the acorn connections, with dr. rick ostfeld: ticks, gypsy moths, songbirds and more
IT’S A CRAZY-ABUNDANT YEAR for acorns where I live and garden, and as I try to remain upright while traversing a carpet of them, I realize how little I actually know about these familiar nuts. Mighty oaks do indeed grow from little acorns, but that one example of cause-and-effect barely scratches the surface of the impact that acorns, and in turn oaks, have when the genus Quercus comprises a main element of a forest community.
Research from the nearby Cary Institute of Ecosystem Studies in Millbrook, New York, reveals how acorns initiate a complex series of ecological chain reactions. And not just the obvious ways, like feeding turkeys or chipmunks or deer, but in influencing Gypsy moth outbreaks and tick-borne disease risk, and even the reproductive success of ground-nesting songbirds.
Dr. Rick Ostfeld, a disease ecologist from Cary Institute, helped me understand what–both seen and unseen–is going on with those tiny acorns and their mighty, wide-ranging influences. Listen (and read along below) to my public-radio show and podcast using the player below, or at this link; it’s the October 19, 2015 edition.
my q&a on acorns’ impact, with dr. rick ostfeld
Q. Seen any acorns lately, Rick Ostfeld? [Laughter.]
A. Quite a few; too many to count.
Q. Sometimes I have been out doing chores lately, and it’s like a shower.
A. And they have that pointy little tip to some of them. So if they hit you on the head at just the right angle—you know it.
Q. They’re weaponized. [Laughter.] We’re both in the Northeast, and are we having what’s called a mast year—and what is that, anyway?
A. We sure are. This term “mast year”—and I have heard various etymologies for where that term originates—is a year of heavy seed production. What the oaks do is they undergo this kind of strange behavior. In some plants, but not all that many, in which periodically let loose with a huge bumper crop of seed, but most years there is usually there is very little seed production–maybe just a trickle, or even a failure of the seed crop entirely.
That’s what our oaks do, and this is one of those years when there might be 100 acorns or more per average square meter of forest floor. That’s why it looks or feels like you’re walking on ball bearings when you’re trying to take a hike.
Q. It does. When this happens, are other species like beeches concurrent, doing this at the same time because of weather triggers? Or is it not that, and just one kind of plant one year and another in another year?
A. It’s the latter. I’ve been working with Charlie Canham, the forest ecologist here at the Institute, for over 20 years, monitoring seed production by our forest trees. He’s done this elsewhere, in other parts of the country and other parts of the world. In the Northeastern forest there is no synchrony between the different species of trees.
In fact, we have a handful of different common oak trees in the genus Quercus, and they don’t appear to be synchronized, even with each other. The sugar maples will have a great mast year every once in a while, but it doesn’t tend to coincide with oak trees. Hickories the same; beeches the same. We’ve seen no evidence of them coinciding.
What’s kind of interesting is that there are places in the world, particularly tropical forests in Indonesia and elsewhere, and you might have 40 or 50 species of trees that are all producing bumper crops of seeds at the very same time. Not only the same year, but in the same two- or three-week period. So you can get that kind of synchrony, but we don’t get it here in the Northeast.
Q. Well, it seems I’m having a bumper crop of a number of things—including apples—so I have large ball bearings to the left of me, and small ball bearings to the right of me…
Q. It’s a wild year. Just to start with the more obvious impact of acorn production on a community: Some animals eat acorns outright–including some who eat them before they are even ripe, still on the tree. And then there are animals who in turn eat the acorn-eaters and so on, so it’s part of the food chain quite literally.
A. When you pick up some of these acorns that are all over our woods these days, you might notice that some of them have little, tiny holes in them. Those are the exit holes of weevils, which attack the acorns when they are just developing, up in the trees. At some point the weevils reach maturity and drill an exit hole and leave the acorn—it still falls to the ground, and it’s still edible by other animals. But that’s one example of an invertebrate, one insect species, to focus on these acorns. It’s not all that well studied—what eats those burgeoning populations of weevils that do so well in an acorn year.
We study tree-seed production using seed baskets—these seed traps that we put out in the woods. We have to very carefully keep them up off the ground on monofilament-line leads onto these nylon posts to keep things from climbing up into them and eating the seed out of our traps.
A. So many things love to eat acorns. You point out that from these tiny seeds do mighty oaks grow, but compared to most other trees seeds around here, they’re quite big. If you think of maple seed, or ashes, or birches and the like, acorns are some of our biggest seeds, and they are a big package of nutrition. There’s a high protein content, high lipid, high micronutrient vitamins. At least some of the acorns like the red oak and black oak acorns have a very long shelf life, because of tannins, which preserve them against decomposition.
