City-dwelling wildlife demonstrate 'urban trait syndrome'

"The most pronounced changes among city-dwelling organisms are in reproduction and foraging," said co-author and researcher Frank La Sorte at the Cornell Lab. "For example, city birds tend to be smaller, eat a wider variety of foods, and produce smaller clutches than their rural counterparts. Smaller clutch sizes in urban birds have been associated with higher survival rates and increased growth."

But not all groups of species share the same urban trait syndrome. Beetles, birds, and reptiles tend to have smaller body sizes in the most urbanized areas compared to their country cousins. The mobility of ground beetles was higher while that of reptiles and birds was lower in more urbanized areas.

An organism's level of mobility plays a role in how it looks for food. The study authors identify four different types of foraging behavior among the urban creatures studied. These food-finding strategies are shown in the graphic as they relate to birds, bees, beetles, bats, and reptiles. The "mobile specialist" behavior is inferred but was not documented in the study.

Both birds and bees are "central place foragers," meaning they have a base of operations and make daily trips out and back to find food.

"The most common dietary strategy for birds in urban areas is to be a generalist -- in other words they'll eat a variety of different foods instead of specializing. You see this clearly among such common city birds as the Rock Pigeon, European Starling, and House Sparrow. The specialists gradually disappear," La Sorte explained.

As the dietary specialists disappear, biodiversity goes down in cities around the world, and the species mix becomes increasingly homogenized. Yet biodiversity is what keeps an ecosystem heathy.

Urban ecology is a growing field of study driven in part by human population growth and the expansion of many of the world's largest cities. One of the biggest hurdles when addressing questions in urban ecology is a lack of comprehensive information from within cities. Much of the existing ecological data, apart from birds, is scarce and not gathered in a systematic way that allows for accurate comparisons across cities. The growth of citizen-science programs such as eBird, in which volunteers compile observations of the natural world, have the potential to address this information gap.

"My perspective is that preserving habitat is critical," said La Sorte. "Ecosystems in cities are heavily transformed and managed and intact native vegetation tends to be scarce. The more components of an ecosystem that are preserved and supported, the healthier the overall urban environment will be."

That support could be in the form of expanded parks and green spaces or by supplying artificial nesting resources as ways to partially compensate for habitat lost to city expansion. It's a more nuanced approach to urban conservation aimed at keeping cities healthy for nature and people by accounting for the needs of many different types of species.

This research was conducted as part of the Urban Biodiversity Research Coordination Network (UrBioNet) funded by the National Science Foundation.