Deer often are blamed for the spread of tick-borne Lyme disease; many local programs focus on deer populations as a method to control Lyme disease. A new thesis suggests these programs are not targeting the actual driving force, coyotes. Researchers note that records from the past three decades link rising numbers of Lyme cases not with booming deer populations but with spreading coyotes.
The theory is that as coyotes have progressed into new ranges, red foxes have retreated. Since Coyotes do not pack a landscape as tightly as foxes, and since they do not kill and cache rodents in flush times as foxes do, then as the red fox population drops more rodents survive, including white-footed mice and others known as hospitable hosts for the Lyme pathogen and the ticks that spread it. Thus, the spread of coyotes indirectly increases the survival of Lyme disease vectors. (See prior posts on how differences in the "personality" of individual animals in a species can support population diversification across geographic areas.)
To test the thesis that predators of small rodents might influence the disease increase, researchers pieced together state information on wildlife sightings and hunting from Minnesota, Wisconsin, Virginia, Pennsylvania, and New York. In the first four of these states, Lyme cases climbed as coyote populations rose and red foxes declined, hunter records suggested. Deer records, however, did not show any consistent pattern. Researchers also zoomed in on regions within a state where possible. A decade of wildlife sightings recorded in five regions of Wisconsin, for example, suggested red fox populations dwindled as the incidence of Lyme disease climbed.
Since the researchers looked only at levels of coyotes and foxes when assessing Lyme disease rates, they note that the next step would be to try and quantify the actual influence of this pattern on rodent populations.
Critics note that while this thesis is new and interesting, deer may still play an important role. Except on islands where deer populations can be radically reduced when necessary, in the free-ranging setting of nonisland locations even reducing deer populations by 60-70% can have little to no influence on tick populations.
BTW. It is worth noting that Lyme disease does not spread directly from person to person, but the bacteria is instead passed along by a small Ixodes tick. This tick is sometimes misleadingly called a deer tick; also, these ticks bite a wide range of creatures, which may make control of Lyme disease difficult at best.
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