Abstract
Biological invasions have major impacts on a variety of ecosystems and
threaten native biodiversity. Earthworms have been absent from northern
parts of North America since the last ice age, but non-native earthworms
were recently introduced there and are now being spread by human
activities. While past work has shown that plant communities in
earthworm-invaded areas change towards a lower diversity mainly
dominated by grasses, the underlying mechanisms related to changes in
the biotic interactions of the plants are not well understood. Here, we
used a trait-based approach to study the effect of earthworms on
interspecific plant competition and aboveground herbivory. We conducted
a microcosm experiment in a growth chamber with a full-factorial design
using three plant species native to northern North American deciduous
forests, Poa palustris (grass), Symphyotrichum laeve (herb), and Vicia
americana (legume), either growing in monoculture or in a mixture of
three. These plant community treatments were crossed with earthworm
(presence or absence) and herbivore (presence or absence) treatments.
Eight out of the eleven above- and belowground plant functional traits
studied were significantly affected by earthworms, either by a general
effect or in interaction with plant species identity, plant diversity
level, and/or herbivore. Earthworms increased the aboveground
productivity and the number of inflorescences of the grass P. palustris.
Further, earthworms countervailed the increasing effect of herbivores on
root tissue density of all species, and earthworms and herbivores
individually increased the average root diameter of S. laeve in
monoculture, but decreased it in mixture. In this study, earthworm
presence gave a competitive advantage to the grass species P. palustris
by inducing changes in plant functional traits. Our results suggest that
invasive earthworms can alter competitive and multitrophic interactions
of plants, shedding light on some of the mechanisms behind invasive
earthworm-induced plant community changes in northern North America
forests.