This is a post by our Research Associate Elsa Youngsteadt about her new paper in Global Change Biology.
The first time we came back to an empty cage in Highbridge Park, I thought there was a problem.
This was a cage cobbled together out of a fry basket from a restaurant supply store plus a square of hardware cloth, and it was firmly tacked to the ground with landscape staples. With its snug, quarter-inch mesh, it should let most insects move freely, while keeping vertebrates out. With holes any bigger than a quarter-inch, mice could squirm through.
Exclusion cage made from a fry basket to keep food scraps safe from rats. Photo: EK Youngsteadt
And they would want to, because the cage held a chunk of Nilla Wafer, a Ruffles Original potato chip, and a slice of Oscar Meyer turkey frank. Yum.
The purpose of the cage was to let us measure how much arthropods* alone were capable of eating in this Manhattan park, without interference from mice, rats, sparrows, and whatever other urban critters might shuffle by with the munchies.
(*Arthropods meaning “bugs,” like sowbugs, mites, millipedes, and insects—including ants.)
So I was dubious when we found an empty cage in Highbridge Park on the first day of our study. Could bugs alone have eaten our little pile of food in a day? When we did a trial run back home on the NC State campus, the food never disappeared completely like that. Had a mouse gotten in after all? Had a human passer-by toyed with our experiment?
So we tried again. We moved our setup to a new spot nearby, tucking it discreetly among the trees and undergrowth behind the ball field. We fortified the cage until it bristled with cable ties and landscape pins. And came back a day later.
This time, our arrival was timely. The cage was surrounded by a silent, busy swarm of pavement ants, trundling off the last fragments of potato chip, the last few cookie crumbs. Ants. Not mice, not tampering humans; ants.
This was a scene we would see over and over in the next several days. And maybe we shouldn’t have been surprised.
Ants feasting on hotdog, chip, and cookie in the cage. Photo: EK Youngsteadt
After all, ants are notorious visitors to kitchens and picnics, and they are incredibly important scavengers and hunters in natural habitats all over the world (except Antarctica).
But it seems that I’m not the only one impressed by our tiny urban neighbors. Our study of garbage-eating ants was published today in Global Change Biology, and The New York Times, The Washington Post, National Geographic, and Wired have already published their versions of the story. More news in the works from Germany to South Korea, and I’ve described our study to more than a dozen reporters this week.
This process has been delightful and a little exhausting and even puzzling. I mean, sure, I was surprised when ants cleaned up the food the first time—but everybody else is amazed, too? It’s news that ants eat crumbs?
Or is it news that some scientists were nutty enough to weigh hot dogs and potato chips and set them out in experiments around the most densely populated city in the US–in Central Park, in the middle of Broadway, in view of the Statue of Liberty?
Or is it news that ants are competing with rats for this junk we drop? That this little drama plays out over the crumbs to which we never gave a second thought?
I’ll let you decide. Here’s what we found—the bullet-point version:
- Bugs in street medians ate two to three times more junk food than those in parks.
Co-author Ryanna Henderson sampling a median. Photo: EK Youngsteadt
Street medians are little strips of trees and groundcover planted between lanes of traffic, and they host fewer ant species than do parks. Ecological theory predicts that more diverse ecosystems are more efficient, so we were surprised that bugs in medians ate more than those in parks. We indulged in some extrapolation and calculated that the bugs inhabiting the 150 city blocks of medians on Broadway and West Street are capable of eating more than a ton of food per year–the equivalent of 60,000 hot dogs.
- Pavement ants (Tetramorium species E) are big eaters. We didn’t watch our cages for 24 hours to see exactly which species collected how many crumbs—but at sites where we found pavement ants, more got eaten. Pavement ants originated in southern Europe and the Middle East, but have been living in American cities for more than a century. They love nesting near pavement, so they’re more common in medians than in parks. (But Highbridge Park had them, explaining its big, median-like appetite.)
- Bugs compete with vertebrates (rats!) for our food garbage. Alongside our cages, we offered a second set of uncaged foods, available not only to bugs but also to any hungry vertebrate. More food got eaten from this open treatment than from the cage. That means that both groups of animals (bugs and vertebrates) want the stuff we drop. In other words, they compete for it; what one group gets, the other group doesn’t. I would love to know how much of that uncaged food went to ants and how much to rats–and whether that depended on the kind of food, the size of the food, the time of day, the habitat… But those are goodies for another study someday.
- Hurricane Sandy was a disaster, but not for garbage-eating bugs. Several of our study sites were inundated with saltwater during Hurricane Sandy, seven months before our study. We thought this would matter to the wildlife. But we detected no effect of flooding on how much food got eaten. Maybe there was still an effect on some kinds of bugs—we’re still figuring that out—but certainly not on those that eat our crumbs.
I admit I’ve often tossed an apple core into the woods, not even thinking of it as litter. It’s “biodegradable,” so it doesn’t count, right? Now, my friends, we have met our urban biodegraders. To them it counts, and their work behind the scenes is making our garbage disappear.
Comic by Dorit Eliyahu