Herbivores in a small world: network theory highlights vulnerability in the function of herbivory on coral reefs

1. The application of network theory within the field of ecology has predominantly been based around the examination of species interactions (e.g. food webs). Extension of networks to spatial ecology is currently limited, yet the approach has significant potential as a framework for analysing habitat connectivity and ecosystem stability. Examination of network structures characterizing the spatial dynamics of organisms has the potential to highlight the relative robustness or vulnerability of the ecosystem processes they support. 2. On coral reefs, grazing by large herbivorous fishes is critical in maintaining the balance between coral and algae and is widely recognized as a key process in reef ecosystem functioning. However, the spatial dynamics exhibited by roving herbivores, and the implications of these dynamics for the overall stability and resilience of herbivory on reefs are not well understood. 3. Here, we present a novel extension of network theory to the analysis of movements of marine herbivorous fishes within a reef system. Using an array of remote acoustic receivers, over a period of 12 months, we tracked the movements of fishes belonging to the three dominant species of roving herbivore within a particular coral reef community. 4. Analysing these movements in terms of network graph theory, we found that individual fishes clustered their activities within small sections of the available linear reef habitat, and movements within and outside of these home ranges were made along predictable routes. All three species were heavily reliant on particular, well-connected parts of the reef (nodes), mirroring an 'ultra-small-world' network. This underlying dynamic creates vulnerability in the system, making targeted removal by fishing or the degradation of individual pockets of reef habitat more likely to result in a complete collapse in the overall process of herbivory at that particular area of reef. 5. Our results highlight an inherent vulnerability in the movement dynamics of herbivorous fishes and, by extension, in the overall process of herbivory in reef ecosystems. They suggest that maintaining high herbivore abundances across all reef habitats is critical in preserving the integrity of the grazing function on reefs. The approach of using network graph theory to analyse acoustic telemetry data is illuminating and is likely to have a range of applications across diverse ecosystems.