Discussion: Increased acorn production in 2010 means increased Lyme Disease

This was taken from a posting in my landscape designers newsgroup. I thought it might be of interest to orienteers as well.

Acorn Glut Signals Lyme Risks | Poughkeepsiejournal.com

This fall, some of you might have noticed it's difficult to walk on sidewalks or hilly trails near oak trees. The acorns underfoot — nature's ball bearings — are so numerous that even sturdy shoes are no match. In our research sites at the Cary Institute and throughout the Hudson Valley, we are seeing acorn production of unprecedented proportions.

Oak trees, like many hardwoods, tend to drop few or no seeds in most years. Then episodically, they produce a bumper crop of acorns, known as a "mast year."

Each tree species has its own rhythm, so it's rare to see multiple species masting together. This year, though, our four most common oak species — red oak, black oak, white oak and chestnut oak —are all producing acorns at the same time. In the 20 years we've been monitoring tree seed production, this is the first time we've seen such an acorn glut.

Why do oaks behave this way? Acorns are among the biggest seeds produced by any of our eastern forest trees. Their large size allows newly germinating tap roots to penetrate thick layers of leaf litter to reach the soil below. It also provides plenty of resources to support seedling growth in the first year. So, large seed size gives oaks an advantage early in life over other species such as maples and birches.

But with these benefits come costs. Acorns are full of protein and fat and are a highly prized food for many wildlife species. You've undoubtedly seen squirrels and chipmunks scurrying around, cheeks bulging, in a frenzy to store as many seeds as possible before the weather turns cold and snowy. Other animals, including mice, deer, raccoons, turkeys, blue jays and bears also scour the forest floor eating or storing every acorn they find.

If oak trees produced a modest acorn crop each year, they'd run the risk of having every one consumed and leaving no descendants. But if they make few seeds in most years, then let loose with a bumper crop, it's likely some acorns will survive the onslaught of hungry consumers. This is called "predator satiation" and is exactly what we find in our northeastern forests. The carpet of oak seedlings we see every three or four springs is a vivid demonstration of safety in numbers.

Many acorns now covering the forest floor will wind up in the stomachs or burrows of white-footed mice. These mice are the most common mammal in our region. With big stores of acorns, mice don't need to move around very much during winter and can avoid predators such as owls, hawks and foxes.

Being so well-nourished, they can start their spring breeding season earlier than usual. Both factors lead to very large populations of white-footed mice the summer following a good acorn year. We expect the forests and fields to be teeming with mice in the summer of 2011.

Research we've conducted with Bard College biologist Felicia Keesing shows newly hatched blacklegged ticks that feed on white-footed mice are much more likely to survive than are ticks that feed on any other mammal or bird host. We've also established that about 90 percent of the larval ticks that feed on mice become infected with Lyme disease bacteria. So, we predict that in 2011 large mouse populations will lead to an increase in the survival and infection of ticks.

This is bad news for human health. Mouse-fed larval ticks from 2011 will sit quietly on the forest floor for almost a year until they emerge as nymphs — the stage responsible for transmitting the vast majority of Lyme disease cases. Our research predicts the acorn bumper crop of 2010 will cause a mouse population explosion in 2011, which in turn will result in abnormally large numbers of infected nymphal ticks in the summer of 2012.

We have been tracking acorn production by Hudson Valley oaks for 20 years and have consistently seen a spike in cases of Lyme disease in Dutchess County residents two years after large acorn mast years.

Risk of exposure to Lyme disease and other tick-borne diseases is a constant part of life in the Hudson Valley. But acorns — an ecological leading indicator — provide an early warning of the years when the risk of these diseases will be particularly high.

Very interesting! Thanks for posting that.

Is the mast year local to New England or does it occur more broadly?

I have a query. As I understand it, Borrelia isn't trans-ovarial in the mammal host, so that would suggest that an explosion in the mouse population would lead to an overall dilution of the Borrelia-infected mouse population, resulting in fewer infected larval ticks? Presumably the 90% of nymphal ticks which become infected are those you've studied feeding on known infected mammal hosts? I can see that the numbers of ticks will increase as a result of the increased mouse population, and therefore more tick bites in humans will result, but I'm wondering if the overall percentage of infected ticks would be lower? Thanks..

more on masting:
http://www.americanscientist.org/issues/id.970,y.0...

The pups may come into the world without the bacteria, but ticks feeding first on an infected adult and then spreading to uninfected pups would pretty quickly reverse that. The CDC factsheet about lyme disease points out that a tick may feed on multiple hosts in a given life-cycle.

Also, if the number of infected ticks rises regardless of the rise of total number of ticks, there will still be an increased risk of catching the disease. You may be right that the percentage of infected ticks will be lower, but the risk of getting lyme should still go up.

That article you shared, ndobbs, had a couple great photos of California blue oak forest. I've mapped a good deal of that forest for orienteering and it's great. Like running through a park. Next time I head out there I'll have to keep an eye out for woodpecker acorn stashes.

I think the last time blue oak masted out here was fall of 2007, and considering that our spring 2008 orienteering trip included 3 rattlesnake sightings in one day, I'm more curious about how an acorn/rodent explosion would affect snake populations than tick populations.

I honestly know nothing about epidemiology, but I just wonder that if MikeW is right and there is a decrease in the percentage of ticks carrying Lyme disease, even if there are many more ticks all told, that the risk of getting Lyme could be lower--if transmission is non-linear with respect to the number of infected ticks? Aren't there "tipping points" of sorts in disease transmission? (I don't know the technical term...)

MikeW: I don't know. It's not my study, I just posted it after finding it interesting in how it related the population dynamics of acorns, mice and ticks, and thus Lyme Disease. If you wanted to have an indepth epidemiological discussion, you may be able to trace the story back through the Poughkeepsie Journal website. You do raise some interesting questions though.

It sounds like normal host-parasite population dynamics. More acorns leads to more mice. At first the new mice are free of ticks hence free of infection with Borrelia, but in short order more mice leads to more ticks and more ticks and mice leads to more Borrelia leads to more Lyme disease.

Or I suppose there's the chance that more acorns => more fat, lethargic mice that lie in their burrows sleeping all day. Hence are not exposed to ticks very much.

I'm thinking of my German Shepherd that seemed to pick up fewer ticks in his later years, but laid around the house all day.

The mice don't have you to open cans of acorns and serve them in a dish.

By the way, in the Western US, the main vector of the Lyme bacteria is the Western black-legged tick. It tends to feed on Western fence lizards, the blood of which kills the bacteria. This reduces, although does not eliminate, Lyme disease in California.
http://www.calacademy.org/science_now/archive/wild...

I do not suppose Western fence lizards would survive in the Chicago woods - we do not have many fences.