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Abstract A reduction in the numbers of macroinvertebrates present in soil may have a negative effect on soil structure, infiltration rates, and gas exchanges. Soil pollution by metal is known to have a detrimental effect on soil macrofauna. The aim of the present study was to evaluate (1) direct and indirect effects of soil pollution on soil macroinvertebrate bioturbation and (2) effects of the two macroinvertebrate communities found in a polluted and a nonpolluted area (one supposed sensitive, the other tolerant to metals) on burrow systems parameters. Macroinvertebrate porosity was studied using X-ray tomography. Three-dimensional reconstructions and characterisation of the burrow system were obtained using image analysis. Results showed that metal pollution principally affected the spatial distribution of macropores (more macropores were found near the soil surface) and the shape of the burrow system (branching rate was higher in the polluted soil), whereas soil macroinvertebrate composition principally affects burrow density parameters (the number of burrows was higher for the sensitive macroinvertebrate community).

Abstract. Earthworm burrows (i.e. macropores) are organised in burrow systems with various geometric properties. These burrow systems have a significant effect on soil processes, particularly the movement of water in the soil. But the relationships between earthworm burrow systems and their hydraulic properties are not well established because experimental studies of burrow geometry are difficult to perform. Although X-ray computed tomography has revolutionised the 3D description of burrow systems, this method is both time consuming and expensive. This paper presents a new, cheap and rapid approach. A computer model simulating the burrowing behaviour of earthworms was developed from experimental studies. A saturated flow model was then superimposed on the structure of the simulated burrow system to explore the relationships between the burrow systems and their hydraulic properties. Simulations of individual burrow systems were obtained for two different species of earthworm belonging to different ecological groups. Structural parameters of burrow systems (pore space, interconnectedness, connectivity and the number of openings at the limits of the space) were calculated and linked with estimates of permeability using regressions. In our results connectivity gave the best prediction of the difference in permeability between the two sets of burrow systems determined by the burrowing behaviour of the earthworms. Pore space, interconnectedness and the number of burrow openings explained fewer variations.

Abstract. The behaviour of earthworms belonging to two different species and ecological types (Aporrectodea nocturna and Allolobophora chlorotica) was studied using two-dimensional (2D) terraria. Two experiments were set up to gain insight into the nature of interactions between these earthworms. Firstly, the evolution of the burrow systems was analysed with the density of the earthworms varying from one to five individuals. Secondly, a burrow system was first established by using one earthworm which was then removed before the introduction of a second earthworm. This second earthworm therefore encountered a burrow system created either by a conspecific earthworm or by an earthworm of the other species. These experiments showed that: (1) intra- and interspecific interactions occur between earthworms, (2) these interactions are dependent on the physical presence of the earthworms, and (3) spatial avoidance can occur (A. chlorotica avoiding burrows created by A. nocturna). The results suggest that earthworm burrow systems are "individual structures", rarely used by other earthworms when inhabited. When abandoned, the burrows may be recolonised depending on the ecological type of the earthworm under consideration.

Abstract:  The unintentional introduction of a new earthworm species (Aporrectodea nocturna) into a Swiss pre-alpine meadow resulted in a great increase in earthworm density in the newly colonized area (386 m–2) compared with the density observed in the natural area (273 m–2) where an earthworm community was already present. To investigate the impact of this introduction on the burrow systems, eight soil cores (length 25 cm, diameter 16 cm) were taken (four in the colonized area and four in the natural area) and analysed with computer-assisted X-ray tomography. The resulting images were processed to obtain the 3D-skeleton reconstructions of the earthworm burrow systems. Due to high variability in these burrow systems, only slight differences were observed between the two areas. The total burrow length and the mean burrow lengths tended to be greater in the colonized area. Moreover, the distribution of pore numbers with depth showed different patterns with a maximum for depths between 10 cm and 15 cm in the colonized area and a maximum for depths between 20 cm and 25 cm in the natural area. These differences may have been related to: (1) the particular behaviour of A. nocturna, which was observed to cast at the surface in this site, and (2) the predominance of juvenile earthworms around the colonization front. These differences were sufficient to create significant effects on the continuity of the burrow systems (assessed by the number of different pathways between virtual horizontal planes) for the two areas. The colonized area was characterized by a greater pore continuity, which could have resulted in enhanced transfer properties.

