Invasive earthworms reduce chemical defense and increase herbivory and pathogen infection in native trees

Recent research shows that earthworms can alter defense traits of plants
against herbivores and pathogens by affecting soil biochemistry. Yet, the
effects of invasive earthworms on defense traits of native plants from
previously earthworm-free ecosystems as well as the consequences for
multitrophic interactions are virtually unknown. Here we use a combination
of an observational study and a complementary experimental study to
investigate the effects of invasive earthworms on leaf defense traits,
herbivore damage, and pathogen infection in two poplar tree species
(Populus balsamifera and Populus tremuloides) native to North American
boreal forests. Our observational study showed that earthworm invasion was
associated with enhanced leaf herbivory in saplings of both tree species.
However, we only detected significant shifts in the concentration of
chemical defense compounds in response to earthworm invasion for P.
balsamifera. Specifically, leaf phenolic concentrations, including
salicinoids and catechin, were lower in P. balsamifera from
earthworm-invaded sites. Our experimental study confirmed an
earthworm-induced reduction in leaf defense levels in P. balsamifera for
one of the defense compounds, tremulacin. The experimental study
additionally showed that invasive earthworms reduced leaf dry matter
content, potentially increasing leaf palatability, and enhanced
susceptibility of trees to infection by a fungal pathogen, but not to
aphids, in the same tree species. Synthesis. Our results show that
invasive earthworms can decrease the concentrations of some chemical
defense compounds in P. balsamifera, which could make them susceptible to
leaf-chewing insects. Such potential impacts of invasive earthworms are
likely to have implications for tree survival and competition, native tree
biodiversity, and ecosystem functioning.