Spatial distribution of earthworms in acid-soil savannas of the Eastern plains of Colombia

The horizontal spatial distribution of earthworms in a native savanna and in a 17-year introduced pasture from the area of Carimagua (Colombia) was assessed by means of aggregation indices and geostatistical analysis. Morisita's I d and Taylor b indices were calculated from field data obtained through 1 m2 quadrats during a sampling period of 17 months and both semi-variograms and contour maps were obtained at three different dates through a rapid sampling of 64 soil blocks (40x40xl5 cm) regularly distributed in the nodes of a 8x8 grid (i.e., 70x70 in). Earthworms tended to exhibit contagious distribution in both land use systems and a multivariate relationship appeared between the size of the earthworm and aggregation indices employed. Land use had no significant effect on the spatial distribution of earthworms. Earthworms belonging to different ecological categories and from diverse sizes were spatially distributed in patches of several tens of meters. Semi-variograms reflected, occasionally, the size of these patches although no spatial structure was perceived at some dates. This might be due to the intersample distance employed (10 m) so that changing the scale of observation could reflect unseen patterns. In this study our purpose was first to determine the strength of spatial aggregation in some neotropical earthworm species and then to observe the movement of population patches at the scale of the studied area. The use of these types of spatial analyses in conjunction may be very useful to establish the dynamics of earthworm populations in the field