Island area and climate jointly impact seed plant richness patterns across the Zhoushan Archipelago

Aims: Islands are an ideal platform for investigating ecological processes that shape biodiversity assemblages because of their distinct boundaries. An island’s physical characteristics, climate, human impacts and other drivers shape an island’s biodiversity pattern by affecting species selection, dispersal and other processes. However, our knowledge around how such drivers concurrently affect island plant richness remains limited, especially on oceanic islands with strong human impacts. Methods: We complied a comprehensive plant distribution database across 92 islands in the Zhoushan Archipelago, the largest archipelago in China. We then used general linear regression and generalized linear model (pseudo-Poisson distribution) to evaluate the effects of island’s physical characteristics (area, isolation and shape index), climate (temperature, precipitation and seasonality) and human impacts on the richness of native seed plants, and across different growth forms and leaf phenology types. We also used beta regression to evaluate the influence of environmental variables on the ratio of evergreen broad-leaved woody plant richness to all broad-leaved woody plant richness. Results: In total, there were 1,158 seed plant species, including 108 tree species, 318 shrub species and 732 herbaceous species. The strongest driver of plant richness was island area, followed by isolation and annual precipitation. Tree richness decreased with increasing isolation, and this trend was most notable among trees than among shrubs and herbs. The richness of evergreen and deciduous broad-leaved woody plants was overall consistent with that of all plant richness. We found that annual precipitation had a stronger effect on evergreen broad-leaved woody than on deciduous, but the ratio of evergreen woody was only strongly affected by temperature seasonality. Conclusion: We found that island area, annual precipitation, and temperature seasonality are the primary determinants in shaping the richness patterns of all plants and different functional groups (growth forms and leaf phenology types) across the 92 islands of China’s Zhoushan Archipelago.