Historically, the "environmental movement" has focused on forests, reefs, rivers and other ecosystems that can and have existed separately from human beings.

But why not consider cities? If an ecosystem is considered a unit - a biological community together with its physical environment, then an urban ecosystem is, simply put, people in a city.

But urban ecosystems aren't simple. While humans are the dominant species that control the physical structure of the urban ecosystem, they are not the only species. And urban ecosystems aren't all natural. They are dominated by human-built environments with buildings and roads, along with water and power supply systems, built mostly on pavement and other hard surfaces that don't absorb rainfall.

Cities were built to meet human needs, often by controlling nature and sheltering humans from the natural world. Even the parks, open spaces, lakes, streams and other somewhat "natural" features in cities often have been changed significantly from their natural states. They have been landscaped, repopulated with non-native species, dredged, graded and reconfigured.

Urban ecosystems support commerce, culture, education, industry and science. They also generate both wealth, consumption of resources and waste. The effects on the environment are profound, both in urban areas and in surrounding ecosystems.

Urban ecosystems bring beauty to the city, but their importance goes well beyond aesthetics. They also protect public health and deliver "ecosystem services" that would otherwise need to be provided by expensive human-engineered systems. For example, according to a 1999 study by American Forests, Seattle's urban forest--our street trees as well as our forested parks, greenbelts and natural areas--provides $42 million worth of absorbed air pollution and filtered storm water per year!

The world, and the ecosystems it supports, is more fragile than we once thought. But we as individuals can take positive steps to preserve our urban ecosystems. Personal choices, however small, can influence your family, your neighbor, your book club, your co-workers and your government representatives. These choices can even influence the decisions of large corporations.

This guide will explore those relationships and how individual actions can make a difference.

NEXT: 101 Eco-Actions: Positive Steps Toward a Sustainable Urban Ecosystem