No evidence of co-facilitation between a non-native Asian earthworm (Amynthas tokioensis) and invasive common buckthorn (Rhamnus cathartica) in experimental mesocosms

Abstract

Non-native invasive earthworms are known drivers of forest change in north temperate forests. Much understanding of earthworm invasion is based on species of European origin, but concern about Asian pheretimoid earthworms (e.g. Asian jumping worms, Amynthas spp.) is increasing. Some effects of Amynthas spp. on soil properties and biota have been studied, but little is known about interaction of Amynthas spp. with plants. Potential interaction between Amynthas spp. and invasive buckthorn (Rhamnus cathartica) is of particular interest given hypothesized co-facilitation between R. cathartica and European earthworms—cited by some as components of an “invasional meltdown”. We used reciprocal mesocosm experiments in Wisconsin, USA, to test for co-facilitation between Amynthas tokioensis and R. cathartica. We asked: (1) Are jumping worms more successful in environments invaded by buckthorn? (2) Does jumping worm activity increase buckthorn germination and establishment? Counter to expectations, co-facilitation was not supported, and we found evidence to the contrary. There was no increase in litter loss (indicative of consumption by jumping worms) or jumping worm fecundity in buckthorn-invaded environments, and buckthorn germination was unaffected by increased jumping worm densities. Counter to our hypothesis, jumping worm fecundity was greater in buckthorn-free soils than in buckthorn-invaded soils. Our results show no experimental evidence of co-facilitation by either of these invasive species, and highlight potential differences in ecological impact of non-native invasive earthworm taxa that vary in life-history and functional dynamics.