Abstract:  To study intraspecific and interspecific interactions between different ecological types of earthworm, the burrowing behaviour of two earthworm species (the anecic earthworm Aporrectodea nocturna and the endogeic earthworm Allolobophora chlorotica) was observed in a microcosm. Earthworms were either alone in the microcosm, together with a conspecific earthworm, or with an earthworm of the other species. Observations under red light, including those of the position of the animals and the burrow dug, were recorded 4 times a day for 8 days and provided the data needed to reconstruct the probable trajectories of each earthworm. Differences in movement and burrowing behaviour were observed. Comparisons between the two species confirmed the expected behaviour of each ecological type: A. nocturna reused its burrow system regularly, whereas A. chlorotica rarely did. Moreover, it was shown that A. chlorotica burrowed less and explored a smaller surface when in the presence of A. nocturna. Besides, A. nocturna burrowed less and explored a smaller surface when in the presence of another A. nocturna. If these interactions occur under natural conditions, they could affect the structure of the burrow systems of the earthworm species examined.

Abstract: Varroa jacobsoni is an ectoparasite of honey bees which reproduces in capped brood cells. Multi-infestation is frequently observed in worker brood and can be interpreted as an aggregative phenomenon. The aim of this study was to determine whether the distribution of V. jacobsoni in worker brood cells relies on a random or an aggregative process. We studied the distribution of Varroa females in capped worker brood at similar age by comparing, by a Monte Carlo test, the observed frequency distribution of mites per cell to simulated distributions based on a random process. A complementary approach, using the "nearest neighbor distances" (NND) with Monte Carlo tests, was investigated to study the spatial distribution (a) between mites in different cells and (b) between infested cells in brood. The observed distributions did not differ significantly from that expected by a random process, and we conclude that there is no aggregation during invasion of V. jacobsoni in worker brood.

In a pre-alpine meadow in the Toggenburg (Switzerland), casts of the earthworm Aporrectodea nocturna, initially detected 20 years ago around a newly planted hedge, now extend 170 m from the hedge. The abundance of A. nocturna decreased between 140 and 170 m from the hedge from 237 (site: An+) to 0 (site: An-) individuals m-2. The worm's activity and the macroporosity it has created have been studied using CAT scanning followed by automated image analysis of pores from 1.5 to 9 mm diameter. Brightness values in images from CAT scanning showed distinct local frequency maxima for stones, soil matrix and macropores. Measured diameters of pores were strongly linearly correlated with diameters of needles used to produce artificial pores. Pores from 1.5 to 3 mm were most abundant. The maximum number of pores from 1.5 to 9 mm (about 1600 m-2) was larger than the maximum number of all earthworms (about 480 m-2). Modelling the cast production of A. nocturna indicated that 23.8 kg m-2 were deposited on the soil's surface within 3 months. The measured macroporosity (1.5-6 mm) was one-quarter to one-sixth of the pore volume corresponding to the removed casts. Thus, net production of pores by earthworms was a result both of the burrowing activity and of the refilling with eroded cast material. Depth distributions of bulk density, total porosity and pores from 1.5 to 9 mm were different at An+ from those at An-. However, diffusion of butane was equal at An+ and An-. Three-dimensional reconstruction suggested that the disposition (continuity, interconnectivity) of pores was more important for gas diffusion than the structure (size distribution, frequency) and bulk soil parameters.

Abstract:  Four soil cores (length, 20 cm; diameter, 16 cm) were sampled in a Swiss pre-Alpine meadow with high earthworm abundance (>400 individuals/m2); two cores were taken in October 1993 and the other two cores in April 1994. The cores were described using computer assisted tomography which gives a series of section images every 3 mm. A method for reconstructing the three-dimensional (3D) skeleton of the earthworm burrow system is presented and discussed. This method provides an image of the structural organisation of the burrow system and was found to be adequately sensitive for use in ecological and functional studies. The seasonal variation of these 3D skeletons was investigated using two approaches, i.e. the analysis of: (1) global burrow system characteristics, and (2) individual burrow characteristics. At the scale of the global burrow system no difference was found between seasons (same number of burrows and same total burrow length) except for the vertical segment distribution, which was homogeneous in spring and decreased with depth in the fall. The study of individual burrow characteristics revealed that burrows tended to be more vertical in spring and that their branching intensity was higher in this season.