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This report focuses on areas of low and undependable precipitation within the tropics and subtropics. (1) Much of the area is occupied by savannahs and thorn-bushlands, often characterized by a rich diversity of grasses. The prominence of grasses in tropical rangelands in many instances reflects the repeated use of fire in hunting or range renewal (Sauer, 1952), as well as the coevolution of grasses and wild herbivores (Harris, 1969). Substantial tracts of forest are associated with tropical rangelands in some regions; in others, extensive swamps created by the seasonal overbank flooding of exotic rivers are features of considerable regional importance.

Tropical rangelands differ greatly from rangelands in temperate regions, and social adaptations to these differences are reflected in the management of range resources. Differences of climate (Trewartha, 1954), soils (Sanchez, 1975), vegetation (Davy, 1938; French, 1957), and other environmental factors are well documented and generally well understood. The management of tropical rangelands is further affected by the prevalence of livestock diseases. Rinderpest, foot-and-mouth disease, contagious bovine pleuro-pneumonia, anthrax, east-coast fever, trypanosomiasis, and sheep pox have historically taken heavy tolls in the tropics (Pratt and Gwynne, 1977). Strategies to blunt the impact of disease include increasing livestock holdings to levels that assure the survival of a breeding nucleus. The relatively high levels of social, economic, and political differentiation within the tropics similarly affect the exploitation and management of range resources.

Range classification

Range management issues are usefully considered within the context of ecoclimatic zones. In this report, these zones are defined largely on the basis of land potential and moisture availability (Pratt and Gwynne, 1977). Within the tropics, five such zones can be distinguished:

1. Humid to dry subhumid (moisture index not less than -10).(2) This zone is characterized by forest and derived grasslands and bushlands, with or without natural glades. The grestest potential is for forestry (perhaps combined with wildlife and tourism), or intensive agriculture. The natural grasslands of this zone require intensive management for optimum production. Approximately 0.8 hectare is required per livestock unit, depending upon the related grassland association. (3) In this zone, approximately 2.5 livestock units are required to support one subsistence pastoralist; hence, 2 hectares are required to support each individual. The maximum population density per km² is about 50 pastoralists (see table 1-1).
2. Dry subhumid to semiarid (moisture index -10 to -80). The vegetation of this zone includes moist woodland, bushland, and savanna. Forestry potential is low. However, the agricultural potential is relatively high, soils and topography permitting, with emphasis on lea farming. Large areas are generally under range use and, with intensive management, can carry 1 livestock unit per 1.6 hectares. Approximately 3 livestock units are required to support 1 subsistence pastoralist. Thus, 4.8 hectares are required to support 1 individual. The maximum density of pastoralists would be approximately 21 per km². Regular burning is an important management tool in this zone.
3. Semiarid (moisture index -30 to -42). These are areas with marginal agricultural potential, which in some regions is limited to rapidly maturing grains. The natural vegetation is characteristically dry woodland and savanna. This is potentially productive rangeland. Approximately 3.5 hectares are required per livestock unit, except where dry seasons exceed 6 months. The corresponding human carrying capability is 7 individuals per km². Animal husbandry is limited principally by the encroachment of woody vegetation and, in some locations, by leached soils. In many areas, particularly in Africa, the more open country with a high density of wildlife is a valuable tourist attraction.

TABLE 1-1 Relationship between Ecological Zone, Livestock Carrying Capacity, and Maximum Population Density under Subsistence Pastoralism

TABLE 1-1 Relationship between Ecological Zone, Livestock Carrying Capacity, and Maximum Population Density under Subsistence Pastoralism

^a These figures presume that all land is accessible and productive; if actual population density under subsistence pastoralism even approaches these estimates, overpopulation is indicated. Higher population can only be sustained if the pastoralists derive a substantial part of their subsistence from vegetable foods--collected, grown, or procured in exchange for livestock.

SOURCE: Modified after Pratt, 1968.

