Temperate and boreal forests in Canada and the northeastern United States have been invaded by several exotic species, including European earthworms (family Lumbricidae) and garlic mustard. Earthworms and garlic mustard co-occur and are both known to adversely impact some native plant species. However, relatively little is known about potential interactions between these two invaders. In a series of growth chamber experiments, we determined the palatability of garlic mustard and six native herbaceous forest species (shooting star, columbine, wild geranium, sweet cicely, butterfly milkweed, and yellow jewelweed) to the common nightcrawler. We also assessed the ability of the common nightcrawler to bury and digest garlic mustard and wild geranium. When offered seeds from garlic mustard and a native plant species, the earthworms ingested more garlic mustard seeds than seeds from four of the six native species. In a mesocosm experiment, the common nightcrawlers apparently digested 72 and 27% of garlic mustard and wild geranium seeds, respectively, that were placed on the soil surface. No seeds were observed on the soil surface at the end of the experiment but the majority of recovered seeds for both species were found within the top 10 cm (3.94 in). More seeds were recovered in 0- to 10-cm and 31- to 40-cm sections for wild geranium than for garlic mustard. No difference in seed recovery was detected at the other depths. Garlic mustard seed is readily consumed by common nightcrawlers and appears to be preferred over some native plant species suggesting that common nightcrawlers may reduce the size of the garlic mustard seed bank.
Nomenclature: Common nightcrawler, Lumbricus terrestris L.; garlic mustard, Alliaria petiolata (M. Bieb.) Cavara & Grande ALAPE; columbine, Aquilegia canadensis L.; butterfly milkweed, Asclepias tuberosa L. ASCTU; common shooting star, Dodecatheon meadia L.; wild geranium, Geranium maculata L.; yellow jewelweed, Impatiens pallida Nutt; sweet cicely, Osmorhiza claytonia (Michx.) C.B. Clarke.
Interpretive Summary: The common nightcrawler is an introduced earthworm species that has been linked to reduced native plant diversity and to altered soil processes in invaded temperate and boreal forests in North America. The common nightcrawler can consume large quantities of seeds but the effect of this invasive species on the seed bank dynamics of native and invasive plants in its invaded range is unknown. In this study, we examined the palatability of garlic mustard, an invasive plant species, and six native species to the common nightcrawler. We also assessed the ability of the common nightcrawler to bury seeds below the soil surface. The earthworm preferred garlic mustard seeds over the seeds of four of the six native plant species and consumed or buried large quantities of garlic mustard seed. Although the net effect of the common nightcrawler on native plant species in invaded forests may be negative, our findings suggest that this earthworm has the potential to reduce the size of the garlic mustard soil seed bank.