Despite the hostile environment, animal and plant life manage to develop in cities. It occupies parks or fallow lands; not only rats and cockroaches proliferate, but also foxes and birds of prey. In certain neighborhoods of Zurich, for example, the density of foxes is 10 times higher than in the countryside! If for some, there is no nature in the city, the city being by definition the opposite of nature, there is however a very particular form of biodiversity.
The urban ecosystem distinguishes itself from natural eco-systems due to the creation of artificial environments that change, amongst others, the climate (bursts of heat, less powerful winds) and disturbs the water cycle (water runoff,…) Air pollution directly affects plants; noise, vibrations, lack of light and space underground harm the development of trees.
Maybe expectedly, studies show that salt spread about in order to melt snow is the first enemy of vegetation in cities. Indeed, salt penetrates the plant and prevents it then from absorbing water and minerals that it needs. Therefore, in Paris, during the great snow-clearing of 1986-1987, nearly 3,000 plane trees died because of salt!
Cities are often polluted environments. Transportation and industries pollute the air. The air we breathe in the subway and RER corridors is sometimes more polluted than the air outside. The high advisory board of public health of France reported as well in 2000 high concentrations of particles, formed for the most part by metal dust generated by the train traffic (see air pollution sheet)
Deposits from urban air pollution affects soil (heavy metals, dioxides, acids….). In cities, the grounds of certain industrial fallow lands are highly polluted. Urban renewal of these fallow lands can then present a danger to the health if a clean-up has not been completed prior to that.
Insufficient treatment of used water, industrial or domestic, pollutes water supplies. Depending on the surface on which it falls and that it drains through, rain water can also become filled with toxic pollutants (hydrocarbons on parking lots for example). Finally, weeding done by individuals or groups of people to beautify their parks is another source of water pollution (see water pollution).
In cities, the impermeabilisation of the ground changes the natural flow of water. Soil can no longer absorb it, which can cause, in cases of strong precipitation, flooding, sometimes devastating. These can be a source of pollution when waste water overflows. Finally, cities produce significant quantities of waste, which must be stocked or treated appropriately. (see waste sheet)
Sound and light pollution
Two types of disturbances need to be added to this list. The first is sound pollution, which has numerous sources (construction and building sites, air and road traffic, mobile phones…). Its health consequences are varied: hypertension, insomnia, depression, hearing problems, and even irritability.
The second is light pollution—a notion that emerged in the 1980s. It designates the abnormal presence of light in a night environment. Light pollution has a negative impact on certain species (nocturnal insects and birds for example) and ecosystems.
Stresses on the rural areas
Physically, the « expanded » city is a factor of environmental degradation [see urbanisation sheet]. It devours agricultural and natural land. Cities are also big consumers of resources and supplying it affects rural areas.
Certainly, there are needs for energy. In third world countries, the biomass supplies between 25 and 90% of domestic energy supplies, above all in small urban centers. It often involves wood, which increases deforestation. Energy consumption in cities follow different evolutions: in third world countries, energy consumption per inhabitant increases, because urban households buy more home appliances. In developing countries, energy consumption per inhabitant (much higher than what it is in the South) tends to decrease, mainly due to cost savings that are characteristic of metropolitan areas.
Many metropolitan areas, even those that have very large water reserves, are faced with more and more acute shortages. In Mexico, for example, urban growth and the increase in needs force cities to still go look for water further away. Actually, Mexico must go obtain water more than 150 km away from the capital and must dig more than 200m deep!
Regarding food, the concept of food kilometers reflects the way in which food supply trade, particularly with big distributions, depends on transportation. In Germany, for example, a strawberry yogurt must travel 9,115km from to arrival from its manufacturer to the consumer. An alternative is thus to encourage local consumption. However, such a rationale endangers production in third world countries that are finding opportunities in the Western markets.