Synergy among introduced plants and earthworms in woodlands of the upper Midwest

Background/Question/Methods

The introduced plant species Rhamnus cathartica (European or common buckthorn) and various species of non-native earthworms are associated with pronounced changes in plant communities of savannas and woodlands in the upper Midwest, as well as alteration of nutrient cycling in the same areas.  R. cathartica and earthworms each have their own particular effects, but they also seem to positively affect each other’s success expanding their ranges and establishing populations.  Earthworms (either cultured Eisenia fetida or wild-caught Lumbricus terrestris) were presented with leaf litter from R. cathartica, Acer saccharum (sugar maple), or Quercus macrocarpus (burr oak), the native dominants of regional woodlands and savannas, in combination or singly in mesocosm experiments.  Amounts of litter consumed and changes in population number and size distribution were recorded.

Results/Conclusions

E. fetida (redworms) choose leaf litter from R. cathartica over leaf litter from Q. macrocarpus and A. saccharum.  When presented with only one species of leaf litter, E. fetida consumed much more of R. cathartica than either of the natives.  E. fetida’s population growth rates were considerably higher on a diet of leaf litter from R. cathartica than when fed leaf litter from either of the native species.  Wild-caught Lumbricus terrestris display the same feeding preferences in parallel mesocosm experiments, even though the animals had no prior experience with R. cathartica because they were collected from stands of A. saccharum that completely lack R. cathartica.  Additional experiments are underway to measure the effect of R. cathartica on L. terrestris’ population dynamics.  These results suggest that earthworm populations may have different population dynamics and growth rates when R. cathartica is present than other wise, which could increase their potential effect on nutrient dynamics within such sites.