Climate controls all living organisms. It is the chief factor determining the distribution of plants and animals on the surface of the earth. Extremes of temperature, progression of day lengths, and amount and distribution of precipitation are climatic factors controlling living organisms, particularly those that cannot (metabolically) control their own temperatures. In addition, the periodic onslaughts of extreme climatic conditions have pronounced effects on long-lived organisms, especially trees. The once-in-50-years cold spell that freezes otherwise tolerant species and the very occasional ice storm that completely debranches mature trees have the same kinds of control over the distribution and establishment of long-lived tree species as do the yearly extremes of weather. A drought that occurs once every 30 years is sufficient to eliminate a species that requires 60 drought-free years to reach maturity. All organisms have temperatures below which or above which they will not grow, or will die.

Man is no exception to this rule. He differs from most organisms in being able to regulate his body temperature metabolically (a characteristic he shares with other warm-blooded animals) and in being able to manage his immediate environment. (Garments, igloos, tents, houses, apartment buildings, and heating and cooling systems all function essentially to help man to regulate his body temperature.) A change of only a few degrees in a man's body temperature is sufficient to produce death. White oaks, sugar maples, purple violets, tulips, and hydrangeas have a critical temperature as well as minimum and maximum light and water requirements. All these factors affect the growth and development of individual species, and the distribution of these species on the surface of the earth. The city and the urban environment can be altered somewhat by man's regulation and management. Though man cannot completely change the climate, he can alter some factors of it—water, for example.

The regulation of water can be detrimental or beneficial. In paving streets, impermeable surfaces may block the flow of water to tree roots, irrigation systems can provide much needed water for turf and ornamentals during intervals between rain. The cost of irrigation in the city may be high, but it is more easily absorbed into the economics of the city than is irrigation of the surrounding hinterland.

Since the city is constructed of hard materials which can gain and lose heat more readily than surrounding natural vegetation, extremes of temperature may be greater in the city than in the countryside. The city weather report always differs from that of nearby farmland and the airport.

Many plants are sensitive to day length and control their flowering by the progression of day lengths. Street lights provide enough light to alter this process.

Man can engineer into the urban environment almost any factor he deems necessary to the enhancement of the growth and development of city vegetation.

Living organisms do not live alone. They exist in communities whose constitution and distribution are determined by the climate. Organisms that can share the various niches of the environment are most likely to form stable communities, and, in turn, may form a substrate in which organisms that would otherwise perish can survive. Biological organisms may passively exist together, they may enhance the growth of their neighbors, or they may metabolically destroy them through parasitism and disease. The principal biotic factor of the urban ecosystem is man, for man can alter the physical and biological attributes of the environment to a considerable degree. Man can enhance or man can destroy the community of organisms in which he lives, either by design or through thoughtlessness.

The ravages of chestnut blight have virtually eliminated the American chestnut. There is little or no prospect of overcoming the disease, because it is spread via airborne spores. Dutch elm disease and elm phloem necrosis, on the other hand, can be controlled, because the fungus and virus respectively causing these diseases are spread by insects and the insects can be controlled.

The plant organisms in the urban environment are prone to the same kind of biological attack as are the organisms of the agricultural environment. For when pure stands of plants are established, plant disease epidemics are especially destructive. Streetside plantings utilizing a goodly number of varieties or species can be good insurance against plant epidemics.