548 ECOLOGY AND BEHAVIOR 



Table 1. Average monthly surface temperatures 



Upwellings of cold subsurface water are not uncommon along the eastern 

 and southern Cape coasts, so inhabitants must withstand sudden changes 

 of water temperature (of the order of several degrees Centigrade) as well as 

 the more gradual changes taking place with the passing of the seasons. This 

 may well be the chief reason for the high degree of endemism among the 

 fauna and flora of this region. 



The inshore currents of the east coast are mainly eddies of the Mozambique 

 and Agulhas currents and are most pronounced where the continental shelf 

 is relatively wide, as it is on the central Natal coast. Off northern Natal and 

 the northeastern Cape the shelf is narrow and inshore current systems are 

 poorly developed. The inshore waters are usually cooler than those of the 

 main current, possibly because of upwelling by cool subsurface water in the 

 centers of the eddies. As a result, animals living on the outer continental 

 shelf and further offshore are usually in warmer waters than will be found 

 inshore at the same latitude; tropical species can thus penetrate further to 

 the south if they are not restricted to inshore waters. 



The Natal coast is shown in more detail in Figure 2. The topography of 

 inland Natal should be considered briefly. The central and southern parts 

 are extremely steep, falling from the plateau in Lesotho at an altitude of 

 more than 3000 m to sea level in a horizontal distance of about 250 km. The 

 numerous rivers flow rapidly as a result of this steep topography and the 

 high rainfall of about 80 to 100 cm per year (chiefly during the summer 

 months), and large quantities of sediment are deposited on the continental 

 shelf. The northeastern part of Natal consists of the southern tip of the 

 Mozambique coastal plain and is notable for its low, flat topography and 

 scarcity of rivers. The sea of that part of the coast to the north of St. Lucia 

 (commonly known as the Tongaland coast) is therefore remarkably free of 

 sediments and also has a very narrow continental shelf. The fauna and flora 

 of this region are markedly tropical compared to those of the rest of the 

 Natal coast. 



From south of St. Lucia to Durban the continental shelf is wider, with the 

 result that the Agulhas Current is forced to flow further offshore and cooler 

 inshore countercurrents are formed. The amount of sediment shows a spec- 

 tacular increase, and the relatively cool and dirty water results in a marked 

 drop in the numbers of tropical species as compared to the Tongaland coast. 



