References

300 P. Lavelle et al.
Fig. 4. Comparison of genetic structures of bacteria based on PCR IGS analysis of 16S and
23S DNA, in soil microsites: Ew– control with no earthworms; Ew + non-ingested soil
treated with earthworms; CAST earthworm casts; BUR burrow wall of earthworm gallery;
GC earthworm gut content
Changes in the activity and composition of microbial communities in
earthworm biogenic structures have been observed by many authors (Parle
1963; Scheu 1987; Tiwari and Mishra 1993; Tiunov and Scheu 1999). Microbial
numbers and biomass increase by a factor of 2.3–4.7 in burrow walls
of Lumbricus terrestris mainly due to changes in the individual bacterial
cell volume (Tiunov and Scheu 1999), with a 3.7–9.1 increase in respiration
rate. These results confirm earlier estimates by Bhatnagar (1975) who
claimedthatupto5–25%oftotalsoilmicroflorawasincludedinthis
microenvironment in temperate pastures of central France. Recent studies
using molecular techniques have shown evidence of the occurrence
of selective microbial communities in Lumbricidae earthworm casts and
burrows (Fig. 4; Kersanté 2003). The bacterial community in the gut contents
differed the most from the original soil, casts and to a lesser extent,
burrow walls, having intermediate communities. This suggests that digestive
processes would select a rather specific microbial community through
the Sleeping Beauty effect, and progressive changes would occur in ageing
structured communities that would slowly converge towards a common
“bulk soil” type of community. An alternative hypothesis, although difficult
to support, could be that earthworms harbour in their guts a specific