The effects of irrigation, cultivation and some insecticides on the soil arthropods of an East African dry grassland.
The following is based on the authors summary of this account of investigations on the changes in the soil arthropod fauna of an area of dry grassland in northeastern Tanganyika caused by a permanently high moisture content, cultivation of sugarcane with irrigation and treatment with aldrin and dieldrin. The fauna was surveyed by means of a flotation technique. The native arthropod fauna was poor in all groups, the total of 14, 800 per m2 being only 4.5% that reported from temperate grassland in England [cf. RAE A 38 228]. Acarina were dominant (66%), and Collembola comprised only 8% of the total. Permanent moisture increased the total arthropod numbers to 53, 600 per m2, Acarina being increased twofold and Collembola ninefold. Also, many more species were present in wet than in dry grassland. The animals, were concentrated beneath grass clumps to a depth of 7.5 cm in both. Ants were the dominant insects apart from Collembola, especially in moist conditions. The cultivated area was examined 27 months after the soil had originally been broken and planted. The sugarcane was growing on ridges separated by irrigation furrows, and the latter were almost sterile. In the ridges, the arthropod populations were only just significantly less than under grass clumps in moist grassland, and this difference was due entirely to insects other than Collembola and especially to the virtual absence of ants, the nests of which had been destroyed. Sucking insects with specific food-plants were eliminated, but general or saprophagous feeders and predators were not apparently affected. Termites had persisted, and Acarina and Collembola were the most abundant orders. These populations had built up from the residue of the sparse dry-land populations that had survived cultivation, assisted by high temperatures and the absence of dry periods. Because the furrows were almost sterile, the total numbers in a field of sugarcane were only about two-thirds of those in moist grassland. Comparison with other surveys in Africa [cf. 40 202; 44 1] showed results in general agreement in pasture, but considerably higher populations in the cultivated soil in the present investigation. In all cases cited, Collembola and Acarina were the principal groups and in moist conditions both multiply, Collembola to a much greater extent than Acarina. Aldrin at 2 and dieldrin at 1 1b per acre applied twice, 27 and 7 months before the survey, had no detectable effect, although they were concentrated in the inhabited zone. This is in contrast with findings in temperate areas and is attributed to the relatively low application rates per unit volume of soil and the much more rapid loss of both insecticides, combined with a more rapid conversion of aldrin to dieldrin, in Tanganyika.