ECOLOGY:

Given that humans are continuing to urbanize the planet, it's important to understand how the rest of life of the planet is adapting to such changes. Ants, for example, can be quite adaptable; one ant species has adapted to both urban and natural habitats multiple times.

Such studies of adaptation to urbanization often focus on gardens and other environments that primitively mimic nature. However, much of the heavily urbanized world is comprised of pavement and other potential habitats that are frequently, fundamentally disturbed by people.

If scientists learn how critters adapt to difficult-to-colonize locations (especially ants, which colonize just about every available habitat, and are able to make quick decisions), they will be more likely to learn how to maintain biodiversity in more readily habitable locations. With this goal in mind, Marko Pecarevic (Columbia University, United States; and State Institute for Nature Protection, Croatia) and coworkers have studied ant life on street medians in New York City.

Why study ant life on street medians?

Current hypotheses state that wider street medians should host more species. This is an extrapolation from analogous studies in natural environments.

However, there's good reason to suspect that unique features of urban life may result in different ecological adaptations. Human disturbance, for example, may be more prevalent on wider street medians, disrupting the local urban ecology.

Furthermore, it's possible that ants treat street medians either as habitable islands (surrounded by inhabitable pavement), or a connecting patchwork linking more habitable environments (e.g. plants in sidewalk cracks). These are the kinds of questions probed by Pecarevic and coworkers.

Ant collection and environmental variables.

The scientists sampled ants from 44 street medians in New York City in July 2006 (Park Avenue, 12 medians; West Side Highway, 9 medians; and Broadway, 23 medians). Equal sampling efforts were defeated due to occasional human disturbance of the traps.

Each of the medians is surrounded on all four sides by roads, and the ants do not cross from one median to another (i.e. they are geographically isolated). Ants were either trapped in plastic cups containing antifreeze or trapped via aspiration.

The scientists noted some natural and artificial environmental variables likely to affect the ant communities. These include but are not limited to arthropod abundance excluding ants, percent of canopy over or under 2 meters high, the number of garbage bins, and the number of subway vents.

Ant species richness in street medians.

The scientists collected over 6600 ants from 13 species and 11 genera (the next highest classification above species). Ninety-four percent of the ants were from one of three species. namely Tetramorium caespitum (the pavement ant), Solenopsis molesta (the thief ant), and Lasius neoniger (the cornfield ant).

The first is non-native, and the latter two are native. Most of the street medians featured a similar ant species richness; 31 of the 44 total medians had from three to five species.

This means that there was little variation between medians, with respect to ant species richness. Why environmental factors tend to not contribute much to ant species richness is puzzling.

There were some trends. Specifically, Tetramorium caespitum ants were more prevalent on medians with fewer trees, and Nylanderia flavipes ants were more prevalent on medians with more trees.

The scientists further found that rare ant species in diverse-species medians contributed the most to differences in diversity from one median to the next. In other words, species richness is driven by rare species, rather than a large number of more common species.

Why this is the case is again unclear; perhaps a more rigorous ant sampling method (e.g. catching soil-dwelling and other ants species) may unravel more diversity than the scientists were able to uncover. In the meantime, it's reasonable to speculate that only a small number of ant species are able to successfully adapt to heavily urbanized environments.

Implications.

Contrary to common scientific opinion, it's now clear that non-native ants are not automatically favored over native ants in disturbed environments. On the other hand, this study closely follows others in that many of the dominant ant species these scientists found are also common in other North American cities.

The fact that New York ants are overwhelmingly dominated by only three ant species strongly suggests that it isn't "easy" to be an ant in a highly urban environment. Further studies should focus on why this is the case, with the goal of enhancing biodiversity in an increasingly urban world.

NOTE: The scientists' research was funded by the United States Department of Energy, the National Science Foundation, and the National Aeronautics and Space Administration.

Pecarevic, M., Danoff-Burg, J., & Dunn, R. R. (2010). Biodiversity on Broadway - Enigmatic Diversity of the Societies of Ants (Formicidae) on the Streets of New York City PLoS ONE, 5 (10) DOI: 10.1371/journal.pone.0013222