Every other Friday on Morning Edition, Outside/In host Sam Evans-Brown answers a question from a listener about some quirk of the world around us.
(Do you have a question for Sam? Submit it here!)
This week, Grant from Lee, New Hampshire asks: “Last year we saw a huge spike in the squirrel population. I was wondering if we can expect any ripple effects from that this year.”
The seasons change, the years go by, but here we are again talking about squirrels.
To review: "mast" refers to nuts (hard mast) and fruits (soft mast) that grow naturally out in the forest, and mast years are years in which a lot of mast is produced at once.
The evolutionary function of mast years is that they create a boom and bust cycle in acorn consuming species. Food is abundant one year and populations rise, and then the food disappears and populations crash.
This ensures that when the next mast year arrives trees can overwhelm their “predators” (squirrels and other things that eat the tree nuts) with a huge crop of seeds - producing so many nuts that the squirrels can't possibly collect and eat them all - ensuring some sprout and survive.
But before the crash, mast years mean good times for squirrels, and a lot of other creatures too.
“Turkeys, deer, raccoons, bears, chipmunks, flying squirrels… various kinds of creatures love to eat those acorns and will focus on them when they’re in abundance,” says Richard Ostfeld of the Cary Institute for Ecosystem Studies.
Up the Food Chain
So how would that ripple out through the food chain? One might think that the next step would be to see the population of predator species spike dramatically, right?
Well, not so fast. It’s certainly not bad for predators to have abundant prey, but there are a couple of fundamental reasons why you’re not likely to notice a huge boom in bobcats out in the woods.
“What determines that largely are how specialized that predator is on that prey species,” explains Ostfeld, “So if the predator eats a bunch of things then it’s going to relatively insensitive to fluxuations in any one prey species. And most of our predators in the Northeast are not highly specialized.”
We do have a couple of critters that Ostfeld thinks might being possibly specialized enough that it might be possible to see a bump in their population, specifically, weasels and owls.
But we don’t have great data on the populations of those creatures. They are nocturnal and very secretive, and getting a good count for a census is rather difficult.
But more fundamentally, every time you move up a step in the food chain, effects are smaller because of basic physics. When a squirrel eats an acorn, around 90% of the energy from that acorn goes to the squirrel doing squirrel stuff.
And when it gets eaten by a weasel, only a small fraction of the energy consumed by the squirrel actually makes it into the weasel's belly. So effects become orders of magnitude smaller, and you’re much less likely to notice them.
Another way up the food chain
This is where we get into the bad news. There is an effect that is quite predictable, that has been measured, but is thanks to another acorn hoarder: the white-footed mouse. And this is the real reason that I reached out to Richard Ostfeld.
“The summer following a good acorn year, the white-footed mice reach a population peak,” he says, “That can be a couple hundred individuals per acre, whereas after a lousy acorn year there might be only a handful … a half dozen or so or fewer individuals per acre.”
And unfortunately, white-footed mice are a major vector for lyme disease, because of a parasite they carry: ticks. If last year there were a ton of white-footed mice, we can expect there to be a ton of black-legged ticks this year.
This relationship is so powerful that Ostfeld has a model that he uses to predict incidence of Lyme disease, and one of the inputs in that model is the date of the last mast year.
“We have statistical evidence that acorn production is a leading indicator of our risk of exposure to tick-borne disease, almost two-years in advance,” he explains.
Now let’s go down the food chain
The ripples go both ways, though. For one, chipmunks and red squirrels are “nest predators” of ground nesting birds. So a big mast year is bad news for thrushes, ovenbirds, redstarts, and anything else that nests on the ground.
But also, mast failure — that’s a really bad year for acorns and nuts — can lead to a collapse in mouse populations. That might mean fewer ticks, but it leads to an outbreak of yet another pest: the invasive gypsy moth.
White-footed mice eat the moths when they are in their pupal stage, and when the mice disappear, the moths erupt.
Nature abhors a vacuum …
... so you should stop vacuuming out in nature. What a weird thing to do. Stop it.
Sam Evans-Brown, is host of NHPR’s Outside/In which you can subscribe to where-ever you get your podcasts. If you’d like to submit a question you can record it as a voice memo on your smartphone and send it to firstname.lastname@example.org, OR call the hotline, 1-844-GO-OTTER.