The island rule- a general pattern of dwarfism in large species to gigantism in small species on islands relative to mainland- is typically seen as a macroevolutionary phenomenon. However, it remains unknown whether the ecological processes associated with abiotic and biotic factors generate a pattern of plant size variation similar to the island rule. Through measuring plant height for 29623 individuals of 50 common woody species in the Zhoushan Archipelago (8500 years old and yet to undergo major evolutionary adaptation) and the adjacent mainlands in Eastern China, we examined whether island area and remoteness, resource availability, environmental stress, plant-plant competition and insect herbivory can explain the pattern of plant size variation. We found pronounced variations in plant height, similar to those of the island rule. Further analyses revealed that islands with low resource availability, such as low soil organic matter content and low precipitation, had a high degree of dwarfism; islands experiencing high environmental stress, such as high soil pH, had a high degree of dwarfism; and islands experiencing less plant-plant competition had a high degree of gigantism. The magnitude of plant dwarfism was also higher on small and remote islands than on larger and nearer islands. These results highlight the importance of ecological processes associated with abiotic and biotic conditions in shaping the island rule-like patterns of plant size variation. Our study therefore suggests that the island rule can be caused by both ecological and evolutionary processes. Given that the age of our studied archipelago is too young to undergo major evolution, our results evidenced that ecological processes likely played a prominent role for generating the island rule-like patterns. Future studies on the island rule need to perform experiments to disentangle evolutionary from ecