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

Aims: Elton's biotic resistance hypothesis (BRH) and Darwin's naturalization hypothesis (DNH) are foundational theories for understanding alien plant invasions, yet their predictions often diverge. Since these discrepancies are frequently attributed to spatial scale, the scale‑dependence of hypothesis applicability remains underexplored. We aimed to (i) assess the respective applicability of BRH and DNH across spatial scales, and (ii) examine how climate and human disturbance modulate their explanatory power in subtropical herbaceous communities.Location: Thirteen national nature reserves, Guangxi, China.Major taxa studied: Herbaceous plants.Methods: We surveyed 467 herbaceous species across 246 transects and 1,046 plots. A phylogenetic tree was used to calculate the phylogenetic diversity of native herbs (PD) and the mean phylogenetic distance between natives and invaders (MPD). Using generalized linear mixed-effects models, we analyzed how the relative abundance, cover, and richness of invasive alien herbs responded to PD and MPD across different spatial scales (i.e., plot, transect, and reserve). We further examined how these response patterns were modulated by climatic conditions (i.e., temperature, precipitation, and elevation) and anthropogenic disturbance.Results: Our results provided evidence for the BRH across all spatial scales, whereas the DNH was only supported at the plot and transect levels. Warmer, low-elevation environments consistently enhanced biotic resistance. In contrast, the DNH received empirical support in cooler, high-elevation environments at the plot scale, although support for this hypothesis reversed at the reserve scale. Human disturbance promoted naturalization while weakening native resistance.Main Conclusions: From a phylogenetic perspective, higher diversity among native herbaceous species was associated with stronger invasion resistance, whereas invaders that were more distantly related to natives tended to colonize more successfully. Notably, the DNH pattern was more sensitive to spatial scale variation than the BRH. Warmer climates enhanced resident resistance, while human disturbance facilitated invasive colonization. These findings offer scale-dependent insights for managing plant invasions and inform conservation strategies in subtropical protected areas.