Jean-Pierre Rossi

Relationships between spatial pattern of the endogeic earthworm Polypheretima elongata and soil heterogeneity

Iu Summary-Spatial distribution of the endogeic earthworm Polypheretima elorigata, soil texture, carbon and nitrogen contents were investigated in a 10-y-old pasture established on a vertisol, by measuring each variable at 57 points randomly located in a 25 x 60 m plot. Soil variables were measured at three depths: 0-10, 10-20 and 20-3Ocm. Principal component analysis (PCA) of the data set showed the absence of relationships between density of P. elongata and environmental variables. The first principal component expressed a marked gradient of soil texture and soil organic matter (SOM). Sampling point coordinates on the first principal component axis were used as a composite variable assumed to account for the major source of soil heterogeneity at this site. The spatial pattern of this variable and three age classes of P. eloiigata were investigated using geostatistics. There was no evidence of any relationship between soil heterogeneity and earthworm distribution. Population of P. elorzgata exhibited a marked patchy distribution with opposite patterns between adults and other age classes (juveniles and cocoons). These results suggest that at the study site processes responsible for the spatial distribution of P. elongata population are of demographic rather than environmental origin. 0 1997 Elsevier Science Ltd

Microspatial structure of a plant-parasitic nematode community in a sugarcane field in Martinique

Abstract The spatial pattern of four species of plant-parasitic nematodes in a sugarcane crop in Martinique was analysed using geostatistics. Aggregation of populations was first described with the variance to mean ratio and Morisitas index. The indices showed that nematodes had clumped distributions. Geostatistical analysis demonstrated that the nematode data were autocorrelated and allowed a fine description of the species density patterns. Correspondence analysis was used to ordinate the data and showed the presence of an antagonism between Criconemella onoensis and the other species. A cross semi-variogram indicated a negative cross correlation between the pattern of Criconemella onoensis and the pattern of the sum of the other species present at the study site.