Invasive rat impacts on insular invertebrate biodiversity: investigating the selective mechanism of rat predation and the taxonomic and functional implications for island ecosystems
Final Report Abstract
Biological invasions are the predominant threat to island biodiversity. Among island invaders, rats (Rattus spp.) are especially harmful, being responsible for 30% of recorded extinctions. While their impacts on island birds are well documented, effects on invertebrate communities – and the mechanisms driving selective predation – remain poorly understood. This Walter Benjamin project applied theoretical advances in nutritional ecology to test the mechanism for selective predation pressure on invertebrates, and to assess consequences for ecosystem functioning. Nutrient intake targets of invasive rats (R. exulans) were quantified. Contrary to expectations of a single, species‐specific intake target, 84% of tested animals aimed at a 10.7% protein intake target, while 16% balanced at 43.5% protein. Statistical analysis confirmed two distinct intake strategies. Follow‐up measurements showed that the high‐ protein group consisted of large adult males and breeding females, suggesting that body size, sex, and physiological state elevate protein demand. Ongoing laboratory analyses will further clarify these links. To relate nutrient intake target findings to natural foraging, metabarcoding on rat stomach contents from an invaded island site were conducted. Sequencing read abundances of animal (high-protein) versus plant material (low-protein) mirrored feeding‐trial clusters: roughly 85% of R. exulans showed low animal intake (< 25% of sequencing reads), whereas 15% displayed up to 92% animal content. R. rattus consumed more animal matter overall, with a significant correlation between animal diet prevalence and rat size and sex. An island rat eradication operation was initially planned to be utilised by this project for investigating the recovery trajectories of island invertebrate communities in relation to the nutrient intake target matching, and for quantifying recovery of arthropod-mediated ecological functions. This operation was unsuccessful, thus precluding before‐after comparisons. However, two years of systematic invertebrate sampling generated a dataset on island invertebrate community dynamics, turnover, and stability. Using a space‐for‐time design across islands (8 invaded, 4 rat-free), trait‐based functional analysis revealed effects of invasive rat presence on invertebrate functioning. Herbivore and decomposer functions were reduced under rat presence, although native forest cover emerged as the stronger predictor for overall invertebrate functional integrity. Together, the results of this project advance a mechanistic, generalizable framework for invasive‐rat predation on island invertebrates, rooted in differential protein demands by invasive rats. The findings have relevance for invasive species management, indicating duality in feeding preferences and, thus, bait attractiveness. Completion of follow-up analyses will solidify causal pathways, enabling robust models of invasive rat foraging and more effective conservation interventions.
