Urbanization and the Disruption of Matter and Energy Flows within Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 425

population concentration is characterized by urban areas occupying only 2.6% of the total
land use, but containing 79% of the total population (Lubowski et al. 2006).
The urban concentration of people, industry, commerce, and transport comes with envi-
ronmental costs. Cities are warmer, the soil is poorer, surface waters and aquifers are more
polluted, and ecosystems are under more stress. Although urbanized land is just one com-
ponent of the mixture of land uses present in urbanized watersheds, the effects or urban-
ization are often felt throughout the entire watershed. If sustainable urbanized watersheds
are the goal, it is therefore essential to understand the disruption of their matter and
energy flows induced by urbanization at the watershed scale. To achieve this understand-
ing, examples of significant human impacts within the four environmental spheres (bio-
sphere, hydrosphere, lithosphere, and atmosphere) are investigated.

12.4.1 Water Infrastructure and Ecosystems

Ecosystems enable organisms to exchange energy and matter between themselves and the
outside environment. Their basic structure consists of organisms arranged vertically by
trophic level into food chains and food webs (Figure 12.7).
These structures organize the intake of external solar energy, produce food, and return
energy in the form of respiration to the environment. At the bottom of the food chain are
the producers, organisms capable of photosynthesis. Matter is transferred upward through
the trophic levels of the food chain or food web by the addition of body mass. The arrows
between some of the organisms in the food web signify the interrelationships between
multiple organisms in an ecosystem. For example, without predators, the population of
certain species becomes uncontrolled, as was the case when rabbits were introduced in
Australia.
Cities take in energy from the outside environment, and mediate flows of matter
and energy within themselves and the surrounding areas. Not surprisingly, there is a
growing trend in the literature and professional planning practice to treat cities as eco-
systems (Newman and Jennings 2008). In urban areas with intensive land use, ecosys-
tems are affected by human impacts and the results are often observable. There is some
logic then to classifying the impacts humans make on ecosystems as a useful first step
toward understanding the depth and breadth of the specific problems confronting urban
watersheds.
Human impacts on ecosystems can be grouped into six major classes (Odum 1971):

• Reduction
• Fragmentation
• Substitution

Trophic
level

Food chain Food web

Fox
Snake
Rabbit
Seed

Hawk
Duck
Slug
Grass

Coyote
Crane
Carp
Algae

Coyote
Crane
Carp
Algae

Consumers

Matter and
energy transfers

Primary producer

FIGURE 12.7
Food chain and food web.