Sustainability

Natural, semi-natural, and intensively managed dryland ecosystems of the world offer a wide range of different goods and services vital to human populations. The list is extensive and includes food production (humans and livestock), construction materials, climate regulation, soil maintenance, nutrient recycling, wildlife habitat, erosion control, tourism/recreation, and aesthetic enjoyment.7 When weighing the advantages and disadvantages of some particular course of action that affects these ecosystems—and hence their "sustainability"—decision-makers need quantitative assessments in order to consider these goods and services. While it is seldom possible (or desirable) to exercise complete control over a landscape, it may be possible to exercise different management regimes on parts of the landscape, and in so doing maintain a disproportionate set of ecosystem services. For example, some services (e.g., plant production) are highly dependent on "key" landscape units such as source zones for water and sedimentation, areas of reserve forage for herbivores, and fertile patch mosa-ics.8 Understanding the interplay between ecosystem services and ecosystem functioning and structure has urgent application to land use planning and management in dryland areas. These are examples of what constitutes the knowledge base necessary to achieve environmental sustainability.

Goodland and Daly9 identify three types of sustainability—social, economic and environmental. While there are obvious linkages and overlaps, Goodland and Daly argue that their true meanings are obvious only when considered separately (Table 6.2). In the context of global change and its potential impacts on drylands, it is inconceivable that one type of sustainability could be realized in the absence of the others since the interdepen-dencies are so strong. While the focus in this paper is on the environmental aspect of sustainability, we believe that its usefulness as a concept is limited without consideration of social and economic issues.

Table 6.1. Amounts (millions of hectares) and percentages of irrigated cropland, rainfed cropland, and rangeland degraded in the world

Irrigated cropland

Rainfed cropland

Rangelands

Continent

Total

Degraded

(%)

Total

Degraded

(%)

Total

Degraded

(%)

Africa

10.4

1.9

18%

79.8

48.9

61%

1,342.4

995.1

74%

Asia

92

31.81

35%

218.2

122.3

56%

1,571.2

1,187.6

76%

Australia

1.9

0.25

13%

42.2

14.3

34%

657.2

361.4

55%

Europe (Spain)

11.9

1.91

16%

22.1

11.9

54%

111.6

80.5

72%

North America

20.9

5.86

28%

74.2

11.6

16%

483.1

411.2

85%

South America

8.4

1.42

17%

21.4

6.6

31%

390.9

297.8

76%

Total

145.5

43.15

30%

457.7

215.6

47%

4,556.4

3,333.5

73%

The area most affected in any individual country is a result of a combination of factors, including population density, climate and land-use history. In the United States, desertification is best exemplified in rangelands of the arid and semi-arid southwest.2 In Latin America, the majority of degraded land is found in highland pastures and grasslands of Argentina, Bolivia, Peru, Ecuador, and Colombia, the central basin of Chile, the northeastern region of Brazil, and in the central plateau of Mexico.30 In Asia, China is the dominant country that must contend with desertification, and the greatest concentration of degraded land is found in the northwestern, northern and northeastern regions.31 Three distinct regions of Africa are at most risk: Mediterranean Africa, the Sudano-Sahelian

region, and the Kalahari-Namib region in southern Africa. Compiled from Grainger, Kassas, and Lopez-Ocaha.

Fig. 6.1. Conceptual model linking changes in ecosystem properties during desertification to changes in global biogeochemistry. Reprinted with permission from Schlesinger et al,28 ©American Association for the Advancement of Science.

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