Planting a tree in the city is not enough

No doubt, the creation of green spaces in cities has environmental benefits. They filter pollutants and dust from the air, they provide shade and lower temperatures in urban areas, and they even reduce erosion of soil into waterways. However, it has also been assumed that planting trees and creating green space in cities is good for attracting species but according to a study from the University of Iowa it may not be enough to ensure biodiversity in these artificially build environments.

The researchers surveyed two types of tree in an urban area in Iowa, and recorded the abundance of two insects that interact with them. They found that while there were plenty of the trees, black cherry and black walnut, they didn't find a corresponding abundance of the insects, in this case fruit flies that feed on the walnuts and black cherries and a parasitic wasp that feeds on the flies.

We found that herbivorous insect densities were decoupled from host tree densities in urban landcover types at several spatial scales. This effect was amplified for the third trophic level in one of the two insect systems: despite being abundant regionally, a parasitoid species was absent from all urban/suburban landcover even where its herbivore host was common. Our results indicate that human land use patterns limit distributions of specialist insects. Dispersal constraints associated with urban built development are specifically implicated as a limiting factor.

The colleagues 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. In addition to this fragmented environment  insects have to deal with highly diverse plant resources that are configured in complex mosaics of disparate habitat types often characterized by frequent disturbance as well as altered biogeochemical, hydrological, and temperature regimes.

The senior researcher of the project concludes:

The responses of the diversity of organisms that could potentially share these developed areas with us can be really idiosyncratic. To promote the full diversity, we really have a lot to learn. That 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.