title: Development of Soil and Litter Arthropod Assemblages in Rainforest Restoration
Creators: name: Nakamura, Akihiro
Publisher: name: Griffith University
Year: 2008
Subjects: subject: biodiversity
subject: rainforest restoration
subject: fauna
subject: invertebrates
subject: reforestation
subject: arthropods
subject: soil and leaf litter
subject: ecosystem
subject: Blackall Range
subject: Australia

description: Broadscale forest clearance is a major human-induced disturbance with devastating consequences for biodiversity. With a rise in public awareness of biodiversity, the last few decades have seen an increasing number of reforestation activities aimed at recreating natural habitat. To date, research on the outcomes of reforestation for biodiversity have mostly focused upon the recovery of vegetation structure and composition, with relatively little attention being paid to the recovery of fauna, especially invertebrates. Arthropods inhabiting soil and leaf litter constitute a considerable proportion of the biomass and diversity of a given faunal community, and exhibit strong associations with ecological functioning (i.e. soil formation, decomposition). The recovery of soil and litter arthropods in restored landscapes is therefore important, not only for the recovery of biodiversity but also for the re-development of a functioning ecosystem, a goal of most types of reforestation programs. However, we know little about the patterns of development of arthropod assemblages in reforested landscapes or the mechanisms underpinning any such patterns. The broad objective of this study was to investigate the effects of selected factors on the colonisation patterns of restored rainforest patches by soil and leaf litter arthropods. Criteria for selecting factors for study included their potential influence on the development of arthropod assemblages and their potential for manipulation by restoration practitioners. The study was conducted on the Blackall Range near Maleny, a mid-elevation (250-530 m a.s.l.) basaltic plateau in subtropical eastern Australia (26°S, 152°E). The plateau supported subtropical rainforest until European settlement in the 19th century, when most of the rainforest was cleared for pasture. The factors selected for study, in relation to the colonisation of restored habitat patches by soil and litter arthropods, were as follows: 1. the isolation of restored habitat patches from remnant forest; 2. the efficacy of inoculation (re-introduction of rainforest soil and litter arthropods) to restored habitat patches; 3. the quality and quantity of substrate (i.e. mulch) used during the initial stages of rainforest restoration; 4. the degree of shading and depth of substrate, and their interaction; and, 5. the short- and longer-term impacts of glyphosate herbicide on arthropod assemblages. To test explicitly the effects of these factors on arthropod colonisation of restored habitat patches, an experimental approach was adopted in this study. The first four factors were addressed by means of a manipulative field experiment. Small-scale habitat patches were created by adding sterilised mulch to an area previously treated with glyphosate herbicide, and covered with shadecloth, to simulate various conditions of forest restoration which may be experienced by colonising arthropods. In order to test for the impacts of a glyphosate herbicide on rainforest arthropods, I carried out a separate field experiment in which experimental patches were established within remnant rainforests. The experimental approach adopted in this study allowed for the construction of replicated units, while controlling for extraneous factors (e.g. heterogeneity of litter composition, habitat area, age of restoration), to enable robust examination of the effects of selected factors. To monitor assemblage composition, arthropods were collected using two methods: pitfall traps and extraction from litter. Responses of arthropods were analysed at two main levels of taxonomic resolution: ‘coarse’ arthropods (all arthropods sorted to Order/Class) and ants, identified to species. Before the field experiments, a survey was carried out to collect reference information on the distribution of soil and litter arthropods in remnant rainforests (undisturbed reference sites) and cleared pasture (disturbed reference sites) in the study region. Regardless of the sampling method (pitfall or litter extraction) or taxonomic resolution employed (coarse arthropods or ant species), the composition of arthropod assemblages clearly differed between rainforest and pasture. The information obtained from this survey generated potential bio-indicators of forested and cleared habitats, assisting interpretation of the data obtained from the field experiments. The effects of habitat isolation and inoculation were tested using ‘restored’ habitat patches which were established within cleared pasture at increasing distances (0, 15, 100 and ca. 400 m) from the edges of rainforest remnants. After nine months, rainforest-dependent taxa were found