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

Abstract

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-, mesoand 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.

Aboveground impacts of a belowground invader: how invasive earthworms alter aboveground arthropod communities in a northern North American forest

“…It has a long history of earthworm-invasion research, including investigations on soil abiotic (soil chemistry and physics) and biotic (micro, meso and macrofauna) aspects [29,30,[35][36][37]. Land-use intensity is low and homogeneous across invasion status areas and the forest last burned in 1909 [29]. We combine community data on earthworms, plants and aboveground arthropods sampled in June and July 2019 on observational plots of the 'EcoWorm' project (described in Eisenhauer et al [30]).…”

“…The forest is dominated by trembling aspen ( Populus tremuloides ) interspersed with balsam poplar ( Populus balsamifera ), with a dense understorey vegetation and a grey luvisol soil. It has a long history of earthworm-invasion research, including investigations on soil abiotic (soil chemistry and physics) and biotic (micro, meso and macrofauna) aspects [ 29 , 30 , 35 – 37 ]. Land-use intensity is low and homogeneous across invasion status areas and the forest last burned in 1909 [ 29 ].…”

“…Earthworm invasion alters soil abiotic conditions [17,19], plant communities [23][24][25] and soil fauna [26][27][28][29]. Moreover, there are reports of consequences for aboveground vertebrates, such as salamanders, birds and deer [18,30].…”

Contrasting protist communities (Cercozoa: Rhizaria) in pristine and earthworm-invaded North American deciduous forests

“…Moreover, the two Barrier Lake forests and the Saint John's forest also considerably differed in other soil abiotic properties. The former two had comparably higher soil water content, carbon and nitrogen concentrations, and thickness of the humus layer (Jochum et al 2021). The understory vegetation was more developed and comprised different species in the Barrier Lake forests compared to Saint John's forest.…”

“…The study sites were chosen as they have been under investigation for two decades and accordingly show a gradient of differing abiotic or biotic conditions along the earthworm invasion history (Eisenhauer et al 2019;Jochum et al 2021; Supplementary Table 1).…”