In the city that never sleeps, it’s easy to overlook the insects underfoot. But that doesn’t mean they’re not working hard. A new study from North Carolina State University shows that insects and other arthropods play a significant role in disposing of garbage on the streets of Manhattan.
“We calculate that the arthropods on medians down the Broadway/West St. corridor alone could consume more than 2,100 pounds of discarded junk food, the equivalent of 60,000 hot dogs, every year – assuming they take a break in the winter,” says Dr. Elsa Youngsteadt, a research associate at NC State and lead author of a paper on the work.
“This isn’t just a silly fact,” Youngsteadt explains. “This highlights a very real service that these arthropods provide. They effectively dispose of our trash for us.”
The researchers were in the midst of a long-term study of urban insects when Hurricane Sandy struck NYC in 2012. In spring 2013, they expanded their study to look at whether Sandy had affected the behavior of these insect populations.
The research team sampled arthropods – such as insects and millipedes – in street medians and parks in Manhattan to measure the biodiversity at those sites. The researchers also wanted to see how much garbage those arthropods consumed and whether they consumed more in some places than in others. One hypothesis was that in areas with more biodiversity, insects would consume more garbage.
To see how much the arthropods ate, the researchers put out carefully measured amounts of junk food – potato chips, cookies and hot dogs – at sites in street medians and city parks. Researchers placed two sets of food at each site. One set was placed in a cage, so only arthropods could reach the food; the second set was placed in the open, where other animals could also eat it. After 24 hours, the scientists collected the food to see how much was eaten.
The researchers found that Hurricane Sandy had no measurable impact on food consumption by arthropod populations in New York, which was somewhat surprising since many of the study sites had been flooded with brackish water.
The bigger surprise was that arthropod populations in medians ate two to three times more junk food than those in parks – even though there was less biodiversity in the medians.
“We think this is because one of the most common species in the medians was the pavement ant (Tetramorium species), which is a particularly efficient forager in urban environments,” Youngsteadt says.
In addition, by comparing food consumption inside and outside of the sample cages, the researchers showed that other animals – such as rats and pigeons – were also eating the junk food.
“This means that ants and rats are competing to eat human garbage, and whatever the ants eat isn’t available for the rats,” Youngsteadt explains. “The ants aren’t just helping to clean up our cities, but to limit populations of rats and other pests.”
The paper, “Habitat and species identity, not diversity, predict the extent of refuse consumption by urban arthropods,” was published online Dec. 2 in the journal Global Change Biology. The paper was co-authored by former NC State undergraduate Ryanna Henderson; Dr. Amy Savage, a postdoctoral researcher at NC State; Andrew Ernst, a research assistant at NC State; Dr. Rob Dunn, an associate professor of biological sciences at NC State; and Dr. Steven Frank, an associate professor of entomology at NC State. Youngsteadt has also blogged about the research here.
The work was supported by NSF RAPID grant number 1318655 and by the Department of the Interior’s Southeast Climate Science Center, under cooperative agreement numbers G11AC20471 and G13AC00405. The center is based at NC State and provides scientific information to help land managers respond effectively to climate change.
Note to Editors: The study abstract follows.
“Habitat and species identity, not diversity, predict the extent of refuse consumption by urban arthropods”
Authors: Elsa Youngsteadt, Ryanna C. Henderson, Amy M. Savage, Andrew F. Ernst, Robert R. Dunn, and Steven D. Frank, North Carolina State University
Published: Online Dec. 2, 2014, in Global Change Biology
Abstract: Urban green spaces provide ecosystem services to city residents, but their management is hindered by a poor understanding of their ecology. We examined a novel ecosystem service relevant to urban public health and aesthetics: the consumption of littered food waste by arthropods. Theory and data from natural systems suggest that the magnitude and resilience of this service should increase with biological diversity. We measured food removal by presenting known quantities of cookies, potato chips, and hot dogs in street medians (24 sites) and parks (21 sites) in New York City, USA. At the same sites, we assessed ground-arthropod diversity and abiotic conditions, including history of flooding during Hurricane Sandy seven months prior to the study. Arthropod diversity was greater in parks (on average 11 hexapod families and 4.7 ant species per site), than in medians (9 hexapod families and 2.7 ant species per site). However, counter to our diversity-based prediction, arthropods in medians removed two to three times more food per day than did those in parks. We detected no effect of flooding (at 19 sites) on this service. Instead, greater food removal was associated with the presence of the introduced pavement ant (Tetramorium sp. E) and with hotter, drier conditions that may have increased arthropod metabolism. When vertebrates also had access to food, more was removed, indicating that arthropods and vertebrates compete for littered food. We estimate that arthropods alone could remove 4 – 6.5 kg of food per year in a single street median, reducing its availability to less desirable fauna such as rats. Our results suggest that species identity and habitat may be more relevant than diversity for predicting urban ecosystem services. Even small green spaces such as street medians provide ecosystem services that may complement those of larger habitat patches across the urban landscape.