TABLE 29–2 The Number of Seed Plant Species in Various Regions of the Worlda

Biogeographic Unit

No. of Species

Area (1,000 km2)

Species (per 1,000 km2)

Tropical Africa

30,000

20,000

1.5

West Tropical Africa

7,300

4,500

1.6

Sudan

3,200

2,505

1.28

Southern Africa

22,977

2,573

8.93

Cape Floral Kingdom

8,504

90

94.49

Brazil

40,000

8,456

4.73

Peninsular India

20,000

4,885

4.09

Australia

25,000

7,716

3.24

Eastern N. America

4,425

3,238

1.37

aAfter Gibbs Russell, 1985.

The flora of the Afrotropical realm probably includes some 40,000 species, the richest component of which is found in southern Africa, especially in the Cape Floristic Kingdom, one of the six major floristic divisions of the world’s flora defined by Good (1974). The magnitude of the floristic richness of southern Africa is indicated in Table 29–2, which provides data for a wide range of regions. The 8,500 species of the Cape flora are compressed into an extremely small area. The change of species composition from one patch of vegetation to the next is very high—two sides of the same valley may differ by 45%, adjacent large areas may share less than 40% out of more than 2,500 species. Such rapid changes in floristic composition are unknown elsewhere, not even in the Indo-Malayan rain forest (Kruger and Taylor, 1979). The Cape Peninsula, only 470 square kilometers in area, possesses 2,256 indigenous species—more than half the flora of eastern North America. The unique floristic richness of the Cape is unfortunately matched by unusually serious threats to its survival. A detailed 10-year survey of the conservation status of southern Africa’s 23,000 species of plants indicates that some 2,373 are threatened. The Cape Floristic Kingdom, occupying less than 4% of southern Africa, accounts for 68% of the threatened species (Hall et al., 1984). Satellite imagery indicates that 34% of the region’s natural vegetation has been transformed by agriculture and other human activities. The second most serious threat to the small heathland shrubs characteristic of this region is the aggressive competition exerted by large, introduced woody weeds. In addition, an invasive ant has been found to suppress populations of the native seed-storing ants, thus exposing critical seed sets to predation by rodents or destruction by the intense fires that characterize these Mediterranean-climate shrublands.

The distribution of Afrotropical birds is rather different from that of the patterns of floristic richness. A recent analysis of the distribution of 1,595 species of Afrotropical birds indicated fairly clear correlation between bird diversity and rainfall, vegetation, and other factors in the present environment. But many species of forest birds displayed distribution patterns that could best be interpreted in terms of past climatic and habitat conditions (Crowe and Crowe, 1982). These somewhat anomalous distributions indicate the occurrence of what are known as Pleistocene



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