Impact of native and exotic millipedes on decomposition, nutrient cycling and puriri (Vitex lucens) growth rates in laboratory microcosms

Anne Tomlinson *1, David Wardle 2, Jacqueline Beggs 1

1 University of Auckland
2 Department of Forest Ecology and Management, Swedish University of Agricultural Sciences Umeå Swede

Litter decomposition and nutrient cycling are key ecosystem functions affecting soil nutrient levels and ecosystem productivity.  The decomposer community, comprising micro-organisms and soil invertebrates exerts an important influence on decomposition.  However an increasingly important issue in the context of native forest ecosystems is the effect of exotic invertebrates on litter decomposition and nutrient cycling.  Exotic invertebrates may compete with native fauna for litter resources but may also alter nutrient cycling in native forests which may affect plant growth.  Invertebrate sampling in reserves in North Auckland found that introduced millipedes were abundant in native broadleaf tree litter where they co-existed with native millipedes.  This study used a native millipede (Spirobolellus antipodarus) and an exotic species (Oxidus gracilis) alone and in combination to investigate their individual and combined effects on decomposition rates and nutrient cycling in a microcosm experiment.  Puriri (Vitex lucens) seedlings were grown in microcosms containing: (1) the native millipede (S. antipodarus); (2) the non-native (O. gracilis); (3) low and (4) high density treatments of both species; and finally, controls without millipedes.  The performance of the two millipede species in different treatments was compared as were decomposition rates, soil and foliar nutrient levels, and growth of the puriri seedlings.  Both millipede species reproduced during the course of the study and co-existed at high densities without apparent adverse effects.  Treatments containing the exotic millipede O. gracilis showed significant differences in decomposition rates and soil nutrient levels compared to treatments without this species.

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