Biodiversity Isn't Everything in Urban Environments - Pacific Standard

Biodiversity Isn't Everything in Urban Environments

Paradoxically, insects and other animals eat more junk food in low-diversity median strips than in parks.
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(Photo: theenmoy/Flickr)

(Photo: theenmoy/Flickr)

Ants ate food waste at a rate of up to 975 kilograms a year at sites in the medians of Manhattan's Broadway and West Street, roughly the equivalent of 60,000 hot dogs, 200,000 Nilla wafers, or 600,000 Ruffles potato chips, according to a study just published in Global Change Biology.

The point of the study wasn't just to see how much junk food ants could get behind their chompers, though. What was of particular interest was how much the ants on Broadway and West, mostly a variety known as pavement ant (Tetramorium Species 5), scarfed compared to the much more diverse ants in Central Park and 13 other parks in Manhattan.

Usually, increasing biodiversity goes along with more efficient use of resources, though that seems not to be the case with urban-dwelling ants in New York City.

To reach that conclusion, entomologist Elsa Youngsteadt and colleagues at North Carolina State University deliberately littered, placing Ruffles, Nilla Wafers, and Oscar Mayer Extra Lean Franks out in 21 park sites and 24 grassy street medians. There were two samples at each site, one open and one caged, so only ants and other insects could get in—that allowed the researchers to study what insects would consume in the absence of vertebrates such as rats.

Medians, the team found, were generally less ecologically diverse places, hosting two fewer ant species and fewer arthropod families as well. In particular, pavement ants, which came to the United States from Europe about a century ago, were much more common in medians, showing up in nearly all median sites and only around a third of park sites. That's significant, because the researchers' analysis shows that in sites with pavement ants, animals including insects and other arthropods ate more than in sites without. Independent of that result, streetwise animals ate about two to three times as much in medians as their park-dining relatives. Environmental factors such as temperature and the depth of leaves on the ground also influenced consumption.

Overall, the results suggest that ants and other insects eat enough food to keep the population of less-desirable scavengers such as rats in check. But the more interesting aspect of the results is what they say about biodiversity, the authors explain—namely, that how well an ecosystem functions is simply a matter of biodiversity. Usually, increasing biodiversity goes along with more efficient use of resources, though that seems not to be the case with urban-dwelling ants in New York City.

"We expected that the more diverse arthropod assemblages in parks should consume more food waste. Although we confirmed that park sites supported more ant species and more hexapod families than did median sites, park arthropods ate 2-3 times less food than those in medians," the authors write. "Our analyses point to the importance of species identity and habitat characteristics, rather than diversity, as predictors of food removal," and likely other ecological functions as well.

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