The Urbane Ecologist

What our cities do

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[Apologies for my numbers–they are all references. The superscripts show up in my “Edit Post” page, but not in the published version. Sorry 😦  ]

Despite the fact that most ecologists do not consider ecosystems static and pristine, many ecologists still conduct studies in that way. They do their best to eliminate humans from studies,categorizing them as a “disturbance”. Studies are often conducted in as close to a pristine environment that can be obtained or in greenhouses. In environments that are not pristine (e.g. former military bases), historical human presence is conveniently left out.

Although ecology has progressed quite well by doing science in this fashion, and we benefit from advanced theoretical knowledge (e.g. models of population dynamics), examining the effects humans have on the environment in cities indicates that we are denying ourselves precious knowledge by ignoring them. Despite the abundance of cities and their immense effects, for example, urban ecology has remained a tiny field.

Some facts about population growth in urban areas:

  • Cities currently cover just 1 to 6% of the Earth’s surface but they are growing rapidly in all areas of the world. 1,2
  • In 1900, only 9% of the world’s population lived in “urban environments”. By 2000, it was 50% and by 2025 it will be 66%.3
  • In the past 30 years, the world’s population increased by 2.4 billion. Half of that growth took place in cities.4
  • All the expected population growth from 2000 to 2030 (about 2 billion people) will be concentrated in urban areas of developing nations, largely due to urban migration and high birth rates.4

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Humans redistribute organisms and the fluxes of energy and materials. This creates what is known as an “urban climate”. 1,5 Urban climate is caused by:

  • the reduction of surface area covered by vegetation 5,6
  • the release of atmospheric pollutants and waste heat (e.g. from combustion).5,6 Artificial heat input may approach or exceed that directly derived from the sun. 7
  • the replacement of natural soil by sealed surfaces.5,6 Replacing grass and tress with brick and concrete, for example, changes the quality of the light that is reflected and radiated. It also changes the heat exchange near surfaces, and the aerodynamic roughness of the surfaces themselves.6

The difference in temperature between the city core  and rural periphery increases with city size and increasingly stronger winds are needed to overcome it.7 Here is how that scales:

  • wind speeds of 5 m/s-1 can eliminate the heat island in a city of 250,000 people
  • wind speeds of 10 m/s-1 can eliminate the heat island in a city of 1,000,000 people

The roughness of the city can reduce wind speeds and inhibit this ventilation, reducing wind speeds by as much as 30% in a big city. 7 So a “rougher” city of a million people may need winds faster than 10 m/s-1 to eliminate a heat island than a city than has a more homogenous surface.

In cities over 10,000,000, the mean annual minimum temperature may be as much as 4⁰ F higher than that off the surrounding rural periphery. 7 This difference is even greater in summer partly because tall buildings, pavement, and concrete absorb large amounts of solar radiation which is released at night. 7

Aside from simply making a city warmer, an urban heat island also serves as a trap for pollutants due to the huge strength of winds needed to disperse it. 6 Ozone concentrations also tend too be highest in and around urban areas because the increased temperatures enhance ozone formation. 16,17 These increased ozone concentrations adversely affect crops, with decreased as much as 5-10%. 8,9 Globally, urban contributions to sulfur and  CO2 contribute in turn to the greenhouse effect, to global warming, and to sea-level changes that are likely to have a serious effect on major coastal cities. 6 Given their historically strategic location, most major cities are coastal. Other climatic effects of urbanization include greater cloudiness, fog, dust, hail, thunderstorms, and precipitation, and lower humidity.7 These effects are magnified with increasing city size.5,7

So, in sum, urban climate makes cities:

  • warmer.
  • more polluted.
  • foggier.
  • dustier.
  • rainier and more thundery.

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Urbanization also has major effects on hydrology by:

  •  increasing flood volume.10
  • increasing flood peak.10(/sup>
  • increasing flood frequency.10
  • increasing turbidity and pollution loads.10
  • boosting the rate of runoff and shortening the time lag between the beginning of storms and the beginning of runoff.11 And don’t forget, urbanization makes cities have more rain and thunder! This is due to reduced tree cover. Greater runoff, in turn, changes the morphology of urban streams, thus possibly damaging remnant riparian vegetation.12 These effects become even more pronounced downstream of larger cities, where natural flushing is incapable of preventing long-term damage.17

Most cities are that are not coastal are located near rivers, which are strongly affected by water pollution. Water pollution can severely damage the ecology of the riverine environment.17

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Ecological processes are tightly interlaced with the landscape, which means that urbanization also has important implications for ecosystem dynamics. Human activities affect land cover, which controls biotic diversity, primary productivity, soil quality, runoff, and pollution.1

Urban ecosystems generally have the following characteristics in common:1

  • low stability.
  • complex and highly variable dynamics on all temporal and spatial scales.
  • more non-native species.
  • different species composition (often simplified, compared to less urbanized areas).

