Establishment of one exotic species may facilitate entry of other alien species into an ecosystem. European buckthorn (Rhamnus cathartica), an invasive understory tree naturalized in many forests of eastern and midwestern North America, may aid the establishment of deep-burrowing earthworms of European origin. Through consumption of the litter layer, European earthworms dramatically alter plant species composition and soil properties in North American forests. Buckthorn might facilitate establishment of exotic earthworms either by depositing nutritious litter or by changing soil properties to favor earthworm growth. To test this hypothesis, we reared earthworms (Lumbricus terrestris) in tree tubes containing soils collected from sites in a Minnesota mixed deciduous forest that were characterized by having (a) no buckthorn with abundant sugar maple (Acer saccharum) seedlings, (b) no buckthorn with no tree seedlings, (c) buckthorn seedlings present, but few saplings, and (d) abundant buckthorn saplings and trees. We supplied each tube with either buckthorn litter or litter made of a 1:1 mixture of sugar maple and oak (Quercus rubra, some Q. alba). We assessed the impacts of soil origin and litter type upon an earthworm's growth using the relative growth rate of ash-free dry mass during a month-long feeding period.
In our tree tubes, L. terrestris individuals that consumed the 1:1 mixture of maple and oak litter grew 40% faster than those that consumed buckthorn litter; ANOVA revealed this difference as statistically significant. Though soils collected from the maple-rich location supported relative growth rates of L. terrestris that were 20% higher than did soils from buckthorn-rich sites, this difference was not significant. Earthworms that were assigned to tubes with soils collected from sites with buckthorn present were significantly more likely to have escaped, died, or appeared ill than were earthworms assigned to soils sampled from buckthorn-free sites. However, probability of escape, death, or illness did not differ significantly between earthworms assigned to the different litter types. Our results do not support the hypothesis that R. cathartica facilitates establishment of L. terrestris by providing nutritious litter or altering soil conditions to favor rapid growth of this earthworm species. Further research is needed to determine whether other earthworm species respond to soil or litter influenced by R. cathartica in a similar manner, and to establish the mechanism for any positive association of exotic earthworms and buckthorn in the field.