PRESIDENTIAL ADDRESS SECTION C. 69 



is very great, both on soil and on the vegetation directly. Their 

 tunnelling operations serve to loosen the soil, and they destroy dead 

 wood everywhere as well as sometimes the living plants. 



Their mounds are often the starting point of a clump of trees 

 in the various kinds of Tree Veld. Earthworms and true ants in 

 Natal as elsewhere are important because of their effects on the 

 soil. Locusts and various caterpillars destroy vegetation and 

 usually have a selective effect. Among the mammals there are 

 many which dig for and eat bulbs and tubers. The bush-pi^ 

 (Potamochoerus) is as efficient almost as an ordinary plough and 

 helps very much in the extension of forest by providing a good 

 germinating ground for the seeds of the forest species. The aard- 

 vark or antbear (Orycteropus), which feeds on termites, forms 

 large burrows in the veld. In disused burrows, characteristic 

 species, shade loving at least in their early stages, are found, which 

 cannot invade the undisturbed grassveld. Ferns, such as 

 Nephrodium athamanticum and the tree fern Cyathea dregei, are 

 examples. 



The relations of the insect fauna and various birds to the 

 vegetation in connection with flower pollination is a subject which 

 has been very little studied in Natal. This applies also to the 

 micro-flora and fauna of the soil. There is reason to suspect that 

 a great deal of nitrogen fixation in the soil takes place, but we 

 know nothing about it with certainty. 



In a few rare cases a curious biotic result is seen in connection 

 with the plant succession. A whole plant community, if left alone : 

 seems to commit suicide. The dominant species grows so dense 

 as to prevent any regeneration either from its own or ciher seed- 

 lings, and the individuals stifle one another and become unhealthy. 

 They die in numbers so that the whole community is readily 

 destroyed by fire. In the case of grassland the dominant species 

 (Themeda) occasionally grows so dense as to prevent regeneration, 

 and, if left unburnt and ungrazed, ultimately withers and dies 

 and leaves the ground more or less bare. The phenomenon, how- 

 ever, it may be repeated, is a rare one. 



Did time permit, much might be said regarding man as a 

 biotic factor. His influence is usually destructive. He interferes 

 with the plant succession by burning the grass or by overgrazing 

 or by drainage and lowering the water-level, etc., so that stable 

 climax types of vegetation are replaced by more primitive stages. 

 We thus see how the various contradictory views regarding grass- 

 burning can be reconciled, provided succession is studied over wide 

 areas and generalisations from isolated data are avoided. If the 

 succession is taking place in a forest climatic area and has advanced 

 far enough to reach the beginnings of the scrub stage, the farmer, 

 by burning the veld, can send back the succession to an earlier 

 pure grassland stage. On the other hand, continuous burning of 

 pure grassland itself tends to establish the most primitive stage of 

 all, that is, wire grass or Aristida grassveld. Grass burning, there- 

 fore, may be necessary or it may not, and it is only by an under- 

 standing of the plant succession and the principles underlying it 



