Urban Ecology Seminar:
Technology Enhanced
Blended Learning

The City as an Ecosystem

    From: ECOLOGY, the link between the Natural and the Social Sciences.
    Eugene P. Odum, 1975, 244 pp, Holt Rinehart and Winston.

Now let us consider the ecology of the fuel-powered city within the same frame of reference as taken in our discussions of ponds, meadows, forests, and reefs. Referring back to Fig. 1, we note again that the city as the oyster reef, but not as the coral reef, is a heterotrophic ecosystem dependent on large inflows of energy from outside sources. Actually, most cities contain large numbers of trees, substantial areas of grass and shrubs, and in many cases, lakes and ponds - so they do have an autotrophic component or green belt. However, the organic production (converted solar energy) of the city green belt does not contribute appreciably to the support of people and machines that so densely populate the urban-industrial area. The urban forests and grasslands do have an enormous aesthetic value and they do contribute indirectly to pollution abatement by reducing noise, carbon dioxide, and other waste products of fuel consumption. But fuel and labor expended in watering , fertilizing, pruning, removing wood and leaves, and other work required to maintain the city's private and public green belts, adds to the energy (and money) cost of living in the city. It was already noted in our discussion of the basic kinds of ecosystems that sunlight is more of a liability than a benefit in the twentieth century city, but this situation may change when fuel becomes limited in supply or high price.

In Fig. 2, an American city is compared with a natural ecosystem of comparable size, namely a large lake. This figure is a combination pictorial and tabular model in three dimensions.

    A. structure (zonation or and use;
    B. population of organisms; and
    C. major inputs and outputs of energy and materials.

The hypothetical city has a population of one million people, a population density of 11.2 per acre, and a land-use pattern as shown in the upper left diagram. The latter two attributes represent the average statistics for seven American cities (New York, Chicago, Philadelphia, Los Angeles, Detroit, Cleveland, and Pittsburgh), as tabulated by Abrams (1965).

At a density of 11 per acre a city of a million would occupy about 36,000 hectares. The hypothetical 36,000 hectares lake is modeled to resemble a large, shallow natural lake of moderate fertility. The data in sections B and C of the model are expressed in terms of quantities per acre and per hectare (in parentheses), since total amounts for the large city and lake come to very large figures that would be more or less incomprehensible to the reader. However, to calculate totals all you have to do is multiply everything in sections B and C by 9x104.

Now let us compare the general layout of the city and lake. Both ecosystems tend to exhibit a concentric pattern of zones which, however, often overlap or interdigitate in a complex manner. The peripheral shallow water or shoreward zone of the lake, where light penetrates to the bottom, is known as the littoral zone; rooted water plants such as pond weeds, cattails, water lilies, and so on, are often found here. The outer zone of the classic city is generally residential with rooted plants also conspicuous (but, as indicated above, these are there more for show than for utility). The inner open water zone to the depth of light penetration effective for photosynthesis, is known as the limnetic zone. The littoral, together with the limnetic zone, make up the autotrophic zone of the lake. It is here that solar energy is converted to organic "fuels" that support the inhabitants of the lake (including any fish harvested by man or other animals). The rest of the lake, including the deeper waters and the large area of bottom that is beyond light penetration, is the profundal zone , and comprises the strictly heterotrophic part of the ecosystem.

The city generally has a distinct core of energy-consuming commercial development. However, the highest energy consumption occurs in the industrial areas which may be located in the city center, or in islands or bands in or around the city. A characteristic feature of cities is the network of transportation arteries which, together with the right-of-ways, take up a surprisingly large portion of the land area (20 percent, as shown in Fig. 2-7). Transportation of materials and people leads to intense and rather inefficient energy consumption in terms of incomplete combustion of fuel that leaves poisonous by-products in the air. Transportation energy use is a major cause of the air pollution that plagues all large cities in affluent countries. In contrast, transportation of materials in the lake is accomplished by waves and currents powered by the wind and the sun. Those organisms, such as fish, which do travel extensively, do so under their own power. Thus, there is little pollution resulting from circulation and transportation in the lake. A point to emphasize is that both population density and energy consumption density are very unevenly distributed in both the solar-powered lake and the fuel-powered city. Nevertheless, average figures do provide a useful overall comparison of the two.

