干旱区生态环境知识资源中心

Nature reserves in Xeric Succulent Thicket of South Africa contain a greater diversity of wildlife and correspondingly a greater diversity of disturbance agents than adjacent, unconserved freehold and communal rangeland. Although more lightly stocked, it is unknown whether protected areas contain a higher diversity of landscape patches (i. e., sub- landscape features such as bush clumps, termitariums, bare patches or animal wallows) which could influence the reflectance value of a single pixel depicting a 20 x 20 m area in a SPOT satellite image, than unconserved land. Our key questions were: How does patch diversity in a nature reserve compare with that on commercial and communal rangeland? Can pixel diversity in a SPOT satellite image be used to quantify these differences? And, is there a correlation between reflectance diversity in a SPOT image and patch diversity on the ground? As a first step, the coefficients of variation (CV) for 10 groups of 12 picture- element (pixel) values of a dry season SPOT satellite image were calculated for two commercial farms and a communal rangeland. The same data were collected on a nature reserve, 50 to 100 m inside the boundary between the reserve and the freehold or communal rangeland. Next, we recorded the variety of 20 x 20 m plots on the ground, also in groups of 12 plots, at the same geographical coordinates as the satellite- based measurements. The means of the satellite-based and ground- based indeces were significantly and positively correlated. In addition, the nature reserve displayed significantly higher pixel CVs than the communal rangeland, and also contained significantly higher ground- based diversity indeces than the freehold, and possibly the communal, rangeland. We postulate that the higher landscape patchiness in the nature reserve is a result of the diversity of disturbances caused by wildlife (especially megaherbivores) coupled with naturally low stocking rates, while the lower diversity in the communally managed rangeland is the result of continuous heavy grazing coupled with intensive fuelwood harvesting. The satellite- based technique is useful for identifying potential sites of high biodiversity, wherein more intensive sampling at a finer scale can be undertaken. It is, however, important to use dry season imagery because of the temporary ’masking’ effect of ephemerals during the wet season.