Urban Ecology

Andrew Jennings
Product Manager, Ecology, Earth Science, and AP® Life Sciences

April 2016

Urban environments have long been considered unnatural and human-created (anthropogenic). While this is true, most “natural” environments are also affected by humans in some way, shape, or form. In an urban environment, all of the normal processes of nature take place. There are energy inputs, water flows, and species interactions, but they occur differently from what is taught in standard ecology.

Human populations are rapidly becoming more urban throughout the world. As recently as 2005, less than half of the world’s population lived in non-urban areas. In 2015, 54% (or 3,880,128,000 people) lived in urban areas. Most of this increase has occurred in less developed countries, but even developed countries such as the US have seen continued population movement into urban areas. In 1965, 71.9% of the US population lived in urban environments; the end of 2015 found 81.6% of the US population living in urban areas, with that number expected to rise to 85.1% by 2035 (UN 2015). This continued urban migration can have an impact on environmental science instruction at the high school level. Students do not have hands-on experience in nature before beginning an environmental science course, and often it is nearly impossible to take students into a natural environment if your school is in the middle of a major city.

Despite these constraints, the urban environment can be studied just as easily as a natural environment and has some unique characteristics pertaining to temperature, nutrient cycling, water availability, and biodiversity measures that can, and should, be explored in your classroom. To help you teach about the urban environment, below are several of the differences you will observe when doing environmental studies in a city versus in a natural environment.

Urban temperature and the urban heat island effect

Changing temperature causes a host of differences in any environment. With the continued focus on the impact humans have on climate change globally, cities provide a unique man-made environment in which students can observe the direct effects of anthropogenic change, especially on temperature. Due to the physical construction of cities, the construction materials used, and the high proportion of greenhouse gas emissions per acre, urban areas on average are 1° to 5° warmer than the surrounding non-urban areas (Parlow 2011).

This is called the urban heat island effect (UHI) and is a microcosm for changing temperatures globally. Students can examine the UHI in their city by taking various temperature measurements along an urban gradient, from a city center to an area outside of the city (or exurban area). Another way to examine this is to look at weather reports for cities, suburbs, and rural towns close to one another and see how the temperature changes based on the urbanization level (Parlow 2011).

Urban nutrient cycling

Unlike natural environments, urban areas tend to have an overabundance of basic nutrients. For example, carbon exists in large quantities all over the urban landscape due to emissions. Nitrogen and phosphorus (key elements in terrestrial and aquatic plant growth) also exist in unnaturally high quantities, mainly from fertilizer runoff but also from manufacturing and wastewater effluent.

Urban soils themselves contain nutrients due to both legacy effects and current land use. Lewis et al. (2006) examined yards in Phoenix, AZ, comparing the organic matter, carbon, nitrogen, soluble ions, and inorganic phosphorus between yards that were previously agricultural fields and yards that were previously wild lands. They showed that the legacy effects of agriculture contributed to levels of organic material, carbon, nitrogen, and soluble ions in soils. In yards that were former farms, the nutrient levels were more than twice as high. This effect was found to exist for more than 40 years! Former farmland also had elevated levels of phosphorus, but the effect dissipated after 10 to 30 years due to fertilizer use in residential yards (Lewis et al. 2006).

This increase in available nutrients has major effects on biodiversity and growth rates in urban environments, but it also can lead to eutrophication, or an overabundance of nutrients, most commonly nitrogen and phosphorus. Easily observed large-scale algal blooms are the typical result of eutrophication. These algal blooms tend to deplete the dissolved oxygen in water bodies and lead to die-offs of non-algal organisms, followed by a crash in the algal population, leaving behind a mostly “dead” body of water that will take time to regain its former biodiversity.

Figure 1  An algal bloom can cause massive fish kills and the collapse of entire aquatic ecosystems. In urban environments, nutrient loading associated with runoff typically causes an algal bloom.

Urban hydrology

The hydrology of urban areas is unique in comparison to other environments. Due to the built surfaces and structures, there is very little greenspace or permeable surface that allows rain infiltration. This means that most rainwater does not saturate the soil but instead flows down streets and sidewalks and through storm sewers, until it finally makes its way into an urban stream or river. The streams then have to find a way to hold all of the water that is pouring into them, generally leading to high water marks and flash floods. Researchers have labeled urban streams as “flashy” because of these short-duration spikes in the amount of water that flow through the streams during rain events.

This flashiness also affects the banks of urban streams. Instead of having gentle banks and easily observed flood plains and riparian zones, urban streams have sharp, carved banks often held together by the root structures of trees that would normally be outside of a riparian zone (Fig. 2). Flashiness can cause bank collapse, and the lack of a riparian zone can lead to worse flooding in urban streams compared to streams that have well-maintained or naturalistic riparian zones.

