Earthworms and Collembola Relationships: Effects

1 2 Relationships between anecic earthworms (Lumbricus terrestris and Aporrectodea 3 giardi) and the collembolan species Heteromurus nitidus (Templeton, 1835), which is known 4 to be attracted to earthworms, were investigated in an 8-week laboratory experiment. Our 5 aims were (1) to assess whether earthworms influence the population dynamics of H. nitidus, 6 and (2) to study pathways of influence and how earthworm effects are modified by humus 7 forms and predators. Using microcosms with three defaunated humus forms, then provided 8 with earthworms and predators, we intended to demonstrate that, amongst possible favourable 9 effects of earthworms on springtail populations, earthworm activity may provide greater 10 access and more pathways for springtails to explore soil and avoid predation. We expected 11 that the effects of predators (centipedes) on the abundance of H. nitidus would increase from 12 less (calcic mull) to more (moder) compact soil, and we hypothesized that earthworms would 13 reduce predation pressure on H. nitidus by providing escape routes through increased 14 macroporosity. Humus forms and earthworms only affected the population size of H. nitidus 15 under high predation pressure. When collembolan numbers were higher in calcic mull than in 16 moder, and were increased by the presence of earthworms. These results corroborate the 17 hypothesis that earthworms, by increasing soil macroporosity, improve the escape routes for 18 Collembola and thus evade predators. In moder humus earthworms increased the density of 19 H. nitidus whether predators were present or not, so we cannot exclude that earthworms were 20 also directly beneficial to H. nitidus. However, the hypothesis of a functional relationship 21 mediated by soil macroporosity seems relevant since it was supported by differences observed 22 when considering body size. When two size classes were distinguished within populations of 23 H. nitidus (1) the positive effect of earthworms in moder was observed only on larger 24 Collembola (> 1 mm), (2) the density of the larger Collembola was decreased by predation 25 only in moder and not in mull, (3) the effects of predators on the smaller individuals were not 26 influenced by the presence of earthworms whatever the humus form, and was not decreased 27 by the presence of earthworms. Nevertheless, factors other than macroporosity may operate as 28 the presence of earthworms in acidic mull led to an unexplained decrease in the abundance of 29 small-sized H. nitidus. 30 31