Surprisingly enough the amount of life on a per-unit-area basis is not greatly different in the city and in the lake, as shown by the comparison of Fig. 2-7B. Fish outnumber people, but people outweigh fish by 10 to 1. When we consider the large number of pets, rats, birds, insects and so on that inhabit the city the biomass of animals is greater than in the lake; and there is a greater biomass of plants in the city even if we consider only the photosynthetically active green parts. Although 25 percent or so of the land area of a city may be completely devoid of plants the total cover of trees, shrubs, and grass approximates that of a natural terrestrial ecosystem or a rural area covered with a mixture of grassland and forest. In satellite photos of the earth's surface only the commercial and industrial areas of cities stand out; much of the sprawling urban area resemble, at first glance at least, the surrounding countryside. As already emphasized, it is the level and type of energy and material flow that makes the big difference between the fuel-powered city and the solar-powered natural ecosystem, and this can best be appreciated at ground level. Comparison of major inputs and outputs, as shown in Fig. 2-7C, will serve to bring out these differences.

The huge input of fuel energy required to support the dense population of machines, and to heat and cool the buildings and homes, has no counterpart in the lake system. The small amount of organic matter imported into the lake from the watershed might be considered in this category, but we have placed this in the food compartment in our graphic model. Remember that we are considering a lake with a natural watershed; urbanization ot the watershed can alter the energy and material budgets of the lake quite considerably, making it much more heterotrophic.

In Table 2-2 energy consumption density is estimated for large cities, industrial regions, whole countries, and the worlds as a whole. For our hypothetical model city (Fig. 2-7C) we have set the annual consumption level at 10 billlion kcal per acre (about 2.5x106/m2), which is about halfway between a concentrated city such as New York and a more spread-out city such as Los Angeles.

The per capita consumption rate for such a hypothetical city would be about 8.9 x 106 (1010 : 11.2) or something more than 10 times the national per capita average of 86 x 106, as cited on page 120. Consumption per acre in the city, on the other hand, is more than 1000 times that of the United States as a whole, which is about 1.8 x 10/m or 7.3 x 106/acre (see Table 2-2).

The reason for citing these large numbers is to emphasize that in terms of energy metabolism cities are pinpoint "hot spots" in the biosphere's surface. It is particularly important to note that although energy consumption in the largest cities exceeds the local solar energy input, mankind's burning of fuels is yet but a drop in the bucket compared to solar input on a global basis (see footnote, Table 2-2).

But remember that solar is low utility (that is, in terms of work capacity) and fuel is high utility energy. Possible effects of the increasing intense fuel consumption on local and global climates and heat balances will be discussed in the next chapter, as will the uncertain prospects for running cities on solar energy.

In addition to fuel, the city must import all its food, in contrast to the lake where most, if not all, food required to support the organisms is grown within the ecosystem. The estimated import of food, as shown in Fig. 2-7C, is greater than that required to support the 11.2 people and their pets that live on the urban acre, because much food is wasted.

What the rats and other scavengers do not get goes in the garbage that forms part of the solid waste output, an estimate of which is shown in Fig. 2-7C. Many cities in Europe are trying to convert this waste into fuel, soil mulch, or other energy-saving uses, and we can expect a similar effort in America soon.

The output of many acres of agricultural land is required to supply the city acre with food. As of 1973 about two acres area required to supply the rich diet of an American citizen, which means that some 22 acres of agro-ecosystems are required to feed the people inhabiting an acre of our hypothetical city.