Urban ecosystems also tend to favor organisms that:1

  • rapidly colonize.
  • rapidly adapt to new conditions.
  • have a higher tolerance for people than many endemic, sensitive, and locally specialized organisms.

You may notice that many of those traits are characteristic of invasive species, helping explain why most cities have problems with one or more invasive species.

As a general rule, urbanizing areas host novel combinations of organisms that live in unique communities.1 Diversity peaks at intermediate levels of urbanization and declines as urbanization intensifies.13 These general rules and observations have very compelling but untapped implications for wildlife and rare species conservation.

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Studying urban areas should be justified by their growing impact. But for the more practically-minded, the reminder that we will be increasingly living in cities and that we depend on their proper management in order to maintain an acceptable quality of life for the foreseeable future may be a more compelling argument.14 However, this field has remained small, with less than 1% of research papers in foremost ecological journals dealing specifically with urban species or urban settings.15 A great deal of development in this field must occur before we can adequately understand and maintain our cities.

In one of my next posts, I’ll be explaining how studying urban ecology has to be conducted differently in developed and underdeveloped/developing areas of the world.

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Literature Cited:

  1. Alberti, M., J.M. Marzluff, E. Shulenberger, G. Bradley, C. Ryan, C. Zumbrunnen. 2003. Integrating humans into ecology: Opportunities and challenges for studying urban ecosystems. BioScience. 53(12):1169-1179.
  2. Botkin, D.B. and C.E. Beveridge. 1997. Cities as environments. Urban Ecosystems. 1:3-19.
  3. McDonnell, M.J. & S.T.A. Pickett. 1990. Ecosystem structure and function along urban-rural gradients: An unexploited opportunity for ecology. Ecology. 71:1231-1237.
  4. UN. 2009. World Urbanization Prospects: The 2009 Revision. Department of Economic and Social Affairs. http://esa.un.org/unpd/wup/index.htm (accessed April 2013).
  5. Kuttler, W. 2008. The Urban Climate—Basic and Applied Aspects. Urban Ecology: An International Perspective on the Interaction between Humans and Nature. (Editors: Marzluff, J.M., E. Shulenberger, W. Endlicher, M.Alberti, G.Bradley, C. Ryan, U. Simon, C. ZumBrunnen.) New York, NY, USA: Springer Science+Business Media, LLC.
  6. Berry, B.J.L. and F.E. Horton. 1974. Urban Environmental Management. Englewood Cliffs, NJ, USA: Prentice Hall.
  7. Berry, B.J.L. 1990. Urbanization. The earth as transformed by human action. (Editors: Turner, B.L. III., W.C. Clark, R.W. Kates, J.F. Richards, J.T. Matthews, W.B. Meyer.) Cambridge, UK: Cambridge University Press.
  8. Pickett S.T.A., V.T. Parker, P.L. Fiedler.1992. The new paradigm in ecology: Implications for conservation biology above the species level. Conservation Biology: The Theory and Practice of Nature Conservation, Preservation and Management. (Editors: Fiedler, P.L. and S.K. Jain.) New York, NY, USA: Chapman and Hall.
  9. Alberti, M., J.M. Marzluff, E. Shulenberger, G. Bradley, C. Ryan, C. Zumbrunnen. 2003. Integrating humans into ecology: Opportunities and challenges for studying urban ecosystems. BioScience. 53(12):1169-1179.
  10. Vitousek, P.M., H.A. Mooney, J. Lubchenco, J.M. Melillo. 1997. Human domination of Earth’s Ecosystems. Science. 277:494-499.
  11. Hough, M. 1995. Cities and Natural Processes. London, UK: Routledge.
  12. Pickett, S.T.A, M.L. Cadenasso, J.M. Grove, C.H. Nilon, R.V. Pouyat, W.C. Zipperer, R. Costanza. 2001. Urban Ecological Systems: Linking Terrestrial Ecological, Physical, and Socioeconomic Components of Metropolitan Areas. Annual Review of Ecology and Systematics. 32:127-157.
  13. Blair, R.B. 1996. Land use and avian species diversity along an urban gradient. Ecological applications. 6:506-519.
  14. Grimm, N.B., J.M. Grove, S.T.A. Pickett, C.L. Redman. 2000. Integrated approaches to long-term studies of urban ecological systems. Bioscience. 50:571-584.
  15. Collins, J.P., A. Kinzig, N.B. Grimm, W.F. Fagan, D. Hope, J. Wu, E.T. Borer. 2000. A new urban economy. American Scientist.
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3 thoughts on “What our cities do

  1. Pingback: The Urbane Ecologist

  2. Pingback: Environmental challenges in developing countries are very different from those in developed countries–green, gray, and brown | The Urbane Ecologist

  3. Pingback: Urban Ecology in Developing Nations | The Urbane Ecologist

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