Effects of an exotic ecosystem engineer on microarthropod dispersal and root growth

Background/Question/Methods

Earthworms are ecosystem engineers which have caused substantial changes to soil structure in many invaded northern forests.  Decreases in leaf litter thickness and mixing of organic and mineral horizons due to earthworm activities have been demonstrated to have large effects on forest systems.  However, the effects of earthworm burrows rather than earthworms per se are less well-understood.  We hypothesize they may facilitate some other species by forming a system of interconnected tunnels that act as corridors for dispersal or growth.  We conducted two greenhouse experiments to examine the relative importance of earthworms versus their burrows on microarthropod dispersal and plant root growth.  In the first experiment, conducted over a period of eight days, we measured microarthropod (springtail and mite) abundance after dispersal through previously sterilized soil containing artificial burrows (i.e. soil cracks), abandoned earthworm burrows, inhabited earthworm burrows, or no burrows.  In the second experiment, Achillea millefolium and Campanula rotundifoliawere grown individually in pots with and without earthworms for 3.5 months.  Images were taken of roots in burrows, soil cracks, and the soil matrix using a minirhizotron camera and were analyzed using ArcGIS.

Results/Conclusions

Burrows and soil cracks appeared to facilitate microarthropod dispersal, while the earthworms themselves had limited effects on Mesostigmata and Collembola and negative effects on Oribatida.  This suggests that increased nutrient concentrations in burrow walls due to earthworm castings/mucus are less important than the openings themselves.  In contrast, there was little evidence that root length per unit area differed in burrows versus soil cracks, despite previous data from agricultural systems suggesting that roots may grow more densely in earthworm burrows.  However, further examination of root distribution in the soil matrix is needed to assess whether roots may be more likely to occur close to burrows or soil cracks.  Although earthworms had little effects themselves, the strong influence of soil cracks on microarthropod dispersal suggests that research should consider effects of earthworm burrowing on soil porosity in invaded ecosystems in addition to the more commonly examined effects of decreases in leaf litter depths and mixing of soil horizons.