disease bacteria.
These results suggest that changes in predator communities may have cascading impacts that facilitate the emergence of zoonotic diseases, like Lyme disease, the vast majority of which rely on hosts that occupy low trophic levels.
According to Wildlife Conservation Society researcher Dr. Kim Berger, healthy populations of wolves help to keep coyote numbers in check. In her study, Afraid of the Big Bad Wolf, Dr. Berger stated, "This study shows just how complex relationships between predators and their prey can be. It is an important reminder that we often don’t understand ecosystems nearly as well as we think we do, and that our efforts to manipulate them can have unexpected consequences.”