A. Think about this little morsel of highly nutritious food that you can store, and it will last all winter long. So some of the creatures, like the white-footed mice and the Eastern chipmunks and gray squirrels, other kinds of rodents, will very happily store these either one at a time like the squirrels—in little pits that looks like they’re planting the seed…
Q. Is that scatter caching or scatter something…?
A. Scatter hoarding—that’s right. And then things like the chipmunks and even the mice do what’s called larder hoarding. They have a big, huge pantry of acorns in a rock wall or underground in a burrow. Those things will persist into the next spring—at least the red oaks and black oaks will.
You may notice if you’re walking through an area with white oaks or chestnut oaks, after the last rains we’ve gotten, that a lot of those now have a little taproot coming out.
A. So there is a group of the oaks that don’t have a long shelf life; they’re already sprouting. As they sprout, their nutritional value declines. Those seeds are sort of escaping these seed-consumers by germinating very quickly. It turns out they’re the preferred ones, because they have a lower tannin content, which is a digestive inhibitor. So they’re really nutritious but they’re not inhibiting digestion, so they kind of have to escape by sprouting and turning into a little seedling before the winter sets in.
Q. These seeds are too heavy to blow around and get planted for the next generation—so they also have some interaction with animals. Jays and squirrels, for instance, don’t just go get the nuts because they’re delicious, but also help move them around and plant some in the process.
A. For the jays and the squirrels, they couldn’t care less about helping these trees—it’s not on their minds. Actually, no one’s ever sure what’s on a blue jay’s or a squirrel’s mind…
A. …it would be hard to figure that out. But you’re right: What they’re doing by scatter hoarding is placing them one at a time. If a squirrel happens to forget where they put a seed, it’s perfectly situated to germinate and grow into a little seedling or sapling the next year. And that is indeed how oak trees can disperse across a landscape. They’re too heavy to blow like some of our other tree seeds do—but they do bounce, of course, if you’re ever seen them hit a rock, or someone’s head.
Q. I was reading a book not long ago, of all the crazy things for bedtime reading, about the Eastern timber rattler, which lives in my area. The book suggested that if you wanted to observe rattlesnakes, you might look at or near the base of an oak tree, because they like to have meal of a chipmunk or a mouse, and guess who likes oak trees? This whole cycle—and I use this as just one example—it’s not just the obvious turkey or deer or mice who are eating the acorns, but it’s feeding a lot more creatures down the line.
A. Very much so. We’ve come to see these acorn years as sort of an engine that drives this whole vehicle of the community of different species interacting in oak forests. One thing that we’ve determined—both by simply monitoring this huge variation in acorn production from one year to the next and also experimentally by adding acorns by hundreds of thousands to field plots (in a bad year we added acorns we had stored)— was that one of the slightly delayed effects of these huge acorn years is that it very strongly boosts the populations of white-footed mice, as well as chipmunks.
The mice respond most strongly. They store these acorns over the winter. They’re able to have this highly nutritious food resource in the winter, following one of these mast years. They don’t need to go out foraging widely, and are less subjected to predators of various kinds, so their survival rates go up.
The mice will sometimes even breed in the middle of winter if they have a good acorn supply, which they would never do if there were no acorns in their little larder back in the nest or burrow.
Q. Oh, dear.
A. So they get a jumpstart on breeding in the spring, and what we’ve found is that they reach these very high peaks in abundance in the summer following a good acorn year. Because these mice are so strongly connected to many other species in these oak-forest ecosystems, that peak in abundance (or very low abundance following an acorn failure) extends their interactions—the interaction between oaks and all kinds of other things you mentioned, like Lyme disease, Gypsy moths, songbirds.
Q. The last time you were on the show, you talked about the correlation between the mice, and how they infect ticks with the bacterium that causes Lyme. When does this abundance of acorns sort of go up the chain and influence the Lyme equation?
A. The way it works is that this big acorn year in fall, around October, is followed the next summer by an outbreak in the mouse population. They reach an annual peak around here typically in August, or maybe September, where you might have several hundred mice per acre of forest—whereas in a normal year there might be 15 or 20 per acre of forest.
Q. A big difference.
A. A big effect. August is the time when the new cohort of baby ticks hatch out of eggs. These are the larval ticks that hatch out in August typically every year. These are uninfected with any tick-borne pathogens like Lyme disease because there is no transmission from their mother to them to the egg. But if they happen to feed on a mouse, then they are very likely to survive the attempt to get their blood meal. They do well when they feed on a mouse, they don’t get groomed off and killed as readily as if they feed on another animal host.
