Land-use impacts on biodiversity and ecosystem functioning of complex multitrophic communities

Global biodiversity is rapidly declining, resulting in far-reaching impacts on the functioning of ecosystems and human wellbeing. In recent decades, anthropogenic land use has been identified as a major driver of biodiversity loss, especially through the expansion and intensification of agricultural systems. While the drivers of biodiversity loss have been relatively clearly established, variability in the way that whole ecosystems respond to these drivers is still poorly understood. This is, in part, because we still lack a clear understanding of how species interactions govern the way that complex communities respond to environmental stressors, as well as their role in mediating ecosystem functioning. Species interactions can moderate community responses to land-use change via trophic cascades, whereby extinctions at the top or bottom of a food chain produce cascading effects through the rest of the food web due to the disruption of resource availability or predatory control of consumers. Additionally, species interactions are fundamental for ecosystem functioning as they are almost always directly linked to processes such as decomposition, herbivory, predation, pollination, and seed dispersal. Therefore, an approach to studying biodiversity and ecosystem functioning of naturally complex communities that incorporates multiple trophic levels and their interactions is crucial for predicting future global-change scenarios. Despite the conceptual advantage of a multitrophic approach, this has been rarely applied in the context of biodiversity and ecosystem functioning of ecosystems undergoing land-use change. In addition, while there has been considerable evidence established for the role of biodiversity in maintaining ecosystem functioning in local-scale experiments, there is still very limited knowledge of how this relationship scales up to landscapes in real-world ecosystems. In this thesis, I aimed to achieve a conceptual advance in biodiversity-ecosystem functioning (BEF) research within the context of global environmental change by investigating responses of complex multitrophic communities to land-use change and the resulting consequences for ecosystem functioning. Firstly, in Chapter 2, I combined data from a wide taxonomic range of trophic groups to test how communities of interacting species respond to tropical land-use intensification in Sumatra, Indonesia. I employed structural equation modelling to test if land-use intensification directly impacted all trophic groups or, alternatively, if it affected only lower trophic levels, resulting in bottom-up trophic cascades. Results from this model suggested that direct land-use impacts were generally much stronger than bottom-up trophic effects. Interestingly though, the number of direct effects from land-use intensification decreased considerably from plants to predators, whereas the number of bottom-up trophic effects increased dramatically with increasing trophic level. These findings suggest that the underlying mechanisms of land-use intensification that alter communities highly depend on the trophic level in question, indicating the need for trophic level-specific conservation management strategies. The results from Chapter 2 provided strong evidence for the importance of species interactions in moderating community responses to land use, leading to the question of how ecological processes carried out by multitrophic communities are resultantly affected. One major challenge of BEF research has been to fully incorporate species interactions across multiple trophic levels to quantify a trophically broad measure of ecosystem functioning. In Chapter 3, I overcame this challenge by developing a measure of ecosystem functioning that integrates food web and metabolic theory to calculate community energy flux across multiple trophic levels. By calculating energy flux of multitrophic macroinvertebrate communities, I demonstrated that declining species diversity with increasing land-use intensity led to concomitantly strong declines in community energy flux. Furthermore, I found that the relationship between species richness and energy flux was steeper in the most intense land-use system, oil palm, but this result did not hold when trophic guilds were analysed independently. Thus, these findings suggest that if trophic groups are omitted, it is possible that BEF relationships could be misinterpreted in response to anthropogenic land use. In order to extend the previous chapter’s findings beyond the provisioning of ecosystem functioning of multitrophic communities, in Chapter 4, I investigated the functional stability and resilience of the macroinvertebrate communities to future perturbations. Using a trait-based approach, I determined how communities were assembled among different land-use types. I then calculated functional stability and community resilience by measuring the number of functionally redundant species within functional effect groups (based on traits that determine species’ influence on ecosystem processes) and the dispersion of traits within functional response groups (based on traits that determine species’ responses to disturbances). In doing so, I found that litter invertebrate communities in oil palm plantations were more randomly assembled, as well as having significantly fewer functionally redundant species. However, the jungle rubber agroforest system harboured communities with considerably higher functional redundancy than in oil palm. These results indicate that communities in high-intensity land-use systems are more susceptible to functional collapse given future perturbations, but low-intensity agroforests could help to maintain higher functional stability in anthropogenic landscapes. Finally, in Chapter 5, I investigated how ecosystem functioning varies across spatial and environmental gradients and the mechanisms that give rise to spatial turnover in ecosystem functioning. To test this, I used data on litter macroinvertebrate communities from landscapes in Indonesia and Germany and applied the energy flux calculations developed in Chapter 3 as a measure of multitrophic ecosystem functioning. I then employed structural equation modelling based on distance matrices to establish how environmental and geographic distance drive turnover in species composition, species richness, functional trait dispersion and community biomass, and how these factors consequentially drive spatial turnover in community energy flux in a tropical and temperate region. Environmental distance appeared to be more important in the Indonesian compared with the German region for driving species turnover. However, the mechanisms that determined spatial turnover in ecosystem functioning were remarkably similar between the tropical and temperate regions, such that species richness and community biomass were the most important variables explaining spatial variability in energy flux. These results suggest that mechanisms such as species identity and niche complementarity may become redundant for predicting ecosystem functioning at the landscape scale. Instead, species richness and biomass should be sufficient for predicting multitrophic ecosystem functioning at large spatial scales. Overall, in this thesis I demonstrate that species interactions are important for mediating responses of multitrophic communities to land-use intensification and that the loss of species across trophic levels has drastic consequences for the provisioning of multitrophic ecosystem functioning. Furthermore, this species loss reduces the stability of ecosystem functioning in intensified agricultural landscapes. Finally, I demonstrate that species richness and community biomass are the key components for developing a framework aimed at predicting likely scenarios of functional losses in intensified land-use systems at the landscape scale. Ultimately, by incorporating real-world complexity into studies that integrate across multiple ecological concepts, this thesis presents a significant advance toward understanding how ecosystems respond to anthropogenic land-use change, thus highlighting important areas for future exploration.