The factors of the urban ecosystem peculiar to man's activities are the engineering factors. Utilities, sewers, streets, walks, and buildings are usually located with the highest priority. Man's engineering capacity makes no task insurmountable, and the landscape can be arranged to suit. Soil is compacted, ground water is diverted, great areas of shade are created by large buildings, and rivers are relocated or encased in concrete. Most plants and animals are included in the urban ecosystem only after all other economic requirements have been satisfied: funds for plants have lower priority than those for engineering—a building and a street are functional without landscaping. In budget squeezes these items are usually reduced or eliminated. The plants of the urban ecosystem must be provided for under some provision other than construction or engineering funds. Plants—particularly trees—then must be selected for their adaptability to the respective niches of the community. This has worked out remarkably well in many cases—and very poorly in others (in many downtown areas to the complete exclusion of plants other than those in pots). The obvious conclusion is that the urban ecosystem can be arranged to provide adequate environmental circumstances for the plants—if they are wanted.

And like all other engineering contingencies, preparations for incorporating vegetation into the city must be made before the streets are paved and the foundations excavated, it is probably true that most of our oldest, most valuable street plantings date from a time before the streets were paved. Originating about 1910 or 1915, they grew to maturity without pavement over their roots and with abundant fertilizer from animal traffic.

If construction of facing buildings is carried out in such a way that maturation of streetside plantings is possible, living beauty can easily be designed into the city environment.

Once created, the urban ecosystem is managed and maintained by man. The choice of species in the plant communities of the ecosystem is not left to chance. Species that would otherwise not occur in the geographical area can be introduced. Species that otherwise would be quite rare can be propagated and used in large numbers. Species can be genetically improved. The latter is not a simple problem with species that may require 20 years to produce flowers, but great strides can be made by selecting seeds or cuttings from outstanding individuals of the species. The plant community of the urban ecosystem, like any other part of man's environment, can be designed.