Urban Ecosystems 4: Metabolism of Urban Ecosystems

Urban Ecosystems 4: Metabolism of Urban Ecosystems Photo Credit: Science NetLinks


To understand that most of the materials and energy used by a city come from outside the city boundaries. To understand that the pathway of these materials through the city tends to be linear (as opposed to cyclic in natural ecosystems), and that flowpaths into the city are longer than flowpaths out.


This lesson was developed by Dr. Penny Firth, a scientist, as part of a set of interdisciplinary Science NetLinks lessons aimed at improved understanding of environmental phenomena and events. Some of the lessons integrate topics that cross biological, ecological, and physical concepts. Others involve elements of economics, history, anthropology, and art. Each lesson is framed by plain-language background information for the teacher, and includes a selection of instructional tips and activities in the boxes.

This is the fourth of a strand of five lessons entitled Urban Ecosystems: Continuity and Change:

  • Urban Ecosystems 1: Cities are Urban Ecosystems
  • Urban Ecosystems 2: Why are there Cities? A Historical Perspective
  • Urban Ecosystems 3: Cities as Population Centers
  • Urban Ecosystems 4: Metabolism of Urban Ecosystems
  • Urban Ecosystems 5: In Defense of Cities

This lesson series addresses the concept of cities as urban ecosystems that include both nature and humans in a largely human-built environment. Students will be shown the importance of food surpluses to the historical development of urban ecosystems. They will also learn how the exploitation of forests, irrigation waters, and other resources led to catastrophe for some early cities. One lesson shows that the size and number of modern urban ecosystems is unprecedented and that fossil fuel use is a key factor in this. Material and energy flowpaths into and out of cities will be described and students will have the chance to consider how and where these flowpaths are linear versus cyclic. Finally, students will look at some of the positive environmental features of urban ecosystems.

Urban Ecosystems 4 will show students that most of the materials and energy used by a city come from outside the city boundaries. Students will need to have at least a general working understanding of the concepts of flow (as in energy flow) and cycles (as in nutrient cycles) in order to get the most out of this lesson. The Development section includes elements of a tutorial. A general ecology text, or the ecology chapter of a biology text, could be consulted for more detail.

Dr. Firth would like to gratefully acknowledge Drs. Morgan Grove (U.S. Forest Service), Alan Berkowitz (Institute for Ecosystem Studies), and Matt Klingle (Bowdoin College) for reviewing the Urban Ecosystems: Continuity and Change set of Science NetLinks lessons and providing valuable comments and suggestions.

Contact Dr. Firth at pfirth@nsf.gov.

Planning Ahead

Be sure to visit the websites to familiarize yourself with them in advance of the class.


Take the class on a field trip to the school cafeteria. Make sure they see the huge tubs, jars, boxes, and other containers of food. Ask the cafeteria manager who orders the food for the school. Invite this person to talk with the class.

Give him or her a list of questions (in advance) that you or your students might ask. These could include:

  • What foods that are served in our school are locally grown or raised?
  • Are these foods fresher or less expensive than foods from far away?
  • Are any of them organic (i.e. grown or raised with certified organic farming techniques)?
  • How far in advance do school planners have to make up the menus? How much flexibility do they have in what they serve?

OK, maybe we’ll skip the output field trip for now. But be sure to point out the bathrooms on your way back to the classroom. Where do those pipes go? What happens to what we flush? Be sure that your class understands that all of the food they eat, no matter how awesome or awful, goes to either build or power their bodies, or to the various outputs from their bodies.


Cities have linear metabolisms and are heterotrophic. Big words, simple concepts. In nature, things cycle. The word "cycle" has a Greek root that means "circle" or "wheel."

Students may need a refresher on the cycles of nature. Basically, everything that an organism puts out becomes an input for another organism, or in some way renews and sustains the living environment. There are no wastes as we humans know them. The water, nutrient, and other cycles of nature are together considered to be part of an ecosystem's metabolism.

The metabolism of urban ecosystems is much more linear (line-like) than that of forest or lake ecosystems, which tend to emphasize cycles (circle-like). Humans literally create scores of inputs and outputs. Many of the materials that enter the city may be used once and then discarded to join a pile in a landfill. Other materials—such as human food—enter the city, enter the humans, exit the humans (in a somewhat smellier form), and then exit the city via a sewage system. The poem below provides a good description of the way it was.


"Now from all parts the swelling kennels flow,
And bear their trophies with them as they go:
Filth of all hues and odours seem to tell
What street they sail'd from, by their sight and smell …
Sweepings from butchers' stalls, dung, guts, and blood,
Drown'd puppies, shaking sprats, all drenched in mud,
Dead cats, and turnip tops, come tumbling down the flood."

