While planting trees and creating green space in cities is good for insects and people alike, it may not be enough to ensure biodiversity in built environments. Insect movement may be limited by barriers such as roads and buildings.
Biologists from the University of Iowa turned a common assumption on its head when they found that more trees in urban spaces do not correspond with more insects.
In a recently published study, the researchers surveyed black cherry and black walnut trees in an urban area in Iowa. They found that while there were plenty of both trees, there was no corresponding abundance of insects, in this case fruit flies that feed on the walnuts and black cherries and a type of wasp that feeds on the flies.
Andrew Forbes, associate professor of biology who led the study, argues that the physical structure and landscape of urban environments decreases diversity in a fundamental, intrinsic way. 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.
Despite this rather grim conclusion, the researchers believe that our efforts at promoting biodiversity through green urban spaces have not been in vain. We simply need to learn how to do a better job of it.