Asian earthworm invasion: The stable isotope perspective

Background/Question/Methods

In temperate soils, earthworms are the most significant ecosystem engineers. Recent studies of North American earthworms focusing on invasive European species have demonstrated that by consuming leaf litter and soil organic matter, invasive European earthworms redistribute nutrients in different pools in the soil and accelerate flux rates among the pools, leading to reduction of the understory vegetation and the leaf litter layer while increasing the thickness of organic soil. In recent years, a group of Asian invasive earthworms, Amynthas, has been widely reported invading forests already inhabited by European species in the Mid-Atlantic region, causing a “second wave of invasion” where the soil ecosystem, already modified by European species, is going through another transition. The objective of this study is to understand how the invading Asian species affect the European and native earthworms through interspecific interaction and how these interactions alter soil C dynamics by conducting a lab mesocosm experiment using ¹³C and ¹⁵N double-enriched leaf litter as tracers for litter-derived C and N and following the C and N into earthworm tissues, soil, and CO₂ under treatments of four earthworm species and different species combinations.

Results/Conclusions

Our results indicated that interspecific interactions in earthworms caused diet shift through competition and have non-additive effects on soil CO₂ efflux. The stable isotope signature of the North American native species Eisenoides lonnbergi is not affected by any of the European or Asian earthworms, suggesting that the native species occupies its own unique stable isotope niche, which may contribute to its survival and success in the field under the pressure of invasive earthworms in the Eastern US. When the Asian Amynthas hilgendorfi is present, the European earthworms Octolasion lacteum and Lumbricus rubellus became more depleted in ¹³C and ¹⁵N (by 3.0‰ and 2.3‰ for d¹³C and 15.8‰ and 10.0‰ for d¹⁵N, respectively), while the Asian earthworm became more enriched when the European species are present. This provides strong evidence that the two European species are outcompeted by A. hilgendorfi for leaf litter. Although Amynthas did not significantly change soil CO₂ efflux, it was the major driver of CO₂ efflux when it was present, regardless of the treatments. Our results strongly suggest that Amynthas invasion could potentially change C cycling through its feeding behavior and competition with other earthworms.