Urban Ecosystems 2: Why Are There Cities? A Historical Perspective


  • Popcorn kernels (which will eventually be popped)
  • Photographs/artifacts brought in by the students
Urban Ecosystems 2: Why Are There Cities? A Historical Perspective Photo Credit: Science NetLinks


To understand the importance of food production and food surpluses to the origin and historical development of urban ecosystems. To understand how the exploitation of forests, irrigation waters, and other resources led to catastrophic consequences for some early cities.


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 second 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 vs. cyclic. Finally, students will look at some of the positive environmental features of urban ecosystems.

Urban Ecosystems 2 looks at the conditions that led to the development of early cities (i.e. food production), as well as some of the factors that caused the decline of early cities (i.e. unsustainable resource use). Students will visit a variety of online sites to see pictures and perform exercises. They will try to bring their learning back home again in the summary exercises that focus on their personal family histories and the history of their local urban ecosystems.

A common student misconception related to this topic is that because of technology, cities are not governed by the natural laws that apply to other ecosystems. In fact, because of their intensive resource use in nearby areas, cities throughout history give us very good examples of how unsustainable use of natural resources can result in disaster.

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.

Contact Dr. Firth at pfirth@nsf.gov. Click here for more interdisciplinary environmental lessons.

Planning Ahead

You may wish to read Guns, Germs, and Steel by Jared Diamond (1997) in order to get a more comprehensive background for this lesson. Another excellent source is Nature’s Metropolis: Chicago and the Great West by William Cronon (1992).


To get students thinking about food production—a prerequisite for food surpluses, which led ultimately to the first cities—you can start them with their own diets.

Have your class keep a food diary over a weekend or for several days. Compile a class food list and then take the class to the USDA National Agricultural Statistics Service site where they can look up what foods are grown or produced in their state, and where in the U.S. different foods are grown.

For example, All Rice 2010 is the map of rice harvests that shows how important California, the lower Mississippi valley, and coastal Texas are to this crop.

Discuss where the foods on the class food list came from (or where they might have come from). You can introduce the concept of food surpluses being sold to nearby or far-off markets.


Let us take a moment to consider what life was like before there were cities. There was a long stretch of human history when people lived off the land and seashore in every possible way. They did not cultivate crops or keep animals for food. They hunted wild animals, fished for wild fish, and collected wild fruits, grains, roots, and mushrooms.

Everyone alive today descends from a long line of people who were expert hunter-gatherers. These ancient people—the ones that survived to reproduce anyway—were exceptionally intelligent. They figured out how to use tools. They figured out how to communicate by speaking to each other. They learned the proper timing to collect or capture hundreds of different foods. They made fire work for them. They routinely avoided becoming meals for faster, better-equipped predators. And they studied out how to make clothing and shelters to shield themselves from the elements. They were strong, swift, persistent, and patient compared with many modern humans.

A Timeline in Popcorn
To give your students an idea of the vast magnitude of time that we refer to as pre-history (i.e. before written records), go to the timeline on the HyperHistory Online site. To access the timeline, click on the "History" button on the left of the screen, then on the "Pre-History" option in the list that will appear on the right.

As they will see, the use of tools and fire—and humans as a species, "Homo"—date to somewhere between 1.5 million and 2 million years ago. Modern humans have been around only about 150,000 years. We got dressed for the first time around 70,000 or 80,000 years ago. But the first civilizations didn't come into being until approximately 5,000 years ago!

As a hands-on exercise, have your students count out kernels of popcorn for each of the major events on the timeline. Each kernel represents 1000 years. So the "pile of years" since modern humans first lived on earth would have 150 kernels in it, the pile of years since Homo stood up and walked around would have 2000 kernels. And there should be a measly pile of five kernels representing the years since everything that we consider familiar about our world came into being.

Now, pop the corn and eat it. If you have a food surplus, you can donate it to a less-fortunate class that has not yet discovered the Science NetLinks Urban Ecosystems lessons!

What early humans did not have, in addition to sofas and VCRs, was a sedentary lifestyle. The timing and distribution of their foods meant that they moved around quite a bit. For millions of years, nomadic hunter-gatherers traversed the landscape, taking what they could use. It was not until the first people thought of scattering or planting seeds—and returning later to harvest the crops—that early humans could begin to settle for any length of time in a given area. Keeping herds of animals for food (animal husbandry) also encouraged settlement in a relatively limited area. Crop agriculture and animal husbandry are together referred to as food production.

