Natural ecosystems also feature a wide variety of species. Biodiversity is the consequence of many niches that can be filled. Each plant, animal, fungus or bacteria has its own range of habitats that it can live in.
When there are fewer species in the landscape a single pest can wipe out all the greenery. For example, when I grew up in Chicago the streets were lined with the most wonderful elm trees. The elms formed a green tunnel--a canopy that reached across every street. It was wonderful, shady, quiet, and green. Until Dutch Elm Disease came to the midwest. Then every single tree on the block was killed within a matter of a year or two.
I took this picture in Medicine Hat, Alberta last summer. These are elms lining a street, giving you an idea of what most midwestern towns had about 40 or 50 years ago. I guess the disease hasn't taken a foothold in the climate of southern Alberta, which is consistently bitterly cold in the winter. Perhaps the beetle that carries the fungus that kills the elms can't live in this climate.
There's been a lot in the news lately about Rochester, New York and its ability to survive and thrive after the bankruptcy of Eastman Kodak. Economic experts attribute Rochester's survival to its diverse economy. Though Kodak failed, there are many other sources of economic vitality that keep the city afloat.
In a similar way, diverse communities can survive social, architectural, or economic trends that might otherwise ruin them. Consider our very diverse community of Cambridgeport, Massachusetts, where houses that are almost two centuries old live side to side with contemporary housing projects. Ironically our neighborhood is threatened by upward economic trends (high real estate values) that threaten to chase out poorer residents, artists, the elderly, and families, all of whom contribute to the vitality and diversity of this neighborhood.
How can designers and planners provide flexibility in their designs that will encourage diversity and long-term survival in their communities?