Invertebrate mediated biodiversity-ecosystem functioning relationships:lessons from tropical forest dung beetles

Biological communities are changing across the globe as a result of anthropogenic pressures; abundances of individuals are declining within populations and species are becoming extinct. Biological diversity and trophic complexity in grasslands and soil food webs are positively associated with the cycling of nutrients in soil and water, primary productivity and decomposition. Since these ecosystem processes underpin a number of goods and services to society, human-driven changes in the structure of ecosystems could negatively impact upon human wellbeing. However, the majority of our knowledge of the role of biodiversity in ecosystem functioning comes from studies conducted in temperate grassland systems. Consequently, our understanding of how of higher-level organisms influence ecological processes in different ecosystems is limited. This thesis aims to address these knowledge gaps by investigating how dung beetle traits and functional diversity influence the secondary dispersal of seeds and the emergence and survival of seedlings in the northeastern Brazilian Amazon. My first research aim was to understand the importance of intraspecific variability in dung beetle traits for the accuracy of functional diversity (FD) indices (Chapter 2). This chapter demonstrates that intraspecific differences in dung beetle traits are small compared to between species differences. However, failure to include intraspecific variability resulted in large errors in the calculation of FD indices when describing small and/or species poor communities. Second, I investigated how dung beetle diversity influences secondary seed dispersal, and the role of environmental context in modulating relationships. Here I reveal positive relationships between dung beetle functional diversity and both the probability of seed burial and the dispersion of seeds throughout the soil profile. However, these patterns were dependant on soil type and thus environmental context (Chapter 3). Finally, I explored the multitrophic significance of findings from Chapter 3 by testing how dung beetle communities affect the burial of different sizes of seeds and emergence and survival of seedlings (Chapter 4). Results from this chapter demonstrate how dung beetles could influence vegetation regeneration because beetle diversity negatively affected the likelihood that experimental seeds emerged from the soil surface, but positively impacted on the likelihood that emerged seeds survived until the end of the experiment period. Furthermore, I show that large seeds could be more vulnerable to anthropogenic driven changes in dung beetle communities than smaller seeds. These research aims were realised through field-based experiments from which I sampled and identified approximately 2,650 dung beetles from 180 naturally formed communities, collected more than 17,000 morphological trait measurements and sieved approximately 11 tonnes of soil in search of 1800 seed mimics. Overall, this work demonstrates diversity in dung beetle communities is positively associated with the ecological processes they govern but that environmental context is instrumental in modulating biodiversity-ecosystem functioning relationships. I use the outcomes from this work to discuss the challenges in describing diversity-functioning relationships across trophic levels. Finally, I highlight that ecological processes are the product of complex species-specific interactions, dependent on the biotic and abiotic environment. Therefore, predicting the consequences of anthropogenic-driven species losses for the structure and functioning of natural systems is a major research challenge