Maintaining biodiversity in the critically endangered lowland renosterveld vegetation of the Western Cape : the challenges of fire, herbivory and fragmentation

Abstract

Cape lowland renosterveld is critically endangered with > 80% transformed to agriculture and < 1% statutorily protected. Conservation efforts are complicated by severe fragmentation and the multiple vegetation states recognised in renosterveld. Historical accounts aggrevate the situation with notions that the area was formerly grassland rather than the unpalatable shrubland characterising it today. The mechanisms driving the multiple states were explored by subjecting three renosterveld states (shrubland, tussock grassland and grazing lawn) to combinations of experimental treatments comprising fire (burnt/unburnt) and herbivore grazing pressure (utilized/unutilized by ostrich, bontebok and eland). Plant biomass was recorded in the experimental plots both before treatments were applied and again six years later. Results reveal that herbivore grazing pressure but not fire significantly affects lawn and tussock grassland vegetation composition. Unpalatable shrub invaded grazed lawn areas while ungrazed tussock grassland was encroached by palatable shrub. The shrubland state was affected by fire, herbivore grazing pressure and the interaction between the two. Burnt and grazed shrubland reverted back to its dense unpalatable state while ungrazed plots became open grassland. Unburnt and ungrazed shrubland plots were encroached by palatable shrub species. More insight on the ecosystem processes driving renosterveld vegetation state dynamics was gained but it does not inform to which state conservation efforts should aspire. A heterogeneous landscape representing all states is unrealistic given that the majority of remaining patches are < 1 ha. A shrubland protected from fire and herbivore grazing pressure appears to be the most species rich but is neither sustainable nor acceptable to farmers using the remaining renosterveld patches as grazing.