Termitaria and Vegetation Patterns on the Loita Plains of Kenya

Abstract

Air photographs of parts of East Africa characteristically show extensive areas where regular patterns produced by mound-building termites are prominent. Some of the patterns form a conspicuous feature of the landscape but others are difficult to distinguish. Distinctive vegetation zones occur on the mounds and often on the surrounding ground if it is affected by rain wash from the mounds. The vegetation patterns on the Loita plains of Kenya have frequently been seen from the air but not properly understood. It has long been known that termites can have a profound effect on vegetation and plant communities on termite mounds are often of distinct types. Thus Glover (1937) listed some of the plants found on old termitaria on the 'high-veld' of the Transvaal. It is known also that termite mounds occur in definite soil types; for example, Kemp (1955) mentions large mounds aggregated on grey soil around the margin of swampy ground in parts of eastern Tanganyika which were probably built by Odontotermes rectangularis. Murray (1938) demonstrated that termitaria in the Transvaal had a higher percentage of humus, nitrogen (about twice as much) and colloidal matter (about twice as much) than the surrounding soils. They also had a higher maximum water-retaining capacity but there was no significant difference between the pH of termitaria soil and that of the surrounding country. There was little difference in total water content or holard (Weaver & Clements 1938) in the two soils but the percentage of holard, humus, nitrogen, colloid and maximum water-retaining capacity rose towards the centre of the mounds. Murray concluded that, although the chemical content of termitaria was richer than that of the soil of the surrounding 'purple veld',* the fertile constituents appeared to be unavailable to plants as the living mounds in the studied areas were never covered with luxuriant vegetation. It is true that vegetation is often absent on living mounds but it has been observed that maize, sisal and other crops planted on old broken-up termitaria in cultivated land are greener and taller than elsewhere. We suggest, therefore, from personal observation, that it is not the chemical composition but the hardness and imperviousness of the soil which discourages plant growth on living termitaria, and that vegetation patterns around the mounds are influenced by the amount of clay and salts derived from the termitaria themselves. Glover (1949) observed that on the edges of 'dambos' (open seasonal swamps) in the Brachystegia woodland in the Abercorn district of Northern Rhodesia, the hard texture of the outer soil of termitaria in the early part of their existence inhibited plant growth. As they became older, the hard surface broke down and the mounds flattened out,