Responses of arthropods to shading and litter depth
Previous studies of soil and litter fauna development in re-vegetated sites have reported an initial colonisation by species tolerant of harsh environmental conditions (e.g. Fox & Fox, 1982; Andersen, 1993; van Aarde et al., 1996; Dunger et al., 2001). As the restored habitat developed, these species were gradually or abruptly replaced by others characteristic of undisturbed habitats. In the context of rainforest restoration, our results suggest that successional patterns of this type could occur as a function of increased shading alone. The effects of shading shown here are consistent with the findings of other studies emphasising the importance of structural attributes, including canopy cover, for the organisation of soil and litter arthropod assemblages (e.g. Holmes et al., 1993; Watts & Gibbs, 2002; Proctor et al., 2003; Lassau & Hochuli, 2004; Grimbacher et al., 2007). Exceptions to this pattern include some groups of arthropods that are linked strongly with biological traits of live plant species (e.g. herbivores, which show strong host-plant affinities, Hunter & Price, 1992; Wardle, 2006).
The observed patterns of arthropod colonisation may be due, at least in part, to ameliorated temperature regimes in shaded plots. Temperature tolerance is important in influencing the distribution of ground-dwelling arthropods (Pearson & Lederhouse, 1987; Addo-Bediako et al., 2000), and a number of restoration studies have shown that arthropods characteristic of rainforest prefer cooler microclimatic conditions (King et al., 1998; Grimbacher et al., 2006). In contrast, soil moisture content did not explain colonisation patterns of arthropod assemblages (as it did not vary significantly among the experimental plots), even though small arthropods could arguably be more sensitive to reduced moisture levels (Levings & Windsor, 1984; Shure & Phillips, 1991). Had we conducted sampling in winter (typically cool and dry), the patterns and role of soil moisture may have been different, although arthropod activity and abundance would have been lower.
In addition to the effects of microclimatic conditions, growth of herbaceous vegetation may have affected arthropod colonisation patterns, particularly those characteristic of pastures. Unshaded plots and some of the plots with 50% shade showed increased levels of colonisation by pasture-associated taxa (Fig. 2c). In these types of plots there was visible regrowth of herbaceous pasture plants, which potentially provided food and habitat for pasture-associated herbivorous arthropods, such as Homoptera and Orthoptera. Colonisation patterns of pasture ant species may have also been influenced by the supply of food resources (e.g. seeds from pasture, honeydew from Homoptera) associated with the growth of pasture plants.
In contrast to the strong effects of shading, our results suggest that the amount of litter may not be a strong determinant of arthropod assemblages in the context of rainforest restoration. Our results were consistent with the findings of other restoration studies of soil and litter arthropods colonising re-vegetated landscapes in various habitats (viz. sclerophyllous and temperate forest, shrubland, grassland, dune forest, heathland and rainforest), mainly from Australasia, Europe, North America and South Africa (53 studies reviewed by Nakamura 2007). Using empirical and anecdotal evidence, these studies evaluated a large number of biotic and abiotic factors that potentially influence the colonisation of arthropod groups, including ants, beetles, and spiders. The effects of differing levels of canopy cover and litter have been well studied, investigated by 14 and 21 studies, respectively. While significant or potential impacts of shading were reported by all of the 14 studies, this was not the case for the effect of litter quantity. Seven of the 21 studies (including studies conducted in rainforest reforestation of formerly cleared landscapes, Jansen, 1997; King et al., 1998) found no apparent responses in the rate of arthropod colonisation to different amounts of forest litter. Although rainforest-like arthropod assemblages may require the presence of at least some litter similar to that typically found on the rainforest floor (approximately 3 cm, see King et al., 1998; Nakamura et al., 2003), our results indicate that further addition of mulch does not benefit their colonisation.
It could be argued that the observed results may have been different had we incorporated other arthropod groups (e.g. spiders, beetles) into species-level analyses (Wassenaar et al., 2005). Furthermore, the use of higher taxonomic levels may have obscured significant responses, as individual species within an order may have responded in different manners. However, our results were consistent with other restoration studies incorporating diverse groups of soil and litter arthropods (Nakamura, 2007, see above), suggesting that the fundamental patterns of arthropod colonisation would not have differed regardless of the arthropod groups investigated.
Effects of the experimental context
Most restoration studies to date have employed a post hoc empirical approach to investigate the effects of factors considered important for the development of colonising fauna (Michener, 1997). The ecological effects of the factors under investigation are therefore difficult to elucidate, because of the presence of extraneous factors (Block et al., 2001; Catterall et al., 2004). This problem is further exacerbated by the fact that restoration projects are generally carried out on a site-specific basis with no spatial replication, limiting inference from the data (Block et al., 2001). This study provided an opportunity to test systematically the factors of interest because the experimental approach allowed for the construction of replicated units in which focal factors (i.e. shading and litter depth) were manipulated, while extraneous factors were controlled.
There were, however, a number of constraints associated with the experimental design, which potentially limited colonisation of the constructed plots by rainforest-dependent arthropods. First, experimental plots lacked some of the habitat components of reforested sites, namely live plants that supply freshly shed foliage and woody debris, both of which may be important for the colonisation and persistence of rainforest-dependent arthropods (Majer et al., 1984; Andrew et al., 2000). Second, the mulch used in the experiment had been sterilised with steam, which may have killed potential food resources (e.g. prey micro-invertebrates, bacteria, fungi), and may have altered the chemical composition of the mulch, making it more or less favourable to arthropods. Third, the spatial and temporal scale of the experiment may have been insufficient for successful colonisation, although the location of plots adjacent to the forest edge maximised the probability of rainforest-associated taxa moving into the plots (Nakamura et al., 2008b). These limitations may be reflected by the low colonisation rates of litter-associated rainforest ant species (e.g. Mayriella abstinens complex, Strumigenys harpya, Discothyrea and Lordomyrma spp., see Nakamura et al., 2007), which were either very low in abundance or absent from the experimental plots. Nevertheless, a diverse array of arthropods, including rainforest-dependent taxa (albeit not as diverse as those commonly found in rainforest), did colonise the experimental plots and their assemblage composition responded differentially to the experimental treatments.
Implications for practice
In order to maximise rainforest biodiversity values (the occurrence of biota and ecological process typical of intact rainforest; Catterall et al., 2004), restored habitat patches need to facilitate colonisation by fauna characteristic of rainforests, while inhibiting re-invasion by taxa characteristic of the matrix habitat (i.e. pasture). Our results show that rainforest restoration using lower density plantings, such as timber plantations, may facilitate colonisation by rainforest soil and litter arthropods even though canopy cover is not developed as rapidly, or to the same extent, as in ecologically designed restoration plantings. However, the establishment of a fully closed canopy (90%) appeared to inhibit invasion by arthropods characteristic of the matrix habitat most effectively. Using deeper mulch did not create more suitable conditions for rainforest arthropods or offset the deleterious effects of less shade.