Different earthworm ecological groups interactively impact seedling establishment

There is increasing evidence that direct interactions between earthworms and seeds impact the assembly of plant communities. However, effects of earthworms of different ecological groups and their interactions on plant germination and establishment are little known. We set up a full-factorial greenhouse experiment in order to explore impacts of different ecological groups of earthworms (epigeic, endogeic and anecic) on the establishment of seedlings. The three ecological groups of earthworms affected seedling establishment in an interactive way with the effects varying in time. While anecic earthworms detrimentally affected the number of established seedlings, impacts of epigeic and endogeic species depended on the presence of earthworms belonging to other ecological groups. Impacts of anecic earthworms were more significant and consistent than those of epigeic and endogeic ones pointing to the dominance of the effect of anecic earthworms for seedling establishment. The initial positive effect of the combined presence of epigeic and endogeic earthworms compared to treatments with only endogeic and only anecic earthworms was likely due to the acceleration of seed germination and elevated nutrient availability for seedlings in earthworm casts. By contrast, reduced numbers of seedlings in presence of both epigeic and endogeic earthworms compared to the control treatment might have been due to seedling predation after the litter layer had disappeared. The results extend current knowledge on interactions between earthworms and seeds by showing that, in addition to anecic species, also epigeic and endogeic species influence seedling establishment with their effect depending on the presence of anecic species. Moreover, the results indicate that impacts of earthworms vary with the developmental stage of seedlings highlighting the importance of interactions between earthworms and seeds.