Little-known fact: 75 percent of the resources used by humans on the planet are used by people in urban ecosystems. In a typical city, much more material comes in than goes out each year. This means that cities actually accumulate mass as time goes by—sort of like a typical child’s bedroom. Many natural ecosystems accumulate mass as well. For example, a forest may accumulate dead wood and leaves, as well as the mass of the growing trees themselves. Periodically, many forests experience fires that burn off the dead material and sometimes even the living trees. Modern cities do not regularly experience this kind of catastrophic removal of materials—thank goodness! And this is another way that they differ from non-urban ecosystems.

Natural for nature, catastrophic for cities
OK, your students have now probably pointed out to you that in relatively recent history, cities have, in fact, experienced catastrophic removal of materials. The Great Chicago Fire of 1871 and the Johnstown flood of 1889 are obvious examples. Many hurricanes also have caused large chunks of cities to be washed away. But on the whole, urban ecosystems have a tendency to accumulate materials.

Let us go back to the term "metabolism" (from the Greek root meaning "change"). In addition to thinking of metabolism in terms of materials going in circles versus lines, ecologists also think of metabolism in terms of where the energy comes from. Consider a green plant. It gets its energy directly from sunlight. Green plants are equipped with extremely cool structures in their cells called chloroplasts.

The very cool chloroplast
Check out Choloroplasts and Pigments, a site with photographs of chloroplasts.

Chloroplasts are the plants’ ticket to an autotrophic or “self feeding” life. They take sun energy and change it into… Doritos®. Well, not exactly. Actually they fix light energy (sunlight) into chemical energy (carbohydrates). These carbohydrates can be used to manufacture most of the foods and fibers that we humans use to support our eating and clothes-wearing habits.

If you are not a green plant, where does your energy come from? Eating plants, of course. Or eating the things that eat plants, or eating the decaying bits that once were plants or plant eaters. Organisms that make their living this way have what is called a heterotrophic metabolism (“other feeding”). Have your students try to name as many different kinds of heterotrophic organisms as they can think of. Most will probably name animals. Don’t let them ignore the unseen masses of microbes such as bacteria and fungi. How many did they come up with?

Who lives here with us?
Little-known biodiversity fact: Scientists have a better understanding of how many stars there are in the galaxy than how many species there are on earth. Estimates of global species diversity have varied from 2 million to 100 million species, with a best estimate of somewhere near 10 million. Attention future taxonomists: Only 1.4 million have actually been collected, examined, and named!

If your class were to name a different organism—animal, plant, fungus, bacterium or other microbe—every 10 seconds of the approvimately six-hour school day, it would take them almost 18 school years (assuming a nine-month school year) just to name all of the living organisms that share the planet with us. How many years would it take if they worked eight-hour days, five days a week including the summer? (Hint: more than 13 years.) How about adding weekends? (Hint: approximately 9.5 years.) OK. They must be convinced that there are a lot of different organisms out there to be named. Why do they think they were only able to name a teeny fraction of them?

The International Biodiversity Observation Year (IBOY) site provides a pretty comprehensive overview of the different kinds of organisms that we know of and how many scientists estimate we do not yet know.

Like organisms, different ecosystems also have autotrophic and heterotrophic metabolisms. In autotrophic ecosystems, most of the energy that powers the food web was fixed by green plants right there inside the system. Recall that the term means self-feeding. A grassland is a good example. The grasses provide food for grazers such as mammals and insects. Predators such as spiders, mammals, reptiles, and birds are supported by this grass-based food web.

If most of the energy that runs the food web was actually fixed outside of the system, you have a heterotrophic ecosystem. Think of a small stream: It is shaded much of the growing season, and in the autumn, leaves from streamside plants drop into the water. These leaves are colonized by stream microbes and then eaten by the insects and other small animals that live in the stream. Fish and other stream predators are supported by this leaf-based food web.

Cities and towns—just like your school—also depend heavily on materials and energy from outside their boundaries. Thus, urban ecosystems are heterotrophic. This is a simple statement, but it is profoundly revealing. Because most of their resources come from outside, cities are dependent on all kinds of processes beyond their borders. The significance of this to human populations is the illusion it encourages: that humans are independent from nature. Unfortunately, it is this illusion of independence from nature that has led to some of the problems of modern cities.

How can cities function more like non-urban ecosystems?
One obvious way is by recycling to take some of the linear material flows and make them circular again. Recycling paper, metal, plastic, and glass not only decreases the amount of waste in landfills, it reduces the amount of new resources that must be mined, cut, or pumped from ecosystems beyond the boundaries of the city.

Earth 911 Reuse & Recycle Sites includes a nationwide search function for recycling locations and programs, with links to information on recycling specific materials and items.

Recycling organic materials such as grass clippings, leaves, and vegetable trimmings from the kitchen is another way that natural cycles can be enhanced in the city. We recycle organic matter by composting, thereby returning nutrients to the soil. See Cornell Composting for more information.