As already noted (see especially Table 2-1), the agro-ecosystem is heavily "fuel-subsidized" so large amounts of fuel are consumed outside the city to produce the food, an energy requirement that does not show up directly in the city energy budget. Recent estimates of the calories of fuel required to produce a calorie of food produced by different crop systems is shown in Fig. 8-2. Although the use of land and fuel is much less lavish in most other parts of the world, many densely populated countries, such as Japan do not have enough food-producing areas to support the population, so food must be imported from other countries. This brings up a point that we will come back to: Densely populated, high energy-consuming areas require low-density, energy-producing areas of much larger extent to support them.

Thus, whether an area is judged to be over-populated or not depends not only on the socioeconomic consequence of crowding (that is, on population density, per se), but also on the capacity and availability of energy sources which may be located in distant regions.

The city's prodigious consumption of energy is coupled with large inflows of water and other materials, and large outflows of polluted water, solid wastes, air pollutants, and heat.

Energy consumption and material flows area inseparably linked; the larger the flow of energy into the city the larger the inflow of materials and outflow of wastes, but the relationship is not linear since utilization my be improved at intermediate volumes. Water, metals, and so on are absolutely essential to the conversion of fuel energy to useful goods and services. Without water, for example, a city would quickly choke to death no matter how much oil or other concentrated energy is available.

Likewise, wastes of degraded energy and end products of material fabrication are a thermodynamic inevitability of energy conversion.

Unfortunately, as long as energy and resources are abundant and available at low cost there is little economic incentive for conservation, so resource use tends to be wasteful, producing an additional increment of wastes above that which is inevitable. Therefore, the output quantities, as estimated for the current (1975 !) American city (Fig. 2-7C) could be reduced, and is reduced in other geographical areas where fuel is scarce. But there is a cost. For Example, to clean up and recycle waste water, substantial amounts of fuel energy and tax dollars would have to be diverted from other uses to do this work. It is cheaper to let nature's hydrologic and photosynthetic systems (both of which run on solar energy) do most of their work free, but this is feasible only if there are no other large cities upstream and downstream. Capacity for waste treatment can be increased by judicious disposal of wastes on land as well as in water areas.

Accordingly, the ecologic and economic budgets of a city are determined not only by energy consumption density and resource availability, but also by geographical location. A city located in a large matrix of seminatural environment is one thing; cities crowded back-to-back results in quite a different situation. We have already called attention to the fact that large cities area mostly located on free natural sewers (rivers, estuaries coastal locations).

The lake resembles the city in having a large "throughput" (that is input-output) of water, but water quality is not so rapidly degraded by the lake in sharp contrast to what happens to water as it passes through the city. There are evaporative losses of water from natural ecosystems which match the "consumption" of water (that is, water lost in transit) in the city, resulting in a lower output than input for both types of ecosystems. Although rainfall is a useful "subsidy" to the lake it becomes a costly nuisance in the city. As is the case with sunlight, rain is not only little used by the city but it causes expensive trouble in terms of storm sewer maintenance and flood damage. Only in the driest climates is runoff from roofs or other artificial catchbasins used for drinking purposes.

It usually is more convenient and cheaper for the city to get its water from a natural watershed.

To summarize, both the city and the lake require large watersheds, but, in addition, the city requires a large "food-shed" and "fuel-shed", that is to say, distant areas that supply energy (the ecological footprint ...).

The far greater rate of energy conversions in the city results in a waste output that stresses any kind of system (whether natural or manmade) that is located downstream or downwind. It is, as stated at the beginning of this chapter, the fuel-powered ecosystem can function only as a consumer (heterotroph) in the matrix of the solar-powered biosphere. Our challenge is to see that the city not only does not become a malignant parasite, but does become a more benevolent symbiont with its surroundings. We hope the data and examples presented in this chapter have convinced you that the fuel-powered city and solar-powered countryside area best viewed as, and in the future managed as, coupled systems. Unfortunately, present-day political and economic procedures area set up to deal with these two systems as if they were separate entities. Political conflicts between urban and rural seem to be an inherent pattern of human behavior even though there is no logic to it.


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