Severely degraded rangeland: Implications for plant diversity from a case study in Succulent Karoo, South Africa

This study aimed to quantify and understand the impact of severe land degradation on plant diversity in part of the Succulent Karoo. Although total number of species declined with heavy grazing, greater evenness resulted in an increase in species diversity. Annuals and geophytes increased while protective nurse plants were decimated and their dependent beneficiary plant species consequently became locally extinct with heavy grazing. The rich biological soil crust dominated by mosses and liverworts with light grazing gave way to brittle physical-chemical or cyanobacterial crusts after heavy grazing. Although heavy grazing can result in extreme levels of degradation in the context of agricultural rangeland use, the implications for plant diversity per se need not be as severe. There appear to be disturbance-dependent indigenous species which may become locally extinct under good rangeland conditions. Our results support the notion of two different species pools, a grazing resistant pool and a grazing beneficiary pool, where changes in grazing intensity can cause substantial changes in composition and some local extinction of species with little or sometimes even positive change in diversity. Beta diversity across the disturbance contrast substantially raised the gamma diversity and hence landscape species diversity.