Association of State Wetland Managers - Protecting the Nation's Wetlands.

The Compleat Wetlander: Lyme on the rise? Blame people, oak trees and coyotes.

By Jeanne Christie

“Now the people are scratching all over the street
Because the rabbits had nothing to eat”
Pete Seeger 

Many wetland professionals I’ve talked to have had lyme disease.  Working in wetlands often requires tramping through fields and forests to get to the wetlands, so it’s not surprising that they encountered ticks and acquired the disease.  I am one of their number.  I’ve had it at least once and possibly twice.  Both times my doctor reacted to my symptoms (high fever, bulls eye rash) with a sustained dose of antibiotics.  I am one of the lucky ones.  I have had no recurring problems.

But it troubles me that increasingly I hear people say they don’t want to go into the woods because they might encounter ticks.  It got me wondering why ticks and lyme were spreading across the country.  A number of reports tie the spread of ticks to increasing deer populations but here where I live in Maine the numbers of ticks and incidences of lyme disease have been increasing annually while deer population has been holding steady and dropping in the state.  It’s not only Maine where ticks are spreading while deer populations are holding steady or decreasing.

 

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Some long term studies have identified one of the chief “culprits” responsible for the spread of disease is—indirectly– acorns, an important food source for the white footed mouse.  A bumper crop of acorns leads to a sharp increase in the mice population and there is a closer correlation of changes in white mice populations to the spread of ticks and lyme disease than to deer.

But there is more.  Acorn crops are highly variable from year to year, and when there is a bumper crop, white footed mice populations increase.  If it is followed by a scarce crop, the population decreases.  This means that the ticks have fewer mice to feed off and therefore more likely to be on the lookout for alternatives. Therefore ticks (who must feed three times during their life cycle) will be looking for a meal from another food source, including people.

 

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Next there is the changing distribution of four legged predators to consider.  Red foxes prey extensively on mice and provide a check on their populations.  But their numbers are down because gray wolves were hunted to the edge of extinction and eliminated from the eastern U.S. This created a vacant ecological niche that has been filled over the past few decades by the eastern coyote – a cross between the western coyote and the red wolf.  It turns out the eastern coyote does not rely on mice as a primary food source, but they are very fond of red fox. See here and here.

It was interesting that I could not find any information about any mammal, bird, amphibian, reptile or insect that fed extensively on ticks.  You’ll find a lot of articles recommending guinea fowl or chickens, but a search for the actual studies behind these recommendations leads to the information the guinea fowl, chickens and other animals will eat ticks, usually only the adults, if they come across them, but none prefer them above other foods.  In addition, all the information I found indicated that the animals studied fed on adult ticks, not the nymphs who are responsible for 80% of the spread of the disease. It seems to me there is a distinct possibility there must be, or more likely, use to be, a large population of a critter or critters in the environment that were voracious eaters of ticks and nymphs.  I suspect, although I did not find studies that hypothesized or attempted to prove it, that something is largely absent from the landscape that once ate a lot of ticks. The trouble is we did not start studying the issue until the tick population was on the rise, and after the critter was largely gone.

The more I researched this topic, the more I became convinced that:

1)   It’s complicated
2)   There is a lot of misinformation on the internet, and
3)   There are missing pieces to the reasons for the spread of ticks and lyme disease.

There are two detailed articles well worth reading to gain a greater understanding of the complexity of trying to address the spread of ticks and lyme disease:  Deer, predators, and the emergence of Lyme disease  and Controlling Ticks and Tick-borne Zoonoses with Biological and Chemical Agents.

But here is what I learned.  Changes in predator and/or herbivore populations create indirect effects that can lead to dramatic changes, called a trophic cascade. This is why changes to wildlife populations, and in particular the loss of biodiversity matters. The upside is the reintroduction of a single animal can have an unexpected and positive cascading effect as shown in this video about the reintroduction of the grey wolf to Yellowstone National park. “How Wolves Change Rivers”

 

 

I don’t want to simplify this issue. It’s complicated and this is revealed by additional work done to understand the changes in Yellowstone Park. With respect to tick populations, there is evidence that deer, mice and other animals influence the spread of tick populations.  But the bottom line is that when actions are taken by people that alter wildlife populations, there are unintended consequences.  And ultimately these may have concrete, negative effects on people.  We need to have a better understanding of the consequences of actions to avoid these.

You don’t need to be a trophic cascade researcher to understand the possibilities. Peter Seeger summed it up nicely in “The people are scratching”  from his 1966 album “God Bless the Grass”.

 

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