Figure 2  Due to the flashiness of an urban stream, the bank is sheer and the normal riparian zone is missing. The exposed roots of non-riparian trees and sheer banks are some of the most visible characteristics of the urban stream.

While stream flows are important for hydrology, the urban environment also affects evaporation and transpiration (evapotranspiration) relative to non-urban areas. Due to the UHI, temperatures in cities are higher than in surrounding areas, and one would expect the rate of evapotranspiration to be higher in cites than in non-urban areas. However, because of the sealing of surface land (through paved streets and built structures) and lower levels of plant life, less water is available in the urban environment, leading to a lower rate of evapotranspiration in urban versus non-urban areas (Illgen 2011).

Biodiversity

The most common way for students to feel connected to their environment is through biodiversity. Seeing many species of plants and animals living together gives one a sense of the greater whole and also can be an indicator of environmental health. As with the other factors explored above, urban ecosystems are unique in species mix, but questions exist regarding species abundance.

Generally, urban areas are associated with decreases in diversity of native species, due to habitat destruction as well as competition or predation from non-native organisms (Adams and Lindsey 2011). Built structures can remove discrete habitat patches and can completely remove the habitat of non-motile plants and animals; built roads can affect migration corridors and lead to accidental deaths of motile animals. While urbanization does negatively impact native species that are not used to living in close contact with humans, plants and animals that have learned to co-exist with humans (synanthropic species) can find niches throughout an urban environment to inhabit (Adams and Lindsey 2011).

Two great examples of synanthropic species are the brown rat, Rattus norvegicus, and the rock dove (pigeon), Columba livia domestica. The brown rat is native to Europe but through trade has been introduced to cities throughout the world. Not only has it been able to find a niche to live in, it has supplanted almost all native rat species in cities due to its generalist requirements for both food and shelter. While not quite as hated as the brown rat, the pigeon is a similarly ubiquitous species found in cities worldwide. Likewise introduced through trade (this time on purpose), pigeons survive in urban environments by living within the built structures of a city and by having a generalist approach to diet. It does not hurt pigeons that humans are more than willing to feed birds and have created a nearly predator-free habitat in which the slow-moving pigeon can forage.

Figure 3  Originally from areas around the Mediterranean, the pigeon is now the most common bird in many cities globally. The rise of the pigeon can be attributed to human-mitigated migration, the pigeon’s generalist diet requirements, and a preference for nesting on built structures that mimic its natural cave habitat.

Although urban areas experience an increase in the diversity of non-native species, this increase, in most cases, is not enough to offset diversity losses—native species loss and a net loss in plant and animal diversity. There is no question about the loss of biodiversity in urban environments; however, several questions remain about how urbanization affects abundance.

Originally it was proposed that while diversity decreased, the abundance of the remaining species increased due to the higher levels of nutrients available in the urban environment. This hypothesis has yet to be accepted as true, yet many urban ecologists still contend it is an accurate picture. Several papers in recent years have contradicted the hypothesis, and a newer hypothesis contends that both biodiversity and abundance decrease in urban environments. Such a prediction is in line with the more-individuals hypothesis, which explains patterns of diversity and abundance in non-urban areas.

Conclusion

The urban environment contains all of the standard components taught in every environmental science classroom across the country. While this is the case, significant differences exist between the urban areas most students inhabit and the non-urban areas most commonly studied in classrooms and researched by field ecologists. Awareness of the differences between urban and non-urban should allow students to feel more connected to the places they live and to realize that the urban environment, while novel, is still a living, breathing ecosystem appropriate for study.

References

Adams, C.E. and K.J. Lindsey. 2011. Anthropogenic ecosystems: The influence of people on urban wildlife populations. In Urban ecology: Patterns, processes, and applications, ed. J. Niemela, 116–128. New York: Oxford University Press.

Illgen, M. 2011. Hydrology of urban environments. In Urban ecology: Patterns, processes, and applications, ed. J. Niemela, 59–70. New York: Oxford University Press.

Lewis, D.B., J.P. Kaye, C. Gries, A.P. Kinzig, and C.L. Redman. 2006. Agrarian legacy in soil-nutrient pools of urbanizing arid lands. Global Change Biology 12:703–709.

Parlow, E. 2011. Urban climate. In Urban ecology: Patterns, processes, and applications, ed. J. Niemela, 31–44. New York: Oxford University Press.

United Nations, Department of Economic and Social Affairs, Population Division. 2015. World urbanization prospects: The 2015 revision, key findings and advance tables. Working Paper No. ESA/P/WP.241. New York: United Nations.

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