Urban Ecosystems 3: Cities as Population Centers

What You Need

Urban Ecosystems 3: Cities as Population Centers Photo Credit: Science NetLinks


To understand that throughout history cities have been centers of population, but that the size and number of modern urban ecosystems is unprecedented. To understand that human exploitation of fossil fuels was key to the growth of large cities worldwide.


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 third 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 3 looks at the enormous increase in size and number of cities in the very recent past and the influence of fossil fuel use in particular on urbanization. In this lesson, students will visit a variety of websites that deal with urban population, fossil fuel consumption, and the signals (i.e. carbon dioxide emissions) that can be used to track population changes.

A common student misconception related to this topic is that “what is now has always been.” In fact, the last two generations have seen astronomical growth in the size of cities, in the proportion of the world population that lives in cities, and in fossil fuel use.

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

You also may wish to print the Change in U.S. Metropolitan Population and Land Area (1950-2050) chart from the Ecological Cities website that the students will use to determine urban population and land area.

A listing of links to many population, demography, and geography sites is found at World Population: A Guide to the WWW.

The United Nations Population Information Network provides links to sites that discuss population trends, including future projections.

The World POPclock Projection on the U.S. Census Bureau site gives up-to-the-minute estimates of global population.


Take a personal approach to introducing change through recent time in this lesson: Ask students to bring in as many family birth dates as possible.

Distribute the Median Birth Years student sheet and divide the class into three teams. Each team will determine the median birth year of one of the three generations being used. Recall that the median is the middle value in a distribution, above and below which lie an equal number of values.

Team 1 will collect the dates of its own generation from all students in the class (i.e. their birth years and the birth years of their siblings) and use this information to determine the median birth year of the present generation. Team 2 will collect the dates and determine the median birth year for its parents’ generation (parent birth years, aunts and uncles). Team 3 will collect and determine the median birth year for its grandparents’ generation (all known grandparents; also great aunts and great uncles).

After students complete the student sheet, make a wall chart as follows and as shown on the Population Graph: Example teacher sheet:

  • The horizontal axis should be time (units are years), marked off in 10-year increments and going back about 100 years.
  • The vertical axis should be population (units are millions of people), from 0 to 7 billion.

Note: Millions and billions can be confusing so you might give students some examples, such as:

  • 1,250,000,000 (read “one billion, two hundred and fifty million”) recorded 1,250 million
  • 5,000,000,000 (read “five billion”) recorded as 5,000 million

Mark off the vertical axis in 1000 million person increments up to 7000 million (that’s 7 billion in plain English).

The teams should record the median birth year of each generation along the horizontal axis of the wall chart as shown by the arrows on the Population Graph: Example teacher sheet.

Now, take the students to World Population 1950-2050 from the U.S. Census Bureau. With the students’ help, put the approximate total population of the planet on the wall chart by date (i.e. 1950: 2,556 million; 1960: 3,000 million etc.).

If you need pre-1930 information, try Historical Estimates of World Population at the U.S. Census Bureau site (i.e. 1920: 1,860 million; 1910: 1,750 million).

Finally, discuss with the class how the urban population of the planet has changed over time. Information can be found in Table 1 and other parts of World Urbanization Prospects: The 1999 Revision on the United Nations site.

In 1950, less than 30 percent of the world population was urban; by 1975, almost 40 percent was urban; and by 2000, the percentage of the world population living in cities was 47 percent. The United Nations projects that over 60 percent of people on the planet will be urban in 2030 (the percentage is even higher, 83.5 percent, in more developed regions of earth). Is the year 2030 about when the next generation of children will be taking your class?


If you used Urban Ecosystems 1: Cities are Urban Ecosystems, the first lesson in this series, you have discussed where cities are and are not (see the composite image of the world at night at Night Lights Around the World).

You can now begin to ask students how one of every two people on earth can possibly live in urban ecosystems. Avoid the quality-of-life issues unless you want to spark a debate of “noisy-dirty-crowded” vs. “exciting-culturally rich-convenient.” Instead, try introducing the idea that for all of pre-history and the vast majority of recorded history, towns and cities had transportation systems and production systems based almost entirely on muscle power. At first, human muscle, but later augmented by dog muscle, horse and donkey muscle, ox muscle, camel muscle, llama muscle, reindeer muscle, and the muscles of a few lesser-known beasts of burden. Water transportation corridors and water power helped out these muscles, and wind power was pretty critical for sailing ships and windmills. But still, most cities were relatively small and most people on the planet lived in village or rural settings. The size of modern cities, in numbers and in physical scale, is unprecedented.

