The Hantam-Tanqua-Roggeveld subregion is part of the Succulent Karoo
hotspot of biodiversity which stretches along the southwestern side of South Africa and
Namibia. Forty Whittaker plots were surveyed in the spring of 2005, in eight vegetation
associations, to gather diversity data for the Hantam, Tanqua Karoo and Roggeveld areas.
Seven plot sizes were used to construct species–area curves using three different models
namely: the untransformed linear function, the power function and the exponential function.
In general, the power and exponential functions produced a more significant fit to the
data than the untransformed linear function. Linear regressions using environmental
parameters indicated that altitude, mean annual precipitation and mean annual temperature
were significant predictors of species richness at the 1, 10, 100 and 1000 m2 scales. To
illustrate the variation in species–area curves and species richness across the landscape, a
transect through the study area is discussed. The transect stretches eastwards from the
Tanqua Karoo across the escarpment into the Roggeveld and crosses five different vegetation
associations. Differences between associations were found in species richness in the
1000 m2 plots. Each association also produced species–area curves with their own characteristics.
Slope values for the samples within an association did not differ significantly,
although the intercept value often did. Comparisons between associations along the transect
revealed significant differences in the slope value between the associations, except for
the Dicerothamnus rhinocerotis Mountain Renosterveld which did not differ significantly
from the associations bordering it on either side.