4. Arid (moisture index -42 to -51). This zone is suitable for agriculture only where fertile soils coincide with a favorable distribution of precipitation, or where rainwater is concentrated in depressions. Many arid rangelands are dominated by species of Acacia or Prosopis. Perennial grasses, such as Cenchrus ciliaris, can be prominent, but succumb quickly to inadequate management. As many as 12 hectares may be required per livestock unit. Wildlife is important, particularly where dry thorn-bushland predominates. Burning requires caution but can be highly effective in range manipulation. Approximately 4 livestock units are required to support 1 subsistence pastoralist, and the maximum population density per km² is 2 individuals.
5. Very arid (moisture index -51 to -57). This zone supports rangeland with relatively low potential. The characteristic vegetation is shrub or grass steppe, with trees largely confined to water courses and seasonally inundated depressions. Perennial grasses, once dominant in many areas, are now localized within a predominantly annual grassland. Growth is confined largely to the seasonal flushes characteristic of summer therophyte vegetative communities, and grazing systems are generally based on pastoralism. Populations of both wild and domesticated animals are restricted by temperature, forage, and available moisture (Schmidt-Nielsen, 1964).

Systems of range classification should be regionally adjusted to include descriptions of the existing vegetation in physiognomic terms, with subdivisions by species composition.

Social system-ecosystem interactions

Most environmental systems are highly modified by human activity. Hence, an understanding of the biological and use potential of these systems benefits greatly from analyses of environmental change over time (National Research Council, 1981). Such analysis is also important in defining ecosystems and in identifying cause-effect relationships that have contributed to changes in the composition and productivity of these systems.

Indigenous social systems, through selection and adaptation, are functionally associated with local ecosystems through flows of energy, material, and information (4) (Rambo and Sajise, 1984). Changes in either the social or environmental system result in changes in the other. Hence, each system must be thoroughly understood if positive change is to be realized. In many, perhaps most, instances, highly disruptive changes are responses to external stimuli. Many examples could be cited. For example, the highly regulated land-use systems of many societies (see the discussion of the hema system in case study 9, Part II) were commonly transformed into open-access systems through the imposition of European public-domain law often combined with land expropriation, a situation that, in many regions, has led to intense use pressure and severe environmental degradation. Similarly, colonial era introductions of cattle into inappropriate areas (such as Zone 5 of the above classificatory system) has led to severe degradation and zonal compression (National Research Council, 1983b). The fixing of boundaries, at national and sub-national levels, has reduced or eliminated strategies of mobility that are crucial to these areas. In addition, increasing market integration has converted highly conservative systems of land use into opportunistic systems that impose greater pressure on available resources. In some cases, this has destroyed the subsistence base that supported the coping strategies of local populations, and has reduced the range of economic options available to them. Wildlife, honey and beeswax, gums and resins, cordage, tannin, and medicinals are among the economic products lost through the de gradation of environmental systems in Africa and Asia.

Characteristically more subtle, but equally important, impacts on socioeconomic and environmental systems result from destructive modifications of indigenous systems of values, ideology, knowledge, and social organization. An unfortunate consequence of past efforts in international development is that so much attention was directed toward the transformation of what are now belatedly recognized to be critically important social adaptations, without corresponding effort being made to understand the context or consequences of the changes promoted.

In addressing issues of range management in the tropics and subtropics, many of the most important clues as to appropriate actions for governments and development agencies reside in the analysis of traditional adaptations to local environmental systems. Growing awareness of the importance of traditional adaptations is contributing to a shift of emphasis by governments and development agencies from open-field cultivation and plantation forestry to more biologically complex agroforestry or agro-sylvo-pastoral systems (National Research Council, 1983a). The growing interest in camel husbandry in the drylands of Africa and Asia similarly reflects pre-colonial strategies of rangeland utilization. In West Africa, for example, camel-based livestock systems were commonly replaced by cattlebased systems by colonial administrators unfamiliar with the characteristics of the drylands of West Africa in relation to the requirements of cattle. By so doing, these administrators contributed greatly to the current environ mental emergency in Africa ( National Research Council, 1983a). An overview of selected African and Asian pastoral adaptations is contained in Douglas Johnson's The Nature of Nomadism (1969).

