Figure 2. Caption to come when figure is finalized. Provisioning services

The supply of water from catchment areas, often located just beyond or even within city boundaries, is a good example of a localized provisioning service. The conservation of wetlands (including rivers) and their biodiversity enables natural reservoirs or channels to store and provide water. The management of habitats on Mount Kenya, for example, is estimated to save the Kenyan economy more than US$ 20 million a year by protecting the water catchment area of two of Kenya’s main river systems and ensuring a regular supply of water. Another example of a provisioning service is that of urban and peri-urban agriculture, which can augment food security and generate income for vulnerable urban households.

Ecosystems regulate not only the supply but also the quality of water, air, and soil. Urban parks and vegetation reduce the urban heat island effect. There is additional potential for lowering urban temperatures when building envelopes are covered with vegetation such as green roofs and green walls. Data from Manchester, UK, show that a 10 percent increase in tree canopy cover may result in a 3–4°C decrease in ambient temperature and save large amounts of energy used in air conditioning. Urban green spaces can contribute to climate regulation by reflecting and absorbing solar radiation, filtering dust, storing CO₂, serving as windbreaks, improving air quality (by oxygen emission and moistening), and enhancing cooling by evaporation, shading, and the generation of air convection.

Serving as habitats for species and as storehouses for genetic biodiversity are examples of supporting services that ecosystems offer. Supporting services make other ecosystem services possible—and they can be surprisingly prolific in cities.

Biodiversity in cities exposes people to nature and thereby facilitates an appreciation of nature. It also provides opportunities for recreation, health and relaxation, and community cohesion. Green-area accessibility has been linked to reduced mortality and improved perceived and actual general health. It has been shown that the psychological benefits of green space increase with biodiversity, and that a “green view” from a window increases job satisfaction and reduces job stress. This can have a strongly positive effect on economic productivity and hence regional prosperity. The distribution and accessibility of green space to different socioeconomic groups, however, often reveals large inequities in cities, contributing to inequity in both physical and mental health among socioeconomic groups. Several studies have shown that property values (as measured by hedonic pricing) increase with greater proximity to green areas.

Impacts on natural ecosystems that are due to urban demands can have unforeseen effects on the health and well-being of city-dwellers. Understanding how ecosystems deliver services, who benefits from them, and what happens when an ecosystem changes can provide a key framework for developing sustainable cities within a wider landscape context.

The rate and ways in which the planet urbanizes varies across the regions, and for each specific country and region. Over the next pages, some general and actionable trends are highlighted for the main UN regions.

1. 
   Africa