By Robert Francis

My spading fork turning the earth turns
This fellow out - without touching him this time.
Robbed of all resistance to his progress
He squirms awhile in the too-easy air
Before an ancient and implicit purpose
Starts him traveling in one direction
Reaching out, contracting, reaching out,
Contracting - a clean and glistening earth-pink.
He has turned more earth than I have with my fork.
He has lifted more earth than all men have or will.
Breaking the earth in spring men break his body.
And it is broken in the beaks of birds.
He has become and will again become
The flying and singing of birds. Yet another spring
I shall find him working noiselessly in the earth.
When I am earth again he will be there.

Historically, cities were smaller, as mentioned above. Some of them were much more similar to autotrophic ecosystems than present cities. For example, many medieval cities were ringed by food gardens and orchards, farms, grazing land, and forests. In China, people put nightsoil (human waste) onto local farmland to maintain soil fertility. Even today, many Chinese cities administer their own, adjacent areas of farmland and, until the recent rapid urban-industrial growth, were largely self-sufficient in food.

Flowpaths in are long, flowpaths out are short. We learned above that urban ecosystems are heterotrophic, with most of their energy and materials coming from outside their boundaries. The ability of cities to reach out for materials and energy has changed greatly over time. Early cities depended upon their immediate hinterland for building materials (e.g. wood, stone, and clay brick), for food (e.g. crops, herds, and fisheries), and for exchange items that supported the local economy (e.g. salt, spices, minerals, fiber, and fabrics). As scientists study the life and death of early cities, they often find clues to decline in the exhaustion of local resources, the poisoning of supporting ecosystems (e.g. soil salination, water contamination), and catastrophic floods and droughts linked to regional deforestation.

A $30 Cab Fare to the Sea
Ephesus was founded in the 7th Century B.C. in what is now Western Turkey. It was a major seaport and center of trade for hundreds of years. It was also home to the Temple of Artemis, one of the seven wonders of the ancient world. Unfortunately, deforestation in the river basin caused erosion of soils and the harbor and river silted in. Eventually, the city was abandoned. Its ruins are located 6 km from the present coastline. It is hard to believe that Ephesus was once a thriving seaport!

Take your students to Ephesus the Seaport to view an image of the ruins of the ancient city of Ephesus.

As transportation networks changed, so did the flowpaths of materials into and out of cities. One of the big patterns of human history has been the punctuated nature of transportation technology. What this means is that major advances in abilities of people to carry materials (how much, how fast, where, when) have been followed closely by increased availability of items, changes in price of items, and changes in demand for items. Students may find it interesting to consider how such improvements as the horse (faster, more efficient than oxen), improved roadways, railroads, modern shipping, and air transportation have changed what the average urban dweller could afford to buy.

Incidentally, even though horses were better than oxen, they did not come without problems. Horses were the primary means of transportation until the early 1900s and they consumed an incredible amount of resources (water and feed) while creating problems with their outputs for city sanitation. According to urban environmental historians, horses were for 19th-century Americans as problematic, within their historical context, as cars are for urbanites today!

Those were the days, my friends.
The New Deal Network is a website that will take students through a photo-essay prepared during the late 1930s by the Works Progress Administration.

The photos show how difficult it was for farmers to get their produce to city markets on unimproved roads. Students might be particularly interested in the photos that show school buses getting stuck in the mud and school children walking to school.

Looking at modern cities, it is easy to see that flowpaths in are longer than flowpaths out. Have students suggest examples of materials that came from far away to be sold in cities. Are there things sold in cities that are not easily available in markets of small towns or villages? Why?


Skit time! Divide the class into five- or six-person teams. Teams should be given 30 minutes to develop a brief skit to be performed for the class. Use a timer to be sure that everyone gets a chance to perform—you might limit the skits to 10 minutes or less. The skits may include all of the “actors” in urban metabolism in human or other form and should demonstrate some of the concepts that the students picked up from this lesson. Everyone should have a role to play, not all need be speaking roles! If a team is particularly poetic or musical, a song or verse would be fine too. Humor should be welcomed and celebrated. Does anything rhyme with metabolism?

At the conclusion of this lesson, students will have an understanding of the metabolism of cities, in particular the fact that most of the materials and energy used by a city come from outside the city boundaries. They will also understand that the pathway of these materials through the city tends to be linear (as opposed to cyclic in natural ecosystems) and that flowpaths into the city are longer than flowpaths out.


Follow this with the final lesson in the series: Urban Ecosystems 5: In Defense of Cities.

Bad Stuff Here and There
As mentioned above, as cities draw resources from further and further afield, they also accumulate large amounts of materials within themselves. Concrete, gravel, asphalt, and other materials may be relatively inert. But dangerous chemicals are also part of the abundance of materials that are brought in. Unfortunately, some of these tend to accumulate in the urban ecosystem, building up in soils, streams, and lakes. Cities also may send water and air pollutants far downstream and into adjacent rural environments.

Students can go to the U.S. EPA website Where You Live and type in their zip code to retrieve lots of environmental information about their community. Have them look for superfund sites as well as other forms of pollution. The EPA site also describes what is being done about some of these problems and how people can get involved in their own community.

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