Save yourselves, kill the chipmunks

Growing up in New England, there are several things you learn from an early age to be true. Number One: Yankees suck. Even if you don’t like baseball, you know this to be true. They are arrogant, spoiled, thieving New Yorkers. They suck.

Number two: You can never have enough ice cream stores. Especially in the winter, when a newly scooped cone is least likely to melt if eaten outdoors. Frozen yogurt though, is another story. Like the Yankees, frozen yogurt also sucks.

Number three: Deer are responsible for the recent outbreak of Lyme disease, because Lyme disease loves ticks and ticks love deer – therefore hunting deer is good for everyone and yum, aren’t they delicious?

The first two are obviously true, and if not backed by good science, at least by good taste. The last though, it turns out is not really true – well, maybe the delicious part is — a nice venison stewed for several hours served with winter potatoes and mint jelly….well, it’s hard to argue with that. But the connection between deer and Lyme disease is not as clear as we had all once thought.

So, you are no doubt wondering, what is a blog on climate change doing tackling Lyme disease? Is it going to become more prevalent in the Northeast with a changing climate? Well, there’s some evidence that Lyme disease will spread Northward, as ticks find new territory in the next 10 years (1). But honestly, if this is news to you, I would like to welcome you to the year 1995, because people have been talking since before Dolly the sheep.

It turns out though, that climate and deer are simple to explain in comparison to what I’m about to tell you. Yes, it’s true, Virginia, forests are complex systems.

So, Lyme disease is caused by a spirochete bacterium which relies on the black-legged tick to pass it on. This tick requires three hosts during its life cycle (for a video of the life cycle that will make you vomit, click here). After the tick hatches, the little larvae finds a vertebrate–chicken, lizard, human, duck, mouse, chipmunk – anything really with a backbone–for its first vampirous meal. After sucking on that sweetness for a little while, it drops off and molts into a nymph, and hangs out for a year or so before becoming active again and finding yet another host (again anything with a backbone will do). The nymph then feeds on that host, drops off and transforms into an adult–when it looks for a larger mammal to feed on (hello people!). Though the tick can get infected with the spirochete in either of its first two life stages, the nymphs and adults tend to be the ones who transmit the infection.

Ok, so what about the deer? Hasn’t tick and Lyme disease abundance increased with the resurgence of the deer population in New England? Shouldn’t we try to control deer through eating them? Are you saying they are *not* delicious with mint jelly? What are you, some horrible vegan?

Well, it seems that the connection between deer and Lyme disease is not as clear as previously thought. Though the adult ticks feed happily on white-tailed deer, the larvae tend to prefer white-footed mice and eastern chipmunks. As these little critters have increased in success over the years, so has the risk of Lyme disease.
So, why are mice and chipmunks doing so well? It’s food — what they eat and what eats them.

During a “masting” year, when oak trees randomly dump oodles of acorns onto the ground, the small mammals go nuts. If there are lots of acorns to chomp in a year, then there will be lots of critters for nymph and larval ticks to bite, which means in the following years there will be more adult ticks ready to spread the disease (2) You’ll notice in this scenario, lots of acorns, no deer.

Meanwhile, foxes, the main predators of chipmunks and mice, are getting squeezed out by coyotes who’re less interested in rodents. More mice and chipmunks means more risk of Lyme disease (3). Again, you’ll notice in this scenario, lots of coyote, no deer.

This is problematic for many reasons. First of all, our main source of controlling the disease just went out the window –what are you going to do, gun down white-footed mice? Moreover, what would you do with all the chipmunk heads after culling the species to control the disease? You just can’t hang them on your wall like deer heads. People would think you were weird.

I think the only solution is a new culinary trend that utilizes these little critters, to replace deer hunting as a sport and dish. I’ve heard deep fried chipmunk is actually quite nice. Anyone game?

Further Reading:

(1) Climate change and the potential for range expansion of the Lyme disease vector Ixodes scapularis in Canada
(2) Climate, Deer, Rodents, and Acorns as Determinants of Variation in Lyme-Disease Risk
(3) Deer, predators, and the emergence of Lyme disease

Minda Berbeco has a PhD in Biology from Tufts University and is the Policy & Programs Director at the National Center for Science Education. She wants to apologize for attempting to simplify an extremely complex system and leaving out a discussion of lizards as dilution hosts, reservoir competence in different animals , how climate change will expand the territory of some carrier animals and how climate may also directly influence tick success. Turns out forests are really complex systems. Go figure. Also, she was kidding about the whole eating chipmunk thing. What are you crazy?

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