Best land-use strategies towards sustainable biodiversity and land degradation management in semi-arid western rangelands in southern Africa, with special reference to ants as bio-indicators
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In South Africa, the unsustainable use of natural resources by domestic livestock has led to resource depletion and serious land degradation. Rangeland degradation, especially bush encroachment and soil erosion, is particularly acute in the North-West Province, where all districts show signs of desertification and a loss of biodiversity resulting in a deterioration of human and animal health. This has a major impact on livestock productivity and the economic viability of livestock farming with serious consequences for the livelihoods of pastoral communities. It is important to recognise ecological change before irreversible changes occur. The aim of this study, which falls within the Global Environmental Facility Desert Margins Programme (GEF-DMP), was to investigate to what extent vegetation in combination with ant communities can be used as indicators of ecosystem change due to anthropogenic human induced land-use patterns and how can this information be used in land degradation management and biodiversity conservation in the semi-arid western rangelands of Southern Africa. Sites, representing a degradation gradient (relative poor and relative good rangeland condition extremes) within each of three Tribal-, three Commercial- and three Reserve areas, were surveyed. The impacts of these land uses on the herbaceous species composition, woody-, soil- and ant components were evaluated. Both the woody and herbaceous species components reflected the existence of a rangeland condition/degradation gradient across the larger study area. The herbaceous species composition reflected similar degradation tendencies within the Commercial and Reserve land uses, with sites being associated with low rangeland as well as high rangeland condition scores. The tendencies differed between these two land uses based on the woody degradation gradient. The entire Tribal herbaceous- and woody species components showed a transitional shift towards another state, which differed significantly from the Commercial and Reserve land uses. Both the Tribal herbaceous and woody components were associated with low to intermediate rangeland condition ranges, with no significant rangeland condition gradient existing within the Tribal land use. Understanding and quantification of the soil-vegetation dynamics hold important implications for rangeland degradation management. This study provided criteria for selecting the most appropriate measures when incorporating the soil parameters as additive data in the multivariate analyses with the vegetation, ant and nominal environmental data. Different land use practices resulted in different soil patterns, with significant gradients pertaining to the soil stratum and openness/woodiness groups. There was a significant though neglectable difference pertaining to the rangeland condition/degradation gradient based on the soil component. Ants have been extensively used as bio-indicators, also with regard to the monitoring of the environmental effects of rangeland pastoralism. Ant species compositional patterns and functional groups displayed congruent clustering and diversity patterns as those of the vegetation and soil components. In contrast to the vegetation components, ant assemblages did not reflect a degradation gradient, but rather reflected environmental changes (modifications) to the habitat structure and - heterogeneity as a result of different land use disturbances. Both vegetation and ant diversity measures were mainly associated with the Tribal land use. These diversity indices were indicators of habitat complexity, heterogeneity and moderate disturbance, rather than indicators of a rangeland condition/degradation gradient. The diversity patterns are best described by a dichotomy between the humped-shaped productivity/diversity and the habitat complexity/heterogeneity models. Vegetation and ant diversity measures for this study should be considered as environmental indicators of habitat disturbance rather than as biodiversity indicators. It is suggested that vegetation, soil and ant patterns are best described by the state-and-transition model, which encompasses both equilibrium and non-equilibrium systems. The resilient nature of these rangelands, typical of non-equilibrium systems, was reflected by the low to intermediate differences between land uses with regard to the herbaceous, woody, soil and ant components. However, densitydependent coupling of herbivores to key resources resulted in transitional shifts and modification of the vegetation composition and structure within and between land uses, displaying the equilibrium dynamics pertaining to these rangelands. Small disturbances in these rangelands may result in detrimental “snowball” interactive biotic-biotic /abiotic cascades. Spatial heterogeneous patterns within and between land uses as displayed by the vegetation, soil and ant parameters, necessitate that monitoring and management at patch, paddock and landscape scale should be conducted, cautioning against the extrapolation and over simplification of management strategies across all land uses. Because these arid rangelands are linked socio-ecological systems, it is not possible to address biophysical issues associated with land degradation without including the human dimensions. A “Key assessment matrix” is provided for monitoring and management purposes pertaining to land degradation and diversity aspects within and between the different land uses, and can be used by the land user, extension officer and scientist.
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