Small-island effect is detected for both native and non-native seed plants in the largest archipelago of China

The small-island effect (SIE) describes a particular form of species–area relationship where species richness on small islands is independent of island area or relates to area in a different way than for larger islands in the same system. The pattern has been detected in many taxa and island systems. However, most studies investigating SIEs have focused on native species, neglecting the potential influence of non-native species. Here, we used a dataset of 1,552 seed plant species distributed across 92 islands in the Zhoushan Archipelago of the East China Sea to examine the SIE, using species richness and phylogenetic diversity, as well as for native and non-native groups separately. We calculated three diversity metrics: species richness, Faith’s phylogenetic diversity (PD) and standardised effect size of PD (sesPD). We used a human influence index (HI, the proportion of human-used land to the land area of the island) to measure the level of disturbance by humans on islands, together with non-native plant data to reflect broader dimensions of human impacts. We used piecewise regression models to test SIEs for species richness, PD, sesPD and HI. We detected the small-island effect for both richness and PD, while the difference in area thresholds (i.e. the breakpoint from piecewise models) between all seed plants and native seed plants was small. Area threshold for non-native plants were only weakly affected when cultivated plants were included, but were significantly smaller than those of native plants when cultivated species were excluded. For both total and native plant richness and PD, herbs exhibited larger area thresholds for the SIE than woody plants, partly related to the recent isolation of the archipelago from the mainland, while non-native herbs exhibited lower area thresholds than woody plants. These results highlight the importance of functional groups and discontinuous (threshold) effects in contemporising island biogeography theory, particularly in relation to anthropogenic drivers. Both native and non-native seed plants exhibit small-island effects in species richness and phylogenetic diversity within islands of the Zhoushan Archipelago, China. Different plant functional groups show different responses to island area, which is reflection in both species composition and evolutionary history of the floras. Native herbs have larger area thresholds (i.e. the breakpoint from piecewise models) than native woody plants in species richness and phylogenetic diversity, likely reflecting the history of connectivity of the archipelago. Human impacts in this archipelago, especially for smaller islands, are reshaping patterns in plant diversity and functional groups.