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A recurrent theme emerging from recent studies is that energy either has a primary role in generating spatial
variation in species richness or is an important modulating factor (Currie 1991; O’Brien 1998 Chown and Gaston 1999; Gaston 2000; Morin 2000). Studies have shown not only that energy is strongly correlated with species richness (Currie 1991; O’Brien J998) but also that there is a clear set of mechanisms that can account for the way in which changes in energy availability translate to alterations in numbers of individuals and the way these, in turn, are partitioned between species (O’Brien 1998; Kerr et al. 1998; Chown and Gaston J999; Kerr and Currie 1999; Currie et al. 1999; Kaspari et al. 2000a, 2000b). However, many scientists believe that several other factors may be responsible for some component – and perhaps in a large way – of spatial variation in species richness (Rosenzweig 1995). Of particular interest is the idea that at the highest energy levels energy loses its grip on species richness and other factors become more significant (Kerr and Packer 1997; Chown and Gaston 1999). At least in some terrestrial systems, habitat heterogeneity is thought to account for the remaining variation (Kerr and Packer 1997).
That habitat heterogeneity is likely to explain some proportion of the variance in species richness is not surpri3ing. Thl! literalure is replerc with studies showing that species richness is correlated with habitat heterogeneity and complexity at local, regional and continental scales (e.g., MacArthur 1964; Verner and Larson 1989; O’Connor et al. 1996; Wiebe and Martin 1998; Ricklefs and Lovette 1999; Boone and Krohn 2000b). Moreover, many studies have provided insight into the way heterogeneity might cause changes in species richness by infl uencing the presence or abundance (Verboom et al. 199 J; Vi liard et al. 1995), movements (Wegner and Merriam 1979; Machtans et al. 1996), and persistence (Hanski et al. 1994) of species.
Nonetheless, few investigations have sought to determine simultaneously the relative influences of energy availability and habitat heterogeneity on species richness. This is particularly important because energy availability is thought to have a direct effect on habitat heterogeneity (Wylie and Currie 1993; Waide et al. 1999; Morin 2000), which in turn has an effect on species richness that increases in importance from the local to regional scales (Wright et al. 1993). At least one regional-scale study has suggested that the extent to whi ch habitat heterogeneity serves as a correlate of species richness is also dependent on the spatial grain of the study, increasing in importance with a decline in spatial resolution (Fraser 1998). This may occur for both statistical and biological reasons. A decline in spatial resolution means larger sampling units, which in turn incorporate greater climatic, and hence habitat variability. Thus, fewer sampling units cover a wider range of vegetation types. In consequence, the strength of the relationship between vegetation heterogeneity and species richness is likely to increase as spatial resolution decl ines (see Currie 1993 for discussion of this effect in another context). At the same time, a change in spatial resolution may result in a difference in the importance of habitat heterogeneity as a correlate of species richness because of an increase In the heterogeneity of resource production characteristics, which in turn has an influence on species richness (Wright et al. 1993).
CHAPTER I: Generallntroduction
CHAPTER 2: Species richness, environmental correlates, and spatial scale: a test using South African birds
CHAPTER 3: Spatial congruence of ecological transition at the regional scale in South Africa
CHAPTER 4: Complementary representation and zones of ecological transition
CHAPTER 5: Species richness, human population size and energy: conservation implications at a national scale
CHAPTER 6: Thoughts for the World Summit on Sustainable Development – Priority bi rd areas and human development needs in South Africa
CHAPTER 7: General Di scussion
