The impact of native large herbivores and fire on the vegetation dynamics in the Cape renosterveld shrublands of South Africa: insights from a six-yr field experiment
Renosterveld – a vegetation complex within the Fynbos Biome of South Africa – occurs in multiple structural states, including shrublands and various types of grasslands, lawns and herb-lands. Uncertainty exists over whether the present unpalatable shrubland state might have replaced a historical grassland state. Settled agriculture characterized by fixed burning cycles and overgrazing by livestock has been blamed for this putative change. However, the disappearance of native large herbivores has also been implicated. Understanding the drivers responsible for putative state changes is of importance for conservation of this critically endangered vegetation type.
Renosterveld shrublands and grasslands of the De Hoop Nature Reserve, Western Cape Province, South Africa.
Three structural states of renosterveld (grazing lawn, tussock grassland and shrubland) were subjected to an experiment involving fire (burned/unburned) and herbivory (grazed/ungrazed by native mesoherbivores). Using animal exclosures and experimental burning, we measured the responses of the biomass of major plant functional types over 6 yrs to test whether switches between states occur in order to discern the drivers of these changes. The data were analysed using permutational manova, Fisher's exact contingency tests and NMDS.
Herbivory (but not fire) had significant effects on lawn and tussock grassland biomass. Palatable shrubs proliferated in tussock grassland protected from herbivory. Unpalatable shrubs invaded burned lawn habitats exposed to herbivory. The vegetation composition of the shrubland state was affected by fire, herbivory and by the interaction between the two. Burned and grazed shrubland reverted to a dense canopy unpalatable state, while shrubland plots protected from grazing became palatable shrubland with a grass understorey. Palatable shrub species increased in the unburned and ungrazed shrubland plots, while the biomass of unpalatable shrubs declined in both the unburned grazed and exclosure plots.
We found no evidence supporting the hypothesis that the disappearance of native large herbivores led to the conversion of tussock grassland into unpalatable shrubland. Instead, the removal of native browsers caused palatable shrubs to dominate tussock grassland areas. Unpalatable shrubland is retained through a combination of fire and herbivory, but how this state established and persisted historically remains undetermined. Temporary relief from large herbivore grazing after unpalatable shrubland has been burned is recommended if a more open grassland state is the desired target.