Cities developed along with food production and new technologies for farming. There was probably a period of time when humans were learning about how to sustain soil fertility and manage crops and herds to guarantee harvests and meat on a regular basis. This was the same time period when historians believe that complex forms of human organization and communication also arose. These included political and spiritual organizations, writing and economic transactions, military organizations, and eventually bureaucrats. Food surpluses allowed some members of society to take on other roles: Not everyone had to farm in order for everyone to eat.

Let us turn now to the city of Ur. Ur was located between the Tigris and Euphrates rivers in a land known as Mesopotamia (meso – middle; potam – river). Its ruins are between the modern city of Baghdad, Iraq and the head of the Persian Gulf. Ur was established around 2100 B.C. (four popcorn kernels ago) as the capital of the Mesopotamian Civilization, which arose about 3500 B.C. The people of this civilization built their city around a temple called a ziggurat. The ziggurat was the tallest building in the city. Ur was the home of the biblical patriarch Abraham.

Ur as in Tour
Take your class on a virtual tour of the ancient city of Mesopotamia (Ur). Ask them if anything about it is familiar to them.

The web page Sumeria, The City of Ur gives a brief history of the city, and shows images of some of the treasures found in thousands of fabulously rich tombs that were excavated in the early part of the 20th century.

Have students visit The Royal Game of Ur. This was a board game that was played in the city over 4000 years ago!

Early cities such as Ur were made possible by food production in their immediate vicinity. Their prosperity also depended on forest products. Unfortunately, deforestation occurred on a dramatic scale on the hills around Ur and other Mesopotamian cities, both for timber and firewood and for the expansion of farmland. In the case of Ur, this eventually had appalling consequences. When archaeologists excavated the city, they found that a meter-deep layer of mud had inundated the city in around 2,500 B.C. Other cities in Mesopotamia suffered similar fates.

The Cedar of Lebanon, Cedrus libani, is a slow-growing evergreen member of the pine family. This fragrant tree may grow to 120 feet tall with a trunk diameter of 9 feet. Ancient trees have been described that had a trunk 45 feet in circumference, and with four, five, and even seven gigantic trunks springing from the same base. The beautiful red wood is astonishingly decay and insect resistant. Many early writers were impressed with the majestic aspects of the cedars, and referred to them metaphorically to indicate such qualities as strength, beauty, endurance, grandeur, majesty, dignity, lofty stature, and noblesse. People hurried to cut them down.

Deforestation has followed people and their cities through time and around the globe. At one time, Mount Lebanon was covered with a forest of cedars that were famous for their beauty and strength. Cedars were widely cut (without replanting) for many centuries. Solomon's temple was built of cedar from this area, as were many Phoenician ships. The Egyptians used cedar timber for construction and used the resin for mummification. In the 19th century, people finished off most of the remaining forests by using cedar wood as fuel for railway engines. They generally bypassed more easily obtainable oak wood, since cedar (because of its oil content) burned much better.

The few presently remaining cedar groves were spared mainly because they were in remote areas with difficult access. Here is a photograph of a living Cedar of Lebanon.

The eventual decline of Mesopotamian civilization is linked to another environmental factor: injudicious use of irrigation water.

As you probably know, the Middle East is a dry place. Land there had to be irrigated for cultivation, and the demand for food increased as the population grew. The irrigated land became salinized (salty) and waterlogged. Salination occurs when the small amount of salt naturally contained in irrigation water is left behind in the soil as the pure water passes back to the atmosphere by evaporation and plant transpiration. Records noting “the earth turned white” with salt date back to 2000 B.C. Yields of wheat and barley declined, and eventually the only food plants that would still grow were date palms. By 1800 B.C., the agricultural system collapsed, with catastrophic consequences for the food supply and ultimately the civilization.

Farther to the east, desertification also followed settlement. The fortification of the Great Wall of China gave rise to intensive cultivation of farmland in northern and western China and to the growth of a major travel and trade route that came to be known as the Silk Road. Deserts began expanding in this area as a result of food production and wood demands of a growing population and gradual climate changes. Pictures and maps of the Great Wall may be found at The Great Wall: A Virtual Tour.

Today salinity seriously affects productivity on about 20 million hectares of the world's irrigated land. It threatens the economy of many arid countries, such as Egypt, Iraq, and Pakistan, where irrigation is the backbone of agriculture. In the United States, an estimated 30% of all irrigated land suffers from reductions in yield caused by salt. Salinity also constitutes the most serious water-quality problem in many rivers and groundwater systems that are located in arid and semiarid regions.

