Most scientific studies investigating invasive species have focussed on the effects of a single species, despite frequent reports of systems with multiple invaders. It has been proposed that invasive earthworms, like Lumbricus terrestris (Lumbricidae), might interact with invasive plants because of their feeding mode of pulling leaf litter down into the soil. This could redistribute allelochemicals and nutrients produced by invasive plant leaf litter.
In the case of Alliaria petiolate (garlic mustard), a common invasive plant in the US, the allelochemicals released from leaf litter can suppress plant mutualists belowground. Invader interactions belowground such as this might be particularly important to plant physiology. Photosynthetic rates are often limited by nitrogen and phosphorus availability and the uptake of these nutrients is commonly mediated by arbuscular mycorrhizal mutualists.
In their new study published in AoBP, Cope et al. explore potential interactions between invasive earthworms and Alliaria petiolata leaf litter under realistic field conditions and a multi-year manipulative experiment. Specifically, they focus on how such interactions influence the abundance and physiology of the common US native plant Podophyllum peltatum (Berberidaceae). P. peltatum, or mayapple, is a herbaceous perennial that commonly associates with arbuscular mycorrhizal fungi and co-occurs with the two invader species.
Cope et al. found that in the field A. petiolata presence and higher soil nitrogen correlated with reduced P. peltatum cover but found no evidence for invader–invader interactions. In the manipulative experiment, there was a super-additive effect of the two invaders on plant biomass only when activated carbon was present. In the absence of activated carbon, there were no differences in P. peltatum biomass across treatments. While the field data do not suggest a negative interaction between these invaders, the experiment suggests that such an interaction is possible with greater environmental stress, such as increasing nitrogen deposition. The authors hope that future field experiments may consider manipulating multiple stressors simultaneously, including nutrient deposition and multiple invaders, to fully understand multi-invader interactions and their context-dependence.