According to historians, in 1800, London was the only city in the world with a population of a million people. At that time, cities were relatively small, usually just a few thousand hectares. When coal, and later oil and natural gas, replaced muscle power, the face and physiology of cities changed drastically. Industrialization, suburban sprawl, new transportation infrastructure, and new capabilities for almost every aspect of urban life were all associated with fossil fuel energy sources. By 1990, the world's hundred largest cities accommodated 540 million people. Almost half of the urbanites lived in twenty mega-cities of over 10 million people, each extending to hundreds of thousands of hectares. In 2000, there were hundreds of cities larger than London was in 1800.

Cities Big and Cities Sprawling
Take your class to the Change in U.S. Metropolitan Population and Land Area (1950-2050) chart from the Ecological Cities website. It includes a nice diagram showing the U.S. metropolitan population and land area for 1950, 2000, and 2050 (estimated).

Ask your students to read the chart and tell you how the percentage of the U.S. population that lives in cities has increased from 1950 to the present (Note: it went from 60 percent to 80 percent). This percentage is not expected to increase much in the next 50 years.

Now have the students describe how the land area that cities cover has increased over these years (Note: it increased from 9 percent to 19 percent). Cities are expected to occupy 35 percent of the land area in the U.S. by 2050.

This might be a good time to introduce your class to the idea of scale: The way in which things work may change with scale. Different aspects of nature change at different rates with changes in scale, and so the relationships among them change too. New, and sometimes surprising, kinds of phenomena can appear at extremely large or small scales. Systems that work well on one scale may work less well, or not at all, if greatly expanded or shrunk.

With urban ecosystems increasing so hugely in scale, will changes in how they function follow? What might some of these changes be?

In the 19th and early 20th centuries, urban growth was occurring mainly in the Northern Hemisphere, as a result of the spread of industrialization and the associated rapid increase in the use of fossil fuels. Industrialization involved the shift from muscle power to machines. New agricultural technologies that used machines instead of human and animal labor meant that fewer people were needed for food production, but it also meant that those who were not engaged in food production had to go elsewhere to find work. Many of them moved to cities.

The steam engine, and later the internal combustion engine, burned coal and oil and produced energy for many purposes. Burning these fuels, in turn, produced carbon dioxide. We can thus trace patterns of urbanization from patterns of carbon dioxide emissions.

Signals in the Air
The urbanization of the Northern Hemisphere is vividly illustrated by the NASA Goddard Institute of Space Studies website, which displays carbon dioxide emissions from fossil fuel burning by latitude. Have your class look up the latitude at which they live and find it on the image shown on the NASA website.

Today, some of the world's largest and fastest growing cities are emerging in the Southern Hemisphere because of urban-industrial development and as a consequence of rural decline. Is this growth mirrored by fossil fuel use?

Regional Changes in Urbanization
Take your students to World Petroleum Consumption, 1991-2000 on the U.S. Department of Energy website. This is a table of world petroleum consumption, by region and country, from 1990 through 1999.

Divide the class into seven teams (North America; Central and South America; Western Europe; Eastern Europe and the former USSR; Middle East; Africa; Far East and Oceania). Ask them to calculate the percentage increase of petroleum use for each region as well as the actual consumption increase (in thousands of barrels per day).

Which region had the greatest percentage growth in fossil fuel use? Which had the greatest actual consumption increase?

Within the U.S. there have also been changes in urbanization over time.

Urban Growth in the United States
This U.S. Census Bureau website provides detailed tables of the urban population of the U.S. from 1790 to 1990.

The table that ranks the population of the hundred largest urban areas by state from 1790 to 1990 shows some interesting trends. Have your students study the table and describe what they see in terms of regional growth and decline. One way to do this is to list the states that hosted the top ten cities in each year of the census, then look for trends.

