Redundancy

 
A multiplicity of pathways to accomplish a function or satisfy a need allows a flexible response to changing conditions. Multiple pathways are redundant systems that guard against overall system failure. Each important function in a design should be supported by several elements. Basic needs such as water, food, energy and fire protection require backup systems. In cold climates with harsh weather, it may be necessary to provide heating through several systems -- passive and active solar, heat storage, and an efficiently fueled stove. Water on a site may be captured through a series of dams, tanks, swales and ponds. Likewise, each component of a design should provide more than one function serving to return food, fiber, fuel, income, waste treatment and other valuable services to the users. 

Multiple Elements for Each Function - An Example
The classic American example of non-application of the redundancy principle is the American Elm. It was the near exclusive choice as a street tree in thousands of communities in the Northeast and Midwest. The tree functioned to define the street edge, to provide human scale to neighborhoods, as a shade tree and as a cultural icon to small town American success. By investing so much importance in one element with no backup plantings, community images were devastated as the Dutch Elm disease swept across the country in near-eradication of the species. A diversified approach to street tree plantings would have maintained the functions even if the elms were killed; it might also have slowed the spread of the disease as the hosts would not have been as numerous and close together.

 
 
"This well-known, once abundant species, familiar on lawns and city streets, has been ravaged by the Dutch Elm disease, caused by a fungus introduced accidentally about 1930 and spread by European and native elm bark beetles." from: National Audubon Society Field Guide to North American Trees, Eastern Region. pg. 419.
Multiple Functions for Each Element - An Example
In an article, Permaculture Oasis, Julie Firth tells of visiting a garden in Palestine. She comments on the diversity of plants in a small area. She reports:

 

"The ground was covered with fronds and living mulch. The deciduous trees were orientated to the south. Everything in the garden was used, either useful. For example the dates from the palms are returned to the soil, eaten, or made into something useful. For example the dates from the palms are eaten fresh or dried, the fronds used as fences, sitting mats and baskets, while the palm itself was pumping out the soil's salt and providing top canopy shade for the garden below. A perfect example of a multiple functioning element". (Permaculture International Journal, #55, pg.16)
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