The Middle and Far East were not the only places that suffered from the cascading effects of poor environmental management. The ancient Mayan empire was located in what are now parts of Mexico, Guatemala, Belize, and Honduras. The civilization was strongest from about 250 – 1200 A.D., and featured remarkable architecture (all built without metal tools), astronomy, mathematics, hieroglyphic writing, and routes cleared through jungles and swamps to support an extended network of villages and towns. The agricultural techniques the Mayans used were creative and intensive—clearing hillsides of jungle, terracing fields to contain soil erosion, draining swamps by digging ditches, and using the soil from the ditches to form raised fields.

Eventually, too much was demanded of this system. Soil erosion reduced crop yields, damaged homes, and higher levels of silt in rivers damaged the raised fields. Decreased food production and competition for the remaining resources may have led to the demise of the Mayan civilization beginning about 1000 A.D.

Virtual Archaeology
Take your students on a virtual archeological expedition to the ancient Mayan city of Copán at Collapse: Why Do Civilations Fall?.

In this exercise, they will follow on the steps of archaeologists who studied evidence including pollen, bones, houses, and monuments for clues as to why this city collapsed, and whether the collapse was rapid, or gradual.

History is not all ancient. Students can explore change through recent time in several ways and be asked to relate what they find to their understanding of urban ecosystems in history and how cities change through time.

"Recent" and Local Archaeology
Although the term archaeology (from the Greek arkhaiologi: antiquarian lore) refers to the study of ancient peoples, you can do a little "recent" archaeology in your own backyard.

Ask students to bring in photographs and artifacts of their own lives and the lives of their parents or grandparents. Construct an archaeological exhibit (be sure to label items with student names and date everything as accurately as possible). Ask the class how the people and their "stuff" changed over a period of decades. Are any of the artifacts evidence of an urban ecosystem? How did it change over time?

Now ask the class to consider the environment through time. Have them look for old newspaper articles and library books that describe changes in a nearby urban ecosystem over time. Sometimes aerial photographs are available. The Earth from Space is an excellent website that features astronaut photography. Is your city on this website? Have teams of students try to pinpoint places familiar to them, or that they have visited.

Finally, ask the class how the urban environment has changed over time. Is there more pavement? Are there more buildings? Have any woodlands been logged or parks created? Streams put in culverts or restored to their former channels? Are any of the features of the changing urban environment related to the artifacts from their personal family histories (e.g. cars, lawns, shopping centers, etc.)?


Assess students' understanding by having them look for patterns in nature and human communities that extend across the metropolitan area they used for the lesson, and across the boundaries of the area into adjacent rural areas. Examples might include waterways, roads, and railroads; geomorphic features such as ridgelines, valleys, rich soils, and poor or rocky soils; biological features such as woodlands, parks, and cultivated or pasture lands; and human-built features such as real estate developments, shopping areas, and industrial zones. Students will likely come up with very original and creative ideas.

Ask questions such as:

  • How might these patterns have changed over time?
  • Are any of these patterns similar to those explored in the ancient cities of this lesson (i.e. food production and surpluses, deforestation, erosion, intensive irrigation, cutting or planting of trees)?
  • What can you conclude?

Have the class write a letter describing their observations to be put into a time capsule. Photographs and other materials can be included as well. When the capsule should be opened will be up to the class, but it should be clearly marked on the outside.

After completing this lesson, students should understand the importance of food production and food surpluses to the origin and historical development of urban ecosystems. They will also have a chance to find out more about how the exploitation of forests, irrigation waters, and other resources led to the demise of some early cities. Their understanding will be connected to present-day circumstances in their community by several of the exercises.


Follow this lesson with the next three lessons in the Urban Ecosystems series:

They Were There
Students can interview older family members or acquaintances about their experiences living in the community and their reflections on how and why things have changed over the years. Listening to History, on the NEH EDSITEment website, features a link to a site that gives tips on conducting interviews.

Have students talk with their parents, other older relatives, and/or older neighbors about what they remember about the community from years ago. What do they recall that changed the environment or the people living there (i.e. major highways, dams, bridges, housing developments built; new employment opportunities, or lost employers)? How did they feel about these events? Can the older folks remember changes in attitude toward the environment? Students should take notes on their answers and be prepared to talk with the class about what they found out.

The West Philadelphia Landscape Project is an excellent model that the students can use to think about how an oral history of place might be developed and used.

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Lesson Details

Grades Themes Project 2061 Benchmarks National Science Standards State Standards