Diversity relations in Cape shrublands and other vegetation in the southeastern Cape, South Africa

Abstract

This paper investigates, and seeks explanations for, the diversity relations of Cape shrublands (fynbos and renosterveld), subtropical thicket and Afromontane forest, in the biogeographically complex SE Cape. Global comparisons of richness at the 0.1 hectare scale, of communities in the study area and elsewhere in South Africa with analogous vegetation on other continents, were largely inconclusive. Reasons for this are the unexplained variability of richness within vegetation types, problems associated with the scale of diversity used, and difficulties in defining analogous vegetation types. Diversity comparisons within the Cape Region and within the study area communities showed that alpha diversity of fynbos was not consistently higher than other vegetation types. In the study area highest richness was recorded in renosterveld and highest equitability in subtropical thicket; the most species-poor communities were Mountain Fynbos and Afromontane forest. The results of a correlation analysis showed that an index of phytochorological diversity was the factor most strongly correlated with richness in all vegetation types. Soil nutrients did not emerge as significant correlates of diversity except in fynbos where low levels of available nutrients were associated with low values of phytochorological diversity and low species richness. The diversity of fire-prone and grazed communities could be partly explained by non-equilibrium models of species diversity. Ecological and historical hypotheses were presented as explanations for the richness of communities having island-like distributions in the study area. It was generally concluded that historical and ecological factors should be given equal weight in descriptive studies which seek regional and global explanations of the evolution and maintenance of species diversity.