Microhabitat utilisation and the spatial distribution of rainforest canopy invertebrate communities

The tropical rainforest is renowned for its high invertebrate species richness. Yet our understanding of the spatial distribution of the invertebrate communities inhabiting tropical rainforest canopies is very poor. Most mass-sampling invertebrate biodiversity studies from the rainforest canopy have either focussed on whole trees, via sampling methods such as canopy fogging, or on subsets of the invertebrate community (usually a particular herbivorous taxonomic group) inhabiting a single microhabitat type (usually the leaves), via hand collection and foliage beating techniques. Consequently, previous studies have been unable to examine the fine-scale spatial distribution of canopy invertebrates within individual trees, or establish where in the canopy different invertebrates are concentrated. This information is vital if we are to make accurate predictions about species and community level responses to disturbance and climate change, or make accurate calculations about ecosystem processes and species richness. I carried out a long-term mass-collecting effort from five canopy microhabitats (mature leaves, new leaves, flowers, fruit, and suspended dead wood) using hand collecting and beating techniques from a canopy crane to examine spatial differences in invertebrate density, diversity, community structure, host specificity, and body size in an Australian tropical rainforest.\ud \ud First, I examined microhabitat differentiation in the invertebrate communities associated with each microhabitat. Specifically, I examined variation between microhabitats in invertebrate density, taxonomic composition and guild structure. I also focused on the beetle community to examine differences between microhabitats in species richness, overlap, abundance patterns, and guild structure. I focused on the beetle community since beetles are arguably the most species-rich taxon on Earth and are biologically diverse. Second, I examined the host specificity of beetles inhabiting different microhabitats to test the assumption that most host specific species are herbivores on the leaves that interact antagonistically with the host tree. This assumption is the basis for many global biodiversity estimates and has resulted in the majority of studies in tropical rainforest canopies being restricted to herbivores on the leaves. Lastly, I investigate body size variation between microhabitats. Since microhabitats vary in a number of qualitative and quantitative factors, I examined whether microhabitat choice has influenced the evolution of the body sizes of the invertebrates that utilise them.\ud \ud I collected 40,374 invertebrates from all five microhabitats, including 10,335 beetles which were sorted to 372 morphospecies. Per unit weight, invertebrate densities on flowers were 10 to 10,000 greater than on new or mature leaves. At the species level, flowers were utilised by an estimated 40% of canopy beetle species, despite constituting just 0.06% of crown biomass. Overlap between microhabitats in species composition was also very low, indicating that each microhabitat is utilised by a relatively unique assemblage. In terms of feeding guild structure, invertebrate communities varied between each microhabitat, largely in relation to the food sources provided. For example, herbivores were found predominantly on new leaves and flowers, but were underrepresented among the invertebrate communities inhabiting the other microhabitats. Fungivores and saprophages however, dominated the communities occupying suspended dead wood.\ud \ud Contrary to expectation, host specificity was equally high between an assemblage of herbivorous beetles on the leaves that interact antagonistically with the host tree, and an assemblage of flower-visitors that interact mutualistically with the host plant. Indeed, both herbivores and non-herbivores on flowers were as host specific as herbivores on leaves. Only species that do not interact directly with the host tree (non-herbivores on the leaves) were significantly more generalised in host use. Consequently, the previous assumption that most host specialists on a tree are herbivores on the leaves is refuted, since half of the host specific species were flower-visitors, and half of these belonged to beetle families that are not considered herbivores according to traditional guild assignments.\ud \ud Mean body size of invertebrate taxonomic groups varied significantly between microhabitats. In particular, phylogenetically independent contrasts revealed that, in general, invertebrate taxonomic groups were significantly smaller on flowers compared to mature leaves and new leaves. Size differences between microhabitats were most pronounced among herbivorous taxa (Hemiptera, Lepidoptera), in particular the immature stages, which were significantly smaller than expected on flowers, and larger than expected on leaves. Taxonomic groups that complete larval development on resources other than flowers, especially those that contain a large proportion of strong flying species (Diptera, Hymenoptera, most Coleoptera) typically showed no differences in body size across microhabitats. Since body size variation was most pronounced among herbivores, and these differences occurred repeatedly in many unrelated lineages, it is apparent that microhabitat identity influences evolutionary changes in the body sizes of species that feed on the available resources.\ud \ud These results indicate that invertebrate communities in the canopy are spatially partitioned between microhabitats, and that each microhabitat supports a unique community in terms of composition, guild structure, and abundance patterns. Variation between microhabitats in temporal and spatial availability, nutritional quality, chemical protection and other attributes may have resulted in species associated with different microhabitats being subject to different selective pressures. Evidence for this is shown in the differences in the level of host specificity and distribution of body sizes of invertebrates between foliage and flower samples. Furthermore, my results clearly indicate that extrapolations or assumptions about biodiversity patterns or ecosystem functions based only on knowledge of the spatial distribution of foliage-inhabiting invertebrates may contain substantial error because this group is not representative of the wider canopy community. Based on my findings, I recommend that future canopy invertebrate biodiversity studies account for microhabitat differentiation in the planning stage, and experiments and sampling protocols be designed in an appropriate manner to account for any possible pre-existing bias that sampling only leaf material may introduce. In particular, the flower-visitor community is identified as an especially important (but previously neglected) assemblage. I suggest future studies focus on the spatial and temporal variation in canopy invertebrate abundance patterns and their influence on ecosystem processes such as nutrient cycling, energy flow and the pollination biology of rainforest trees