Saguaro

The giant saguaro cactus is native to the desert near Tucson, Ariz., with an annual rainfall of only about 10 inches and lengthy spring and autumn rainless seasons, it has adapted to the harsh environment with water-storage devices and a low transpiration rate, but seems to be highly susceptible to air pollution from the city.

Elm

The American elm grows on the streets of Eastern seaboard cities from St. Johns, Newfoundland, to Savannah, Ga. (at the same latitude as Tucson). Air pollution is high in many of these cities, but rainfall is about five times as great as at Tucson and is more evenly distributed throughout the year.

The urban community contains not only plants and animals that man desires but also many that he would like to eliminate. Through management, circumstances may be created where unwanted organisms, even though rare, can flourish because they share the optimum conditions of environment with more desirable species. A good example is the dandelion, which thrives in the environment of a city lawn—where it is not wanted. The pigeon, English sparrow, and starling could be cited as "weed" species of animals; the Norway rat is a pestilence in itself, causing economic loss and great costs in human health wherever it has migrated and taken hold. These organisms exist in an environment created by man, and the need to control them has posed the most refractory of biological problems.

Why have songbirds left the cities? Is it because of competition with the English sparrow and starling, or is it that the overall quality of the city has changed and only the English sparrow and starling have been able to survive the change? Man introduced dogs and cats to the city and has essentially removed the horse.

Agricultural activities have largely been banned from cities, which formerly had thriving businesses in cattle, swine, and chickens, with many householders keeping animals. The ban is mainly for reasons of health but is enforceable only where adequate facilities permit storage of freshly killed meat.

Aedes egypti, the yellow fever mosquito, breeds exclusively in containers, such as tin cans, made by man. The Aedes egypti control program entails keeping the area free of such manmade containers. And of course the rat problem seems to be a simple relation between man's handling of garbage and the rat itself.

For better or worse, man has created an environment in the cities that may be favorable for many species of plants and animals detrimental to his well-being, health, and comfort, and he has created environmental circumstances where desirable plants and animals have been driven away. What is obvious is that man controls the biological relations of the urban environment. How he exercises that control will be largely dependent upon his understanding of the urban ecosystem itself.