Management of critically endangered renosterveld fragments in the Overberg, South Africa
Lowland renosterveld is a relatively fertile, shale-derived vegetation type found within the FynbosBiome of South Africa, a biome which is recognized as one of the world’s smallest, yet richest plant kingdoms. Due to the fact that renosterveld tends to be dominated by a handful of small-leaved asteraceous shrub species, it creates the illusion that it is a homogeneous habitat with low levels of alpha diversity and species turnover. This is exacerbated by the widely accepted, although not proven, hypothesis which states that current-day renosterveld is derived from a pristine C4 grassland and that the dominance of asteraceous shrubs is solely due to more than 300 years of mismanagement in the form of overgrazing. This process of degradation is believed to have started with the arrival of the European settlers who exterminated the large herds of free-roaming ungulates and replaced them with livestock (sheep and cattle), which, through their selective feeding habits and their sedentary manner of grazing, altered the system from a grassland to a shrubland. The debate over what renosterveld actually is, combined with a dearth of knowledge as to its ecological functioning, has meant that management recommendations are largely based on a combination of guess-work and inferences from studies in adjacent and similar fynbos and karoo habitats. Additionally, renosterveld has been severely transformed for agricultural development, rendering it a Critically Endangered vegetation type, with 4- 10 % of the original extent remaining.In this thesis, I focus on lowland renosterveld in the Overberg (also referred to as South Coast Renosterveld), Western Cape, South Africa. I investigate, through the use of soil carbon-isotope analyses, the grassy-shrubland vs. shrubby grassland debate and assess whether or not the theory that renosterveld is merely a degraded grassland has merit. I evaluate diversity levels at the alpha, beta and gamma scales and contrast these with comparable Mediterranean-climate vegetation types, while considering the implications for conservation planning for renosterveld in the Overberg. Through the use of a simple model, I investigate whether it is possible to predict species extinction rates at the local (patch) level. I then assess responses to grazing and fire, through a combination of controlled experiments and random surveys, in order to assess management requirements.I found that South Coast Renosterveld was more-than-likely always a grassy-shrubland, and that although data suggest slightly higher C4 inputs historically, renosterveld was never a pure C4 grassland. Alpha diversity was on a par, if not higher, than that of any other studied fynbos habitats and is comparable, if not richer, than its other Mediterranean-climate shrubland counterparts. Similarly, species turnover across habitat and landscape gradients was high, suggesting that multiple renosterveld as a functioning ecological entity at the landscape level.
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