Recherches écologiques dans la savane de Lamto (Côte-d’Ivoire) : Production annuelle d’un ver de terre Millsonia anomala Omodeo

Résumé En Fr

The large earthworm Millsonia anomala constitutes a major part of the Oligochaete fauna in low-lying, wet areas of the Lamto savanna (Ivory Coast). The mean annual number and biomass of this species are 180 000/ha and 170 kg fresh weight/ha respectively. Breeding occurs twice a year at the end of the rainy seasons. Successive generations were identified and separated by graphic analysis of individual weights and numbers of worms in each weight class. Growth and survival rates, calculated for worms from 12 m² samples taken monthly from August 1969 to August 1970, were confirmed by field culture experiments. The production of different generations in calculated for each month by using Boj sen-Jensen’s formula : [FORMULA] where P is the production from time 1 to time 2, N1 and N2 are the numbers of worms present at times 1 and 2, and P1 and P2 are the mean weights at times 1 and 2. At the beginning of the study, in August 1969, 78 % of Millsonia anomala captured resulted from the 1969x generation, raised between December 1968 and March 1969. Other worms raised previously were composited. Two generations appeared between August 1969 and August 1970 : 19692 in October-November 1969, and 19701 between January and February 1970. Cocoon production during this period was 20,5 Kg/ha fresh weight. Worms born before 1969 produced 94,4 Kg/ha fresh weight whereas 19691, 19692 and 1970x generations produced respectively 157,6 Kg, 129 Kg and 58,3 Kg fresh weight/ha. Production was maximal during the rainy seasons. However inondation of the area in June caused a lowering in production. During the very dry months (September, January and March) production was low. Total production for the 12 months starting August 1969 was 460 Kg/ha fresh weight, that is 23,5 Kg dry weight/ha. The ratio of annual production to mean annual biomass was 2,70.

Millsonia anomala, espèce d’assez grande taille, constitue la majeure partie du peuplement d’Oligochètes de la zone de bas-fonds où a été réalisée cette étude : l’effectif moyen est de 180 000 individus par hectare représentant une biomasse de 170 kg en poids frais. La reproduction survient deux fois par an à la fin des périodes pluvieuses. Les générations successives peuvent être reconnues et isolées graphiquement sur les histogrammes construits en portant en abcisses les classes par ordre de poids croissant et en ordonnées les effectifs des classes.