Selective vertical seed transport by earthworms: Implications for the diversity of grassland ecosystems

Earthworms are suggested to play an important role for the plant diversity of grassland ecosystems. Here we tested whether (i) Lumbricus terrestris L. selectively feeds on seeds of grassland species, (ii) feeding patterns match with seed species present in surface casts in a permanent grassland, and (iii) grassland plant seeds deposited in different soil depths are transported by earthworms. Food choice experiments with 10 plant species (3 grass spp., 4 non-leguminous herb spp., 3 leguminous spp.) showed that earthworms significantly selected between the offered seeds and generally preferred herbaceous species over grass species. Seed species in grassland surface casts did not correlate with seed species preferred by earthworms in the feeding experiment. Mesocosm experiments with L. terrestris in sterilized soil where 3500 seeds m−2 of Dactylis glomerata (grass), Taraxacum officinale, Rumex obtusifolius (non-leguminous herbs) and Trifolium repens (legume) were deposited in 2, 10, 20 and 30 cm depth showed a significantly different species-specific transport of the seeds both downward and upward the soil profile. Dependent on the initial deposition depth between 26% (seeds initially at 10 cm depth) and 56% (20 cm depth) of the seeds were transported by earthworms. After 90 days on average 585 germinable seeds m−2 of D. glomerata, 94 seeds m−2 of T. officinale, 38 seeds m−2 of R. obtusifolius but no seeds of T. repens were transported from deeper soil layers to the soil surface. Results suggest that seed herbivory and seed transport are important mechanisms by which earthworms can selectively alter the diversity of grassland ecosystems