Recently, a new clade of invasive earthworm species (Amynthas) has been introduced into North America. Amynthas have been noted to behave more aggressively than other invasive earthworm species and competitively exclude them by depleting microbial food resources. We therefore wanted to determine if Amynthas species shape the microbial community in areas that they invade and also to determine if these effects are specific to the species of Amynthas. To address this possibility, we took earthworm and soil samples from 0.25-meter quadrats along an active Amynthas invasion edge at the Wisconsin Madison Arboretum. We incubated earthworms in sterile petri dishes and allowed them to pass casts, which were then subsequently collected. We dissected the gut section posterior to the clitellum of each earthworm and extracted the microbes from them. We used Illumina sequencing of soil- and worm-associated microbes to characterize differences in bacterial and fungal community composition of soils, earthworm casts, and earthworm gut microbes. We assessed the data with nested analysis of similarity (ANOSIM) evaluating the effects of earthworm species nested in position along the invasion edge, and non-metric dimensional scaling (NMDS).
Results/Conclusions
We found that invasion edge position marginally affects bacterial (p = 0.086, R = 0.333) and fungal soil community composition (p = 0.058, R = 0.329). We found that earthworm species significantly affects cast bacterial communities (p = 0.015, R = 0.194) and that invasion edge position has no significant effect (p = 0.667, R = -0.250). Earthworm species also significantly affects gut bacterial community composition (p = 0.004, R= 0.397), but invasion edge position (p = 0.667, R = 0.000) had no significant effect. Earthworm species had a significant effect on the cast fungal community composition (p = 0.031, R=-0.22), unlike invasion edge position (p = 0.333, R = 0.75). Earthworm species significantly affected gut fungal community composition (p = 0.002, R= 0.566), but invasion edge position had no significant effect (p > 0.999, R = -0.5). These results suggest that Amynthas species introduce novel assemblages of bacteria and fungi through their casts, and these differences are associated with the species-specific community composition of earthworm guts. These results also suggest that Amynthas species are likely affecting the soil microbial community during invasion.