Distinguishing the Direct and Indirect Effects of Island Area and Isolation in Regulating Plant Disease and Herbivory in Tropical Islands

Although habitat area and isolation are considered key factors influencing plant disease and herbivory, their specific mechanisms remain underexplored within island biogeography. We investigated the direct and indirect effects (via community functional traits) of island area and isolation on plant disease and herbivory across 21 tropical islands in the South China Sea. Community-weighted mean leaf area (CWM LA) was the most significant functional predictor at the community level. Decomposing CWM LA revealed isolation primarily drove species turnover (LASTE; 99. 99% of the relative explanatory power), whereas area mainly influenced intraspecific variation (LAITV; 60. 42% of the relative explanatory power). Isolation-mediated species turnover indirectly amplified disease and herbivory, likely through adaptive trait shifts favouring resource acquisition. Island area had no significant effect on disease and herbivory at the community level. This study reveals the complex roles of area and isolation in plant biotic risks, underscoring the utility of island biogeography theory.