Mention the word wildlife to a city dweller and images of animals and birds in remote natural surroundings will probably come to mind – not in an empty parking lot around the corner. But research shows that cities can in fact support wildlife biodiversity, and this can have major implications for conservation efforts.
On a crowded planet, protecting species in their natural habitat is proving increasingly difficult. Humans continue to expand their networks of cities, towns, and farms, leaving few remaining natural animal habitats. By 2030, cities are expected to occupy three times as much land as they did in 2010. With the number of species going extinct on the rise, it is necessary to consider the potential of urban environments to serve as refuges for the survivors.
In 2010, the Convention on Biological Diversity commissioned a new global assessment of the state of biodiversity in urban areas. Their findings, published in the book "Cities and Biodiversity Outlook," which I co-edited, were not entirely bleak. It turns out that cities support biodiversity and provide opportunities for innovative approaches to conservation.
A recent global analysis of urban plant and bird diversity found that cities have lost an average of one-third of the native species found in their surrounding region. While this level is worrying, it is worth noting that two-thirds of the native plant and bird species continue to exist in cities that were never designed with biodiversity protection in mind. In fact, at least 20 percent of the world’s known bird species now occur in urban areas, as do at least 5 percent of the known plant species. More conscious green landscape designs in cities can only help support more of the native species diversity.
While urbanization displaces many species, others have adapted and not only survive but thrive in cities. House sparrows, rock pigeons, starlings, brown rats, and feral house cats are just some examples of species that are ubiquitous in many cities worldwide. More surprisingly, many rarer species are adapting to suburban environments, including San Joaquin Kit Fox of central California.
For many native species, urban habitats may actually be more attractive as refuges. They provide easier and more predictable access to water and food resources, warmer temperatures in the winter, and often fewer predators. Continued breeding can drive the long-term evolution of urban species as they adapt to their new environment.
Species such as the house sparrow have evolved to be strongly dependent on human habitation. And warmer nights and feeding by humans have even changed the migration pathways and geographic ranges of some migratory species. For example, a population of European Blackcap Warblers now winters in suburban southern England instead of Africa.
Noise pollution is another factor influencing urban ecology and affecting the many animals that communicate using sound, such as birds, frogs, and some insects. Birds that have adapted to the urban soundscape show distinct dialects with songs that are simpler, louder, or higher pitched to cut through the background noise.
San Francisco’s resident White-Crowned Sparrows have changed their tune over the past 30 years as the city has grown noisier, losing some distinct notes of their songs. This may have evolutionary consequences, as dialect formation is often the first step toward speciation. Other studies have found genetic differences between urban and nonurban populations of some species, indicating fairly rapid evolutionary changes.
New wildlife communities are coming together in cities, often with accidental manipulation and active management by humans. These communities can play an important role in both the urban ecosystems and surrounding natural habitats. Urban and suburban gardens, for example, can support important reservoir populations of bees and other pollinators that find it difficult to survive under modern intensive agriculture.
The overall picture is not bleak. Cities can provide new habitats that may be quite different from those in natural ecosystems but can still support a variety of species. Species that evolve under such urban conditions may well represent what the future holds for much of Earth’s biodiversity.
Madhusudan Katti is an associate professor of vertebrate ecology at California State University, Fresno. He studies urban biodiversity and has received funding from the US National Science Foundation, and the National Center for Ecological Analysis and Synthesis (NCEAS). He is a contributor to several chapters in the Cities and Biodiversity Outlook (CBO) scientific synthesis and a lead author of the CBO: Action and Policy.