Earthworms create disturbance zones in the rhizosphere of forest soils and garlic mustard (Alliaria petiolata) seeks disturbed areas as a means of invasion. Earthworms may help facilitate this invasion by consuming native seeds and burying them in burrows. As part of a larger effort to determine the relationship between garlic mustard and the non-native worm L. terrestris, we conducted two experiments examining feeding preference and seed spatial distribution. The feeding preference study consisted of garlic mustard and six native plant species: jewelweed (Impatiens pallida), columbine (Aquilegia canadensis), geranium (Geranium maculatum), shooting star (Dodecatheon meadia), cicely (Osmorhiza claytonia) and butterfly weed (Asclepias tuberose) were offered to L. terrestris in combination and separately. Earthworms were placed individually in Petri dishes in a darkened growth chamber at 18°C and offered 50 seeds per species or garlic mustard in combination with each species (25 garlic mustard seeds with 25 native species seeds). The seed spatial distribution study consisted of two species, geranium and garlic mustard, presented separately to the earthworms. Mesocosms with sterilized soil were packed to a depth of 40cm and one earthworm was placed in each mesocosm. On the soil surface, 500 seeds of garlic mustard or 500 seeds of geranium were offered to the earthworms.
Results/Conclusions
When offered a single species, the earthworms consumed more shooting star and garlic mustard seeds than seeds from any other plant species. When offered garlic mustard and a native species, the earthworms consumed more garlic mustard seed than native seed, with the exception of shooting star. The earthworms distributed seeds vertically throughout the mesocosm, but most seed was recovered in the top 10 cm of the soil. More geranium seeds were recovered than garlic mustard seeds, supporting the hypothesis that earthworms predate garlic mustard seeds at a higher rate than the seed of native species. This research also suggests that earthworms may have a larger negative effect on garlic mustard seed than on the seed of native plant species.