to have only colonised the habitat patches closely adjacent to rainforest remnants. Attempts to increase the extent of arthropod establishment by inoculation were unsuccessful: the majority of rainforest arthropods from the raw inoculum failed to persist within the isolated plots. The results indicated that many forest-dependent soil and litter arthropods may have a limited capacity to colonise isolated restoration sites, and/ or small experimental plots in the short term. Inference from the experiment was potentially limited by the relatively small temporal and spatial scales of restoration treatments. Avoiding these limitations in future research may require controlled and replicated efforts in experimental restoration over larger areas, based on collaborations between researchers and practitioners. To test the effects of quality and quantity of substrate on arthropod colonisation, habitat patches were established by adding either sterilised hay (a conventionally used mulching material in restoration projects) or woodchip mulch (a structurally more complex alternative), each at two depths (3-5 cm, 10-15 cm). Habitat patches were positioned within pasture adjacent to the edges of rainforest remnants to minimise the effect of isolation, and were all unshaded to create conditions similar to those during the initial stages of rainforest restoration. Despite its simple composition, hay performed better than woodchips in facilitating colonisation by arthropods characteristic of rainforest. However, neither hay nor woodchip mulch inhibited arthropods invading from the surrounding pasture. Shallow hay was favoured by ants characteristic of rainforest, but other groups of arthropods (e.g. Coleoptera, Isopoda) were associated with deep hay (10-15 cm). The optimum amount of hay may therefore vary among different groups of arthropods. The effects of shading and mulch depth, and potential interactions between them, were tested using habitat patches created with varying degrees of shading (none, 50% or 90% shading) and two depths of woodchip mulch (3-5 cm or 10-15 cm deep). The presence of shading, at both 50% and 90%, encouraged colonisation of habitat patches by arthropods characteristic of rainforest. However, only the more complete shading treatment (90%) inhibited re-invasion of restored patches by arthropods from the surrounding pasture habitat. Effects of mulch depth were significant only for rainforest-associated ant species which responded positively to shallow mulch within shaded plots. These results suggest that moderate levels of canopy closure, as produced by tree spacings typical of timber plantations, may be sufficient to facilitate colonisation of reforested land by rainforest arthropods. However, greater canopy shading (90%) is likely to be needed to inhibit re-invasion of arthropods from surrounding pasture habitat. Using deeper woodchip mulch does not necessarily create more suitable conditions for rainforest arthropods or offset the deleterious effects of the lower levels of shading. Paired herbicide-treated and control plots were created within rainforest remnants to test the short- (approximately three days) and long-term (approximately three months) impacts of herbicide application on soil and litter arthropod assemblages on the floor of the remnant rainforests. The results found no deleterious effects of glyphosate herbicide formulated as Roundup® Biactive™ on rainforest soil and litter arthropods; hence, this herbicide appears suitable for the control of unwanted plants in rainforest restoration projects, from the perspective of arthropod biodiversity. The outcomes of this study also have a number of important implications for the monitoring of the development of soil and litter arthropod assemblages in restored rainforests. First, pitfall traps (a commonly used sampling technique) can provide sufficient information on the state of arthropod assemblages in the context of subtropical rainforest restoration projects, although samples collected by this method alone do not necessarily represent the whole suite of soil and litter arthropods (e.g. cryptic arthropods that live in soil and litter). Second, a combination of higher-taxon sorting of all arthropods, together with species-level sorting of a significant major taxon (ants), provides a feasible compromise between comprehensiveness and detail in monitoring responses of arthropods. Third, ‘composite habitat indices’, such as those developed in this study to quantify the extent to which a site resembles rainforest or pasture in terms of its arthropod assemblage, can help alleviate problems associated with the patchy distribution of arthropod taxa in monitoring samples. The experimental approach adopted in this study provided information that would otherwise have been limited by post-hoc empirical studies alone. This study’s results demonstrated that the selected aspects of different restoration techniques and management affect the colonisation of soil and litter arthropod assemblages in rainforest restoration of old fields.