They’re also much more likely to get infected with the Lyme-disease agent, and other tick-borne pathogens like Babesia and Anaplasma. So what that means is that those larval ticks do well, they survive well—and get infected at a high rate. So the next year—now we’re into two summers following that acorn year—we see the emergence of a high abundance of infected nymphal ticks, the second stage. That’s the stage responsible for transmitting most cases of tick-borne diseases to humans.
So what we see is that two summers following a good acorn year, risk of tick-borne disease goes way up. That’s the time when we need to be particularly vigilant to protect ourselves against exposure.
The good news is that we have a fairly accurate leading indicator of what will be a bad year or what will be not so bad of a year. The bad news is that there are bad years—lots of bad years.
Q. Mark your calendars.
A. Acorns, as aesthetically pleasing and wonderful as they are, they’re a harbinger of bad things to come, too.
A. It’s a very interesting one. It has been well known for maybe 20 years now that white-footed mice like to eat the pupal stage—the cocoon stage—in the life cycle of the Gypsy moth. The Gypsy moth [above] is an imported insect pest that attacks oak trees and can defoliate forests and kill millions of trees if they’re abundant for very long.
It turns out that if mouse populations are moderately high, they can essentially nip population of these Gypsy moths in the bud. There are enough pairs of mouse jaws running around on the forest floor after a good acorn year that the Gypsy moths can’t really get a foothold, and begin to grow. They remain regulated, and don’t go through their outbreaks and never really defoliate tracts of forest.
If the mice crash, and especially if they stay low for a few years in a row, then it’s easy for the Gypsy moths to get started in their popular growth rate and get so abundant that by the time the next mouse peak happens, it’s just too little too late. The moths are off and running and can defoliate the forest.
The mice are kind of the bad guys when it comes to tick-borne disease, but they’re definitely the good guys when it comes to regulating this exotic forest pest.
Q. Nothing is either 100 percent good nor bad; it’s all very Eastern, yin-yang.
A. The broader lesson is that making value judgments about wildlife is not as straightforward as we think. Sometimes they do things we like; sometimes they do things we don’t like.
Q. What about the ground-nesting songbirds—why are they affected by these acorn boom-and-bust cycles?
A. We’ve done some work here at the Cary Institute, led by Ken Schmidt, who is a scientist who’s now at Texas Tech University and a former post-doc of mine. He continues to work in the Hudson Valley at the Cary Institute.
This is again mediated by white-footed mice and Eastern chipmunks. As cute and cuddly as those little rodents are…
A. …they’re pretty voracious predators on the eggs and nestlings of some ground-dwelling songbirds. What we’ve found is that in these post-acorn years where mice and chipmunks are so abundant, that they’re so good at attacking nests especially of the veery, a small thrush that nests on the ground or in low shrubs, that they can prevent the veery from having much fledging success that year.
In years of intermediate mouse and chipmunk, you see pretty good fledging success—these birds are able to replace themselves from one year to the next. It turns out that there’s another ground-dwelling songbird, the ovenbird, a warbler that nests exclusively on the ground….
Q. A favorite!
A. …I love them, too. They’re able to escape; apparently their nests are not detected by the mice and chipmunks, as opposed to the veery’s, which is much more vulnerable. The ovenbirds seem unaffected by the fluctuating populations of nasty rodents who want to eat their eggs or babies, whereas the veeries just take it on the chin when it’s a good rodent year.
Q. Any other connection—we didn’t mention predators coming down to get all these mice and chipmunks who are so abundant. Is this good for populations of, say, raptors?
A. We think so. The raptors and mammalian predators are much harder to study.
Q. I see.
A. The really good, solid evidence for these relationships is often lacking. You notice that when I said the mouse and chipmunk populations were moderate, these ground-dwelling songbirds like veeries do well. It turns out that when mice are really scarce, some of the raptors, especially the accipiters such as the sharp-shinned hawk and the Cooper’s hawks in particular—when mice are very scarce, they tend to spend more time attacking the adult birds. Even though they’re not as avid attackers on the nestlings or the eggs, these poor veeries take it on the chin whenever there are a lot of mice—their babies or eggs get eaten—and when there are very few mice, the Cooper’s and sharp-shinned hawks go after the adult birds.
It’s kind of like a Goldilocks effect: You have to have the “just right” population of mice and chipmunks, that’s kind of in between high and low. That’s when these birds do well; the adults survive, have lots of babies and the populations can grow.
Q. Fascinating—and you have the best job ever, Dr. Rick Ostfeld at Cary Institute. And we’re thankful to you for doing it. Thanks for explaining these intricate relationships outside.
more from dr. rick ostfeld and cary
- Our earlier interview about the tick-borne disease cycle
- Acorn connections on the Cary website
- Visit the Cary Institute website
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