NOTES

1. In this report, the terms "tropics" and “tropical” are expanded to include the subtropics (Tropical and Subtropical Steppe, Tropical and Subtropical Desert, Mediterranean or Dry Summer Subtropical, and Humid Subtropical climatic regions) as well.
2. Moisture indexes provide expressions of climate derived from monthly rainfafl and evaporation, with the estimate of evaporation based upon measures of radiation, temperature, saturation deficit, and wind speed, weighted for altitude and latitude. They are calculated on the basis of Thornthwaite's concept of moisture indexes (1948), combined with Penman's estimate of evaporation (1948) .
3. In many areas of the tropics, a livestock unit is taken to be a mature zebu cow with calf at Soot (averaging about 300 kg liveweight and having a daily dry matter requirement of 6.5 to 8.5 kg).
4. In an ecological context, information is simply organized or patterned energy or material that tells the observer something about the past, present, or probable future state of an ecosystem or its components. Human response to environmental information is unique compared with that of other organisms because it occurs largely at the cognitive level where cultural conditioning affects both perception and the selection of appropriate responses.

References

Davy, J. B. 1938. The classification of tropical woody vegetation types. Papers of the Commonwealth Forestry Institute 13:1-85.

French, M. H. 1957. Nutritional value of tropical grasses and fodders. Herbal Abstracts 27:1-9.

Harris, D. R. 1969. Agricultural systems, ecosystems and the origins of agriculture. In The Dome&tication and Exploitation of Plants and Animals. P. J. Ucko and G. W. Dimbleby, eds. Aldine-Atherton, Chicago, Illinois, USA.

Johnson, D. L. 1969. THe Nature of Nomadism: A Comparative Study of Pastoral Migrations in Southwestern Asia and Northern Africa.

Department of Geography Research Paper No. 118, University of Chicago, Chicago, Illinois, USA.

National Research Council. 1981. Environmental Degradation in Mauritania. Board on Science and Technology for International Development. National Academy Press, Washington D.C., USA.

National Research Council. 1983a. Agroforestry in the West African Sahel Board on Science and Technology for International Development. National Academy Press, Washington D.C., USA.

National Research Council. 1983b. Environmental Change in the West African Sahel. National Academy Press, Washington D.C., USA.

Penman, H. L. 1948. Natural evaporation from open water, bare soil and grass. Proceedings of the Royal Society. Series A, 193:120-145.

Pratt, D. J. 1968. Rangeland development in Kenya. Annals of Arid Zone 7:177208.

Pratt, D. J. and M. D. Gwynne, eds. 1977. Rangeland Management and Ecology in East Africa Hodder and Stoughton, London, England.

Rambo, A. T. and P. E. Sajise. 1984. An Introduction to Human Ecology Research on Agricultural Systems in Southeast Asia. University of the Philippines at Los Banos, College, Laguna, Philippines.

Sanchez, P. A. 1975. Properties and Management of Soils in the Tropics. John Wiley and Sons, New York, New York, USA.

Sauer, C. 0.1952. Agricultural Origins and Dispersals. Bowman Memorial Lectures, ser. 2. American Geographical Society, New York, New York, USA.

Schmidt-Nielsen, K. 1964. Desert Animals: Physiological Problems of Heat and Water. Oxford University Press, Oxford, England.

Thornthwaite, C. W. 1948. An approach toward a rational classification of climate. Geographical Review 38:55-94.

Trewartha, G. T. 1954. An Introduction to Climate. 3rd Ed. McGrawHill, New York, New York, USA.