Diversity, gene flow and conservation of the serpentine flora on the Witwatersrand
Date
2011-03-28
Authors
Reddy, Renee Ansuria
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Abstract
Serpentine soils and their floras are extremely variable in soil composition, age,
elevation and climate and thus make generalizations difficult. The stressful and highly
selective nature of the serpentine habitat, a consequence of the interplay of physical,
chemical and biotic factors, defines the serpentine syndrome. Soils derived from
serpentinites (serpentine soils) usually have very high magnesium to calcium ratios and
high levels of toxic trace metals such as nickel and chromium. Regions containing
serpentine soils are often considered to be living laboratories where the processes of
evolution can be studied in situ because plants respond in various ways to elevated
levels of toxic metals and the lowered nutrient status of serpentine-rich soils and often
have characteristic vegetation growing on them, usually with few species or with stunted
plants. In some species ecotypic races have evolved and survive on these less than
optimum areas. Other species are endemic to these soils, yet others appear to be
indifferent to these toxic soils (bodenvag) and are found growing equally well on and off
serpentine soils. Botanists have been aware of Greenstone outcrops on the
Witwatersrand, but, until now, little work had been undertaken on the Witwatersrand.
The aim of this study was to gain an understanding of the relationship between the
serpentine flora and its associated soils, between the serpentine flora and the adjacent
non serpentine flora on the Witwatersrand, to investigate life histories, breeding biology
and gene flow patterns in selected bodenvag taxa at specific sites and thus observe any
possible processes involved in the formation of ecotypic races and to determine the
implications of the results of this study for the conservation of serpentine flora on the
Witwatersrand. This study found that the serpentine areas on the Witwatersrand were
not very toxic and did not support a unique flora and possessed no endemic taxa.
Species richness and diversity between the serpentine and non serpentine sites was also
not significantly different indicating that soil factors, such as the concentrations of
nickel, magnesium and iron, do not significantly influence diversity. Reproductive and
life history data did not separate the study taxa into ecotypes implying that the bodenvag
serpentine taxa investigated were either pre-adapted for the serpentine condition or,
because the serpentine conditions are similar to that of the Highveld grasslands, there
was no selection pressure for the formation of ecotypes. The AFLP results indicated that
gene flow occurred amongst the sub populations growing on and off serpentine soils and
that these sub populations were genetically similar. It can thus be concluded that other
factors, such as the harsh climatic conditions in the form of severe Highveld winters,
being part of a fire dependent grassland and occurring at a high elevation, exert a
stronger influence on these plants than does a relatively mild serpentine condition thus
resulting in a flora that is similar to the non serpentine flora on the Witwatersrand.
Currently three of the nine study sites receive some protection as municipal parks or
nature reserves and one as a conservancy; but none are nationally proclaimed parks, so
the future of these sites is not assured. However, if the ridge development policy
guidelines are strictly implemented, then the rocky serpentine outcrops may remain safe
from development and contribute towards the richness of the flora of the Witwatersrand
as a whole and act as corridors and refuges of local biodiversity.