Earthworm invasion causes declines across soil fauna size classes and biodiversity facets in northern North American forests

Anthropogenic pressures alter the biodiversity, structure, and
organization of biological communities with severe consequences for
ecosystem processes. Species invasion is such a human-induced ecosystem
change with pronounced impacts on recipient ecosystems. Around the globe,
earthworms invade habitats and impact abiotic soil conditions and a wide
range of above- and belowground organisms. In northern North America,
where earthworms have been largely absent since the last glaciation period
and most earthworm species present today have only been (re-)introduced a
few hundred years ago, invasion impacts have been intensively studied.
However, despite several studies assessing impacts of invasive earthworms
on soil fauna, studies have rarely investigated the simultaneous responses
of different soil-fauna size groups and biodiversity facets which might
respond differently to earthworm invasion and independently affect
ecosystem processes. Our study goes beyond previously-established
knowledge on earthworm-invasion effects by simultaneously assessing
differences in four biodiversity facets, namely the abundance, biomass,
richness, and Shannon index of soil invertebrate macro-, meso-, and
microfauna communities between high- and low-invasion status plots (n=80)
and in relation to invasion intensity measured as earthworm biomass across
four northern North American forests sampled between 2016 and 2017. Across
forests and soil-fauna groups, we found reduced abundance (-33 to -45%)
and richness (-18 to -25%) in high compared to low-invasion status areas.
Additionally, meso- (-14%) and microfauna biomass (-38%), and macro- (-7%)
and microfauna Shannon Index (-8%) were reduced. Higher invasion intensity
(earthworm biomass) was additionally related to reduced soil-fauna
biodiversity. While the studied biodiversity facet was important for the
soil fauna response, soil-fauna size group was comparably unimportant.
Given the global ubiquity of earthworm invasion and the importance of soil
fauna for key ecosystem processes, our observational results help to
assess future impacts of this invasion and the consequences for
anthropogenically-altered ecosystem functioning.