Exploring scale-dependent differences in Elton's biotic resistance hypothesis <i>vs.</i> Darwin's naturalization hypothesis in subtropical nature reserves

Elton's biotic resistance hypothesis (BRH) and Darwin's naturalization hypothesis (DNH) are foundational theories for understanding alien plant invasions, but empirical support remains inconsistent across studies. These discrepancies are often attributed to spatial scale, yet the influence of spatial scale on their applicability has not been comprehensively examined. We tested both hypotheses across spatial scales in a study system of herbaceous plants across thirteen national nature reserves in Guangxi, China. Based on a survey of 246 transects and 1,046 plots, a phylogenetic tree was constructed for 467 herbaceous species. We modeled the relative abundance, cover, and richness of invasive alien herbs as a function of phylogenetic diversity of native herbs and the mean phylogenetic distance between natives and invaders. We also assessed how climate and human activities influence these two hypotheses. Results showed that BRH was generally supported at all scales, whereas DNH was only supported at plot and transect scales. Warm, low-elevation conditions contributed positively to BRH across all spatial scales. In contrast, DNH was supported at the plot scale under cool high-elevation environments, but this relationship reversed at the reserve scale. Additionally, frequent anthropogenic disturbances promoted Darwin's naturalization processes while inhibiting biotic resistance. By demonstrating how plant invasions are shaped by spatial scales, climate, and human disturbance, our study provides both empirical evidence for invasion mechanisms along environmental gradients and scale-sensitive insights for managing subtropical nature reserves.