Belowground competition among invading detritivores
The factors regulating soil animal communities are poorly understood. At local scales, interspecific interactions within a trophic group, especially competition, in theory, can play an important role in structuring communities that are resource-regulated. However, most groups of soil fauna apparently coexist in high species richness with little sign of competitive exclusion. This pattern led to the conclusion that competition is unlikely to be of major structuring force in soil fauna community. However, in earthworms, competitive exclusion has frequently been suggested to be the major community-structuring factor. Despite these claims, direct evidence of competition and its mechanism is still lacking. The ongoing invasion of Amynthas hilgendorfi, an Asian earthworm species of Japan origin, into the deciduous forest in the Mid-Atlantic region in the USA provides a unique opportunity to study interspecific interactions with both native and non-native resident species. We combined laboratory mesocosm experiments using 13C and 15N double-enriched leaf litter and field observations to test the hypotheses that (1) species from the same functional group compete for food, (2) competition between epigeic species is not mediated by soil microbial modification, and (3) competition lead to non-additive effects on the translocation of litter-derived fresh C and N into subsurface soil.
Our study documented the first direct evidence of competition in earthworms and confirmed that food is the limiting resource. Isotope data indicated that Amynthas hilgendorfi led to a reduction of litter-derived C and N assimilation by 26.6 ± 6.8 and 23.9 ± 9.5 % in Lumbricus rubellus and by 31.0 ± 3.0 and 49.2 ± 2.85 % in Octolasion lacteum (P < 0.001 in all cases), suggesting that A. hilgendorfi is the superior competitor for leaf litter. As expected, N translocation from leaf litter into soil was also reduced as a result of competition in the case of L. rubellus (P < 0.001). Furthermore, although A. hilgendorfi reduced bacteria PLFA in the soil (P < 0.001), this change alone, without the physical presence of A. hilgendorfi, did not translate into reduced biomass or litter consumption in L. rubellus (P = 0.25 and 0.44), supporting our second hypothesis. Standard ellipse areas of natural isotopic abundance further indicated that the isotopic niche of A. hilgendorfi is wider than that of L. rubellus (P = 0.005), indicating a wide diet range. All together, we concluded that the invasion success of A. hilgendorfi can be attributed to its competitive advantage and dietary flexibility.