'n Bio-ekologiese ondersoek na die terrestriële Oligochaeta in die Mooirivierbesproeiingsgebied

Abstract

The earthworm fauna in established irrigation soil in the Mooi River irrigation area was investigated. A quantitative and qualitative faunistic survey indicated that there were five different species in this irrigation area. Three of these, Allolobophora trapezoides, Eisenia rosea (Lumbricidae) and Eukerria saltensis (Ocnerodrilidae) are considered, on theoretical grounds, to be introduced species. The other two species, Udeina kinbergi (Acanthodrilidae) and Tritogenia sp. (Microchaetidae) are known to be endemic in this country. The introduced families (especially the lumbricids) showed not only a wider distribution in this area, but they also occupied a numerically dominant position compared with the endemic families. The results of the present survey do not suggest any competition between the endemic and introduced families. The endemics were only found in clayey soils on which there were limited agricultural activities, while the introduced species were found in loamy soils in highly populated areas with a great deal of agricultural activity. The lumbricids apparently had a high reproduction rate measured by cocoon production and the presence of juvenile worms, while their adaptability to unfavourable environmental conditions was greater than that of the endemic species. Fortnightly ecological surveys in three different irrigation plots were undertaken. Two of these plots were situated in irrigation fields (with different soil types) in which lucern (Medic ago sativa) had been cultivated for three years. The third plot differed from these two in that crop rotation was applied to it. Certain fluctuation patterns in respect of numbers and biomass were determined for the earthworm population in each plot. An analysis of variance of the results indicated a significant difference in the earthworm populations (A. trapezoides and E. rosea) of the three plots. A. trapezoides was dominant numerically and with regard to biomass in all three plots. This species also showed a high reproduction rate and adaptability to unfavourable conditions. The fluctuation patterns were closely associated with the seasonal fluctuation in environmental factors among which the soil temperature and soil moisture were the two most important. It appears that crop rotation with the associated physical and chemical disturbances is unfavourable to the earthworm population. No other explanation can be given for the low number of worms on the plot where such disturbances took place. A high percentage of clay and the comparatively lower percentage of available soil moisture were probably less favourable conditions for the earthworms, since low densities were found in such soils. Laboratory experiments showed that cocoons of A. trapezoides and E. rosea both produced an average of 1,8 worms per cocoon. There was a correlation between hatching order and both worm length and number of segments, the latter two both showing ~ progressive decrease.