Parametric intensity and the spatial arrangement of the terrestrial mollusc herbivores Deroceras reticulatum and Arion intermedius


Dr David A. Bohan, IACR-Long Ashton Research Station, Department of Agricultural Sciences, University of Bristol, Long Ashton, Bristol, BS41 9AF, UK. Tel: (44) 1275 549329, Fax: (44) 1275 394007, E-mail:


1. Parametric intensity and spatial arrangement analyses were used to investigate the spatial pattern of the slugs Arion intermedius and Deroceras reticulatum.

2. The spatial lag of sampling (distance between sampling points) was shown to be unimportant in the intensity analyses. Rather, the 0·25 m grain scale was imposed on the whole sampling. The observed slope of the variance to mean relationships was common to both species, possibly determined by egg laying in batches at 0·25 m. However, the variance of the sample, for a given mean, was lower in summer. This corresponded with a reduction in the proportion of zero counts, which could be due to slug movement, possibly increased by predator activity, acting at the 0·25 m scale.

3. By contrast with the intensity analyses, the lag scale was important for spatial arrangement. At 0·25 m, in March 1997, the A. intermedius and D. reticulatum juveniles were aggregated, presumably about where egg batches were laid. At higher scales, the arrangements of D. reticulatum became spatially random, and A. intermedius resolved to a patch arrangement at the 16 m scale.

4. Over time, the D. reticulatum spatial arrangements remained random and independent of the previous sampling date. From March to July 1997, the A. intermedius patch persisted. A crash in abundance of both species, between July and October 1997, appeared to destroy the patch, but subsequent association suggested that the patch persisted until March 1998. The arrangements of the species were independent of one another on all sampling dates.

5. These species-specific spatial arrangements were independent of all measured environmental factors and consistent with differences in the local reproduction, survival and migration of A. intermedius and D. reticulatum.

6. This comparative study indicates that the terms aggregated, random and regular should have separate definitions for parametric intensity and spatial arrangement. Furthermore, spatial scale has different meaning in intensity and arrangement analyses. Spatial arrangements are not described by parametric intensity. Spatial arrangements change with spatial scale. Temporal changes in intensity need not manifest as changes in spatial arrangement.