Ants of Manhattan

Credit: Shelby Anderson

Global urbanization is rapidly expanding and we know that most of the world's humans now live in cities. Most ecological studies have, however, focused on protected areas, such as national parks, as cities are often perceived as not having any ecology.

A group of colleagues decided to test if the theories developed to predict biodiversity in protected areas could also predict species diversity in urban environments. To explore this issue, the researchers decided to focus on ants, partly because ants are ecologically important, but also because the ant species composition in a given area can tell you a lot about its environment.

They collected ant samples at approximately 50 sites in Manhattan, including street medians, urban forests and recreational areas in city parks. They examined each site thoroughly, turning over rocks and sifting through leaf litter and  found not only a wide range of species, but also significant differences in the levels of biodiversity in different urban areas. In fact, they showed that the city has much more diversity than initially expected. In total they've encountered 42 different species across all sites and they exclusively used morphological species determination. I wonder if the results would have been different if they had added DNA Barcoding.

Interestingly the type of urban habitat seems to be more important in determining ant diversity than the proximity between habitats. Sites in urban forests that were far apart had more similar species than an urban forest site and a recreational area site that were right next to each other. 

Overall existing diversity theories from protected areas were fairly accurate at predicting the levels of diversity in urban spaces:

Many predictions derived from less modified ecosystems were supported by our findings: despite being the most intensively sampled habitat, high stress urban medians had less variability in ant composition –both within and among sites – than either urban parks or urban forests, the lowest stress habitat – urban forests-had significantly more accumulated species and a higher number of unique species than higher stress habitats, and urban parks, which have intermediate levels of chronic environmental stress, also had intermediate levels of variation in among-site species composition, accumulated species richness, and the incidence of unique species. The most common species also differed across Manhattan's urban habitat mosaic.

Only one prediction did not turn out as expected. The researchers thought there would be more exotic species in high-stress environments, e.g. street medians. As it turns out exotic species were equally common across all habitats.

This tells us that urban ecosystems are complex and deserve future study -- which could not only inform our understanding of urban ecology, but also our understanding of ecology as a whole. The good news is that it also tells us that existing biodiversity theories can help to guide that future work.