New York, Nov 2 (IANS): Planting trees and creating green space in cities is good for attracting insect species but it may not be enough to ensure bio-diversity, said a study.

The team found insect abundance was lacking in two common, urban trees in Iowa in the US, suggesting insect movement may be limited by barriers, such as roads and buildings.

They found that while there were plenty of black cherry and black walnut trees, they did not find a corresponding abundance of the insects, in this case fruit flies that feed on the walnuts and black cherries and a type of wasp that feeds on the flies.

"In cities, you might have more trees, but you do not necessarily have more insects associated with them," said study co-author and associate professor of biology at the University of Iowa, Andrew Forbes.

"There is 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," Forbes added.

The researchers counted walnut fly larvae to determine the abundance of flies at 250 urban sites, which helped them determine how built environments influence species diversity.

The researchers believe that barriers found in urban landscapes, such as built structures and paved 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.

The study appeared online in the journal PLOS ONE.