Invasive earthworms erode soil biodiversity: A meta-analysis

1. Biological invasions pose a serious threat to biodiversity and ecosystem functioning across ecosystems. Invasions by ecosystem engineers, in particular, have been shown to have dramatic effects in recipient ecosystems. For instance, invasion by earthworms, a below-ground invertebrate ecosystem engineer, in previously earthworm-free ecosystems alters the physico-chemical characteristics of the soil. Studies have shown that such alterations in the soil can have far-reaching impacts on soil organisms, which form a major portion of terrestrial biodiversity. 2. Here, we present the first quantitative synthesis of earthworm invasion effects on soil micro-organisms and soil invertebrates based on 430 observations from 30 independent studies. 3. Our meta-analysis shows a significant decline of the diversity and density of soil invertebrates in response to earthworm invasion with anecic and endogeic earthworms causing the strongest effects. Earthworm invasion effects on soil micro-organisms were context-dependent, such as depending on functional group richness of invasive earthworms and soil depth. Microbial biomass and diversity increased in mineral soil layers, with a weak negative effect in organic soil layers, indicating that the mixing of soil layers by earthworms (bioturbation) may homogenize microbial communities across soil layers. 4. Our meta-analysis provides a compelling evidence for negative effects of a common invasive below-ground ecosystem engineer on below-ground biodiversity of recipient ecosystems, which could potentially alter the ecosystem functions and services linked to soil biota.