Tiny — but mighty — food-cleanup crews

To people living in cities, insects may seem like a nuisance. But some scientists now argue that we might better view these critters as mini sanitation workers. New data show they quickly remove food waste that otherwise would attract larger, disease-carrying animals such as rats and pigeons. One city ant species, for instance, appears to play a big role in controlling populations of these bigger pests.

These are the findings of a study published December 2 in Global Change Biology.

Researchers at North Carolina State University in Raleigh set out to study the role of bugs and other arthropods in controlling food waste. They expected to see the best rate of cleanup by insects in areas with greater biodiversity — those hosting the most species. The reason: Many ecosystems become more efficient as biodiversity increases, explains Elsa Youngsteadt. An entomologist, or scientist who studies insects, she led the new study.

To test the idea, she and other members of her research team headed to New York City. With more than 8 million people living in just over 777 square kilometers (300 square miles), food wastes there can be a problem. Crumbs drop. People discard uneaten food, and not always in trash bins. The result can offer a feast to urban critters.

The researchers contrasted two very different city ecosystems: parks and median strips. Median strips are vegetated patches that separate traffic moving in opposite directions.  

Parks host hundreds of different types of birds, mammals and other vertebrates. They also have a high diversity of insects, spiders and other arthropods. Far fewer vertebrates and bugs hang out in the median strips.

At each location, the researchers set out bait for the urban wildlife. This included bits of hot dogs, chips and cookies. Each portion was weighed before it was set onto a piece of metal mesh. That way, the scientists knew exactly how much grub they were offering hungry animals.

But not all animals had equal access to the food. Over one piece of metal mesh at each site, the scientists placed an upside-down wire basket. This created a cage over the food. Openings in the basket were only 0.64 centimeter (0.25 inch) across. This kept squirrels, rats and other big scavengers from chowing down on the scraps. A polystyrene foam plate glued to the top of the basket worked like an umbrella to keep rain off of the chow. Over another mesh plate of food, they placed a similar foam umbrella, but no mesh cage.

Twenty-four hours later, Youngsteadt and her team collected, dried and weighed any remaining food to calculate how much had been eaten.

At caged and uncaged sites where the team had left small portions of food — amounts similar to morsels accidentally dropped by city snackers — nothing remained. The bugs had devoured it all. This left no food for bigger critters to eat, even if they could reach it. The bugs, however, didn’t finish off larger portions of food — those weighing more than 6 grams (0.2 ounce) — at caged sites. Large portions left uncaged did completely disappear.

This suggests that bugs compete with bigger animals for food. More bugs in the city should therefore help keep rat and pigeon populations down, concludes Youngsteadt.

But her team’s big surprise came when they compared food scavenging in parks and median strips. Youngsteadt expected to find that the wildlife in parks would eat more food. Remember, biodiversity is higher there than in medians. In fact, city wildlife in the median strips scarfed up more of the food scraps.

When Youngsteadt’s team looked closer, one species stood out. The pavement ant appeared to play the biggest role in city food removal. These ants prefer to nest near pavement, so they are common in cities. They also are very good at finding food and taking it home. Although pavement ants are not the only bugs in medians, they proved the most efficient at cleaning up the food.

Youngsteadt now estimates that insects and other arthropods could remove food scraps weighing as much as 60,000 hot dogs, 200,000 vanilla wafer cookies, or 600,000 potato chips each year. And that’s just from the 14.5 kilometers (9 miles) of medians dividing two streets in Manhattan.

“Right under our feet, arthropods are busy cleaning up after us,” says Richard Fuller. An ecologist at the University of Queensland in Brisbane, Australia, he was not involved in the new research. “This study reminds us that urban ecosystems are highly specialized and often work in unique ways,” he says.

City planners should take advantage of these natural street cleaners, Youngsteadt suggests. “Whatever food the ants get won’t sit around to rot or feed rats,” she says. And that will reduce disease, making cities cleaner — and healthier — places to live.

Power Words

arthropod  Any of numerous invertebrate animals of the phylum Arthropoda, including the insects, crustaceans, arachnids and myriapods, that are characterized by an exoskeleton made of a hard material, called chitin, and a segmented body to which jointed appendages are attached in pairs.

biodiversity  (short for biological diversity) The number and variety of organisms found within a geographic region.

ecology  A branch of biology that deals with the relations of organisms to one another and to their physical surroundings. A scientist who works in this field is called an ecologist.

ecosystem  A group of interacting living organisms — including microorganisms, plants and animals — and their physical environment within a particular climate. Examples include tropical reefs, rainforests, alpine meadows and polar tundra.

entomology The scientific study of insects. One who does this is an entomologist.

insect  A type of arthropod that as an adult will have six segmented legs and three body parts: a head, thorax and abdomen. There are hundreds of thousands of insects, which include bees, beetles, flies and moths.

mammal  A warm-blooded animal distinguished by the possession of hair or fur, the secretion of milk by females for feeding the young, and (typically) the bearing of live young.

urban  Of or related to cities, especially densely populated ones or regions where lots of traffic and industrial activity occurs.

unique      Something that is unlike anything else; the only one of its kind.

vertebrate  The group of animals with a brain, two eyes, and a stiff nerve cord or backbone running down the back. This group includes all fish, amphibians, reptiles, birds and mammals.