The individuals of the natural community are ecotypes. As such, they are interchangeable with any member of the same species, or with a member of another species having similar life form and climatic needs. The area will enjoy greater stability if it is occupied by a variety of individual organisms which require similar climatic conditions, but which have a broad spectrum of genetic characteristics.

If all the street plantings of the city come from the same source of seed or the same clone of plants, thus being as genetically uniform as possible, and are planted at the same time, the aspect might be one of horticultural neatness. However, all of the plants will have passed through all of the stages of their life development at the same time, and all will be susceptible to any disease, to any insect attack, or to any physiological disorder for which any one of them is susceptible. In other words, the range of stresses, climatic changes, insect or disease attacks that a genetically uniform population can withstand is considerably narrower than that which can be tolerated by a genetically broad-based community derived from different clones and seeds.

For example, trees grown from a single clone, propagated vegetatively, will have the same genes for resistance or susceptibility to, say, Dutch elm disease. If any one of the trees becomes infected, and if the insect vector is uniformly distributed (and it usually is in an American elm stand), the consequences will be serious. If those cuttings have been used to plant the whole city or a large area of it, the result will be catastrophic, as has been demonstrated in Columbus, Ohio, Champaign, Illinois, and elsewhere. On the other hand, if in addition to American elms, the area includes other species of elms or other forest trees, it is unlikely that any one disease could affect them all.

The city of Washington, D.C., loses some elm trees each year to Dutch elm disease, but its elm population comes from a great many seed and clone sources. As a result, Washington has never suffered a Dutch elm disease epiphytotic of the kind experienced by other sections of the country.

This same strategy that avoids monotypic specimens in street plantings should be applied to shrubs and herbs, to ornamentals, to turf (turf plantings are comprised of a mixture of seeds), and to every use man makes of plants in the city.