As much as green spaces in urban areas are important, this may not be enough to ensure biodiversity in cities, according to a study by the University of Iowa.

Green Spaces not Enough for Biodiversity:

The study surveyed black cherry and black walnut trees in an urban area in Iowa. But they did not find the same abundance of insects, such as fruit flies that feed on these trees and a type of wasp that feeds on the flies.

It said that intense urban and agricultural development alters habitats, increases fragmentation, and may decouple trophic interactions if plants or animals cannot disperse to needed resources.

“In cities, you might have more trees, but you don’t necessarily have more insects associated with them,” Andrew Forbes, associate professor of biology and one of the authors of the paper, said to Iowa Now. “There’s still this real impact on diversity that’s mediated by the landscape. This study implies that cities decrease diversity in some sort of fundamental, intrinsic way.”

“We found that herbivorous insect densities were decoupled from host tree densities in urban landcover types at several spatial scale,” the study said.

The team, led by Amanda Nelson, a graduate student in biology at the university, surveyed 250 sites in Iowa. They counted the fruit flies found in cherries and walnuts as well as the parasitic wasps that feed on the flies in each site. Nelson found that there were fewer flies and wasps in these locations than in agricultural or non-urbanised sites.

The study believes that built structures and paved areas in urban areas “may make it difficult, if not impossible, for the insects to reach other trees, mate with other populations and thus enrich the gene pool”.

This is the first study that examined insect interactions across a broad area. Tens of thousands of fruits and hundreds of thousands of insects were surveyed between 2011 to 2013.

“We think there’s something about the city that changes those dynamics, those interactions between plants and flies and the insects that eat those flies,” Forbes said to Iowa Now.

Even with the result, the main goal is to find a way for insects and people to share an urban space. Nelson said that to promote full diversity, there is still a lot to learn.

“This doesn’t mean our efforts are wasted, but it definitely means that we need to continue trying to learn to do a better job and be thoughtful about it,” Nelson said.

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