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Deterministic processes, community organisation and ecosystem function

Final Report Summary - FUNCTION (Deterministic processes, community organisation and ecosystem function)

This project aims to generate a sequence of highly resolved plant litter food webs combined with associated measures of plant litter decomposition and nutrient flux to fulfil the following objectives:

Objective (1): Identify the deterministic processes driving community organization (within and across habitats distributed over a strong environmental gradient) using information on phylogenetic relatedness, and validate the processes using information on environmental characteristics and trophic structure.

Progress towards Objective (1): A litter bag experiment was established on the mainland Waitakeres Ranges park and Hauturu-o-toi island, to link plant and invertebrate diversity with rates of decomposition and nutrient flux. Both are mixed broadleaf forest ecosystems in the Auckland region which differ in historical and current land management, conservation status and allochthonous nutrients (marine nutrients brought in by nesting seabirds). The experiment used 26 plots established by Auckland Council and Department of Conservation to monitor plant biodiversity, 15 were on the off-shore island Hauturu-o-Toi and 11 in the mainland Waitakere Ranges. The invertebrate fauna from all litter bags has been sorted; all invertebrates have been identified to a range of taxonomic resolutions including phylum, class, order, family and sub-family level, resulting in 43 standard groups across all samples. Beetles and mites were the two dominant groups numerically, and have been identified to species or morphospecies (105 beetle and 54 mite species/morphospecies). All the mite and beetle species have had CO1 and 28S genes sequenced in both directions and phylogenies have been constructed. To determine tropgic position of each species in each environment, 15N and 13C have been measured for all litter species all samples. Beetles, mites and amphipods analysed for change in 15N and 13C stable isotopes in a subset of samples. Analysis of these data sets is complete and three resulting papers are in preparation.

Objective (2): Identify patterns of community organization driving decomposition processes using attributes of phylogenetic, community and trophic structure, and examine the extent to which patterns vary or hold across a strong environmental gradient.

Progress towards Objective (2): The diversity-decomposition experiment was successfully established at 15 biodiversity plots on Hauturu-o-Toi, and at 11 biodiversity plots in the Waitakere ranges. At each plot, the diversity-decomposition experiment was set up at three sub-plots, to generate a spatially stratified sampling design (sub-plot, plot, island/range). The relationship between diversity and decomposition was tested using a litter-bag experiment which uniquely links the composition and diversity of the leaf litter within the bag, the invertebrate community that colonises the litter, and the observed rates of decomposition and nutrient flux at the end of the experiment. Leaf litter was collected from each sub-plot, separated into species, dried, sampled for nutrient content (%N and %C), weighed and divided equally between mixed and single species litter bags according to an additive design (ie. for each sub-plot, each species was the same weight in single and mixed litter bags, such that mixed litter bags contained the weight of all single species litter added together). The mixed and single species litter bags were returned to their original forest site. Natural variation in the litter composition created a large gradient for testing effects of leaf litter diversity on decomposition; subplots had between 2 and 9 litter species, and showed a wide range of species-abundance-distributions. An 8mm mesh size was used for all litter bags, enabling full access to the litter by all New Zealand forest arthropods.

Litter bags were collected after approximately eight months, and the litter fauna extracted on Burlese funnels into 95% alcohol. Litter was again separated into species, dried, weighed and sampled for nutrient content. The change in litter weight was used to calculate the rate of decomposition (g / day) for each species in single and mixed litter bags. All decomposition and nutrient flux data are complete. Decomposition data has been analysed in relation to the leaf litter community structure, and results show strong and positive effects of litter species richness in driving rates of decomposition. Decomposition and nutrient flux data have been modelled in relation to the invertebrate community structure, trophic structure, and phylogenetic community structure and the resulting paper is in preparation.

Objective (3): Identify the best predictors of ecosystem function at different spatial scales (using environmental parameters, taxonomic diversity, trophic structure, functional diversity, phylogenetic diversity) and the processes underlying the predictive relationship.

Progress towards Objective (3): Collection of plant and leaf litter composition data enables the assessment of plant taxonomic diversity measures as predictors of decomposition. Preliminary results suggest that the richness of litter species forming the bulk (>80%) of the litter present can explain a large proportion of the variation in decomposition rates. However, litter species composition, and the degree to which senesced leaves are recalcitrant to decomposition, is equally important in determining decomposition rates. Now that sequencing and stable isotope processing have been completed further measures based on these will be tested as predictors of decomposition and nutrient flux.