Earthworm invasion in North America: food resource competition affects native millipede survival and invasive earthworm reproduction

Abstract

The invasive non-native earthworm Amynthas agrestis (Goto and Hatai 1899) has recently been documented invading forests of the Appalachian Mountains in the southeastern United States. This epigeic earthworm decreases the depth of organic soil horizons, and this may play a role in the decrease of millipede richness and abundance associated with A. agrestis invasion. To investigate the mechanisms behind these effects, A. agrestis and the millipede Sigmoria ainsliei (Xystodesmidae) were placed into microcosms with soil and either L horizon, F and H horizon, or a combination L/FH treatment. Microcosms were destructively sampled and reconstructed with the same treatments every four weeks to assess faunal fresh weight change and survival. Soils from earthworm treatments were wet-sieved for cocoons to assess treatment effects on reproduction. On average, millipede mortality occurred 88 days sooner in treatments that did not have FH horizon material, and within all litter treatments millipedes tended to survive longer when A. agrestis was absent. Earthworms maintained higher fresh weight in L/FH than FH or L treatments. With a single exception, no A. agrestis cocoons were recovered from microcosms that also contained S. ainsliei. The results suggest that A. agrestis and S. ainsliei may compete for food resources, particularly the smaller particle material in the FH horizons of the forest floor. Millipedes may exert some biotic resistance to A. agrestis invasion, as diminished earthworm fecundity was observed in experimental units containing both species.