Invertebrate response to disturbance and liana growth in tropical forests

Charlotte Raven

doi:10.25907/01059

Tropical rainforests support exceptional biodiversity and key ecosystem functions but are increasingly affected by anthropogenic and natural disturbances that alter forest structure. One consequence of disturbance is the proliferation of woody vines, including lianas and rattans, which suppress tree growth, modify microclimates, and generate novel structural conditions. However, the implications of these structural changes for invertebrate communities and ecosystem processes remain poorly understood. Invertebrates play central roles in rainforest food webs and resource cycling and are therefore widely used to assess how variation in forest structure and climate influences ecological communities and functions. The Australian Wet Tropics in North Queensland spans pronounced elevational and climatic gradients, from warm, seasonally variable lowland rainforests (mean annual temperature ~24°C; mean annual precipitation ~2320 mm) to cooler, cloud-affected highland rainforests on the Atherton Tableland (mean annual temperature ~20°C; mean annual precipitation ~2683 mm), providing a natural context in which to examine how climate and disturbance jointly shape invertebrate communities and ecosystem processes. Ants (Hymenoptera: Formicidae) and crickets (Orthoptera: Ensifera) occupy diverse trophic roles as predators, prey, herbivores, seed dispersers, and detritivores, and exhibit high diversity and abundance in tropical ecosystems. In this thesis, I investigated how rainforest disturbance and forest structure, quantified using vine- and tree-derived metrics (including vine-to-tree ratios), together with elevation and climate (mean annual temperature and precipitation), influenced ground-dwelling ant and cricket diversity, richness, and community composition. I also examined how these same gradients affected two key forest-floor processes—arthropod predation and leaf-litter decomposition—to assess whether structural and climatic variation translated into changes in ecosystem functioning. All work was conducted in tropical rainforests of the Wet Tropics, North Queensland, Australia. First, I examined how climate and vegetation structure shaped cricket communities using Hill diversity metrics (q0, q1, q2), ordination, and trait-based analyses. Cricket diversity, community composition, and functional traits varied with both forest structure and climate. Species richness increased with precipitation and above-ground tree biomass, while flightlessness was more prevalent at higher elevations and in denser forests. Acoustic species were more common in high-biomass plots, whereas small, non-acoustic species declined with increasing biomass and elevation. Second, I compared ground-dwelling ant communities across lowland and highland forests and found that diversity was structured by different environmental drivers in each bioclimatic zone. Highland sites supported greater species richness, with mean annual temperature emerging as the strongest predictor of diversity. In contrast, vegetation structure and disturbance proxies were more influential in lowland forests, where diversity increased with litter depth and decreased weakly with increasing vine proliferation. Highland ant diversity was lower where warmer temperatures coincided with high liana densities, indicating an interaction effect between climate and disturbance. Finally, I assessed how disturbance-related structural variation influenced ecosystem processes. Total predation rates on sentinel caterpillars varied little across gradients of vine abundance relative to trees; however, predator groups responded differently to environmental variables. Arthropod predation declined with increasing litter depth, while mammal predation decreased with rattan density and increased with coarse woody debris. Leaf-litter decomposition rates increased with above-ground tree biomass, suggesting associations between forest structural development and nutrient cycling. Overall, this thesis demonstrates that cricket and ant communities and forest-floor processes are associated with forest structure, climate, and disturbance in complex, taxon-specific ways within tropical rainforests of the Wet Tropics.

Invertebrate response to disturbance and liana growth in tropical forests

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