Mapping Grazing-Induced Degradation in a Semi-Arid Environment: A Rapid and Cost Effective Approach for Assessment and Monitoring |
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Authors: | Mark Thompson Jan Vlok Mathieu Rouget M T Hoffman Andrew Balmford R M Cowling |
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Institution: | (1) GeoTerraImage, P.O. Box 295, Persequor TechnoPark, Pretoria, 0020, South Africa;(2) Department of Botany, Nelson Mandela Metropolitan University, P.O. Box 77000, Port Elizabeth, 6031, South Africa;(3) Biodiversity Center, South African National Botanical Institute, Brumeria, Pretoria, 0001, South Africa;(4) Conservation Science Group, Department of Zoology, University of Cambridge, Cambridge, CB2 3EJ, UK;(5) Leslie Hill Institute for Plant Conservation, Department of Botany, University of Cape Town, Private Bag X3, Rondebosch, 7701, South Africa; |
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Abstract: | Improved techniques for measuring and monitoring the state of biodiversity are required for reporting on national obligations
to international and regional conservation institutions. Measuring the extent of grazing-related degradation in semi-arid
ecosystems has proved difficult. Here we present an accurate and cost-effective method for doing this, and apply it in a South
African semi-arid region that forms part of a globally significant biodiversity hotspot. We grouped structurally and functionally
similar vegetation units, which were expert-mapped at the 1:50,000 scale, into four habitat types, and developed habitat-specific
degradation models. We quantified degradation into three categories, using differences between dry and wet season values of
the Normalized Difference Vegetation Index (NDVI) for the three succulent karoo habitats, and the difference between maximum
and mean NDVI values for the subtropical thicket habitat. Field evaluation revealed an accuracy of 86%. Overall, degradation
was high: 24% of the study area was modeled as severely degraded, and only 9% as intact. Levels of degradation were highest
for bottomland habitats that were most exposed to grazing impacts. In sharp contrast to our methods, a widely used, broad-scale
and snapshot assessment of land cover in South Africa was only 33% accurate, and it considerably underestimated the extent
of severely degraded habitat in the study area. While our approach requires a multidisciplinary team, and in particular expert
knowledge on the characteristics and spatial delimitation of vegetation types, it is repeatable, rapid, and relatively inexpensive.
Consequently, it holds great promise for monitoring and evaluation programs in semi-arid ecosystems, in Africa, and beyond. |
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Keywords: | Semi-arid ecosystems Land cover Land degradation Monitoring Remote-sensing South Africa |
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