Hint: New England had the largest cities early in our nation's history, growth of Midwestern and Western cities followed, more recently, the sunbelt has attracted a lot of people to its cities.

Discuss with the class what kinds of factors were important to these changes. Think about what kinds of employment were available in different places, how climate might have influenced changes, and what kind of role natural resources such as fresh water and forests might have played.

Cities do not grow "naturally" at all. They are intentional, human creations and many have arisen in what are environmentally questionable places (i.e. in flood or earthquake prone areas, or near sources of disease vectors). Human desires and human institutions—governments, corporations, boosters, etc.—are what drive urban growth.

Have the class try to locate booster literature from several cities (hint, try a Web search with the name of the city and "Chamber of Commerce." How do cities sell themselves as desirable places to live? Discuss with the class how efforts to sell cities may have changed over time. What kinds of features—housing, energy, climate, real estate, etc.—have been emphasized when and for what reasons? If you were going to try to boost the number of people settling in your area, what would you write about? (See an ad below.)



The Greater Southwest Boosterville Chamber of Commerce serves a 10-neighborhood area of businesses and residences that is unsurpassed in the Boosterville area in quality of living factors. With easy access to downtown as well as to the rest of Boosterville via major freeways, the southwest area is ideal for businesses. It is also home to major shopping districts and entertainment venues, including the new football arena and exposition center.

The history, charm, and neighborly ties of the communities put Southwest Boosterville in the top tier of residential areas anywhere. Southwest Boosterville reflects all the best of what has made Boosterville one of the largest cities in the United States. Updating of roads and infrastructure, new and redeveloping shopping and business centers, and the ongoing renovation—house by house—of older, close-in neighborhoods, are constants in our area. The areas served by the Chamber are perfectly positioned to offer many benefits for residential and business needs alike.

Cities are not the same all over the world. North American cities, for example, differ from European cities in shape and size, density of population, transportation networks, and the patterns in which people live and work within the city. The same contrast is true of cities in Africa, Latin America, and Asia. For example, in North American urban ecosystems, the suburbs are home for many of the wealthy people. In Latin America, the spatial pattern is reversed: the wealthiest people live close to the city centers.

In addition to allowing larger and more populous cities, fossil fuel powered transport also caused many cities to stop relying on resources from their local regions and to become dependent on an increasingly global hinterland. More about this later.


Ask the students to work in teams to develop a "letter to the grandkids"—that is, the people who will be sitting right here in this classroom in 50 or 70 years. The letter should explain, in plain English, what the students learned about global population trends as well as urban population trends. The letter should also tell the grandkids how important fossil fuels were to the growth of cities and how the signal of fossil fuel use showed up in the atmosphere.

Ask the students if the grandkids will be using fossil fuels in 2050 or 2070. If not, what might power the cities?

After completing this lesson, students should understand the importance of human population growth and, especially, fossil fuel use, to the development of urban ecosystems. They will know that what they see today, in terms of the size of cities and the proportion of the world population that lives in cities, is historically unprecedented. They will also have a better appreciation for how people may be attracted to live in particular cities.


Follow this lesson with the final two lessons in the series:

The Asia-Pacific Urban Transition:
As we enter the twenty-first century, Asia and the nations of the Pacific are emerging as major contributors to the global urban transition. The region presently contains three-fifths of the world's population and almost 45 percent of its urban population. Asia is the home to many of the world's fastest growing cities and, despite the recent economic crisis, to an increasing share of its economic activities.

Asia, however, is an extremely diverse region where generalization is difficult. Countries vary widely in size, population, and density. Some of the richest—and the poorest—countries in in the world are in Asia. Before the recent economic crisis, it was also the home to a group of nations that enjoyed the world's fastest economic growth rates while some of them, in South Asia in particular, remained amongst the poorest. Urbanization is the single most important force throughout the region. If the present rate of urban growth is to continue, Asia will need to double the size of its cities or their number.

(Reference: D. Weerapana. "Asia and the Pacific: Home to the World's Fastest Growing Cities")

Select an Asian-Pacific nation and, using the CIA’s World Factbook 2001, study it in comparison with a familiar urbanized nation of similar size or human population. What are the similarities? The differences?

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Lesson Details

Grades Themes Project 2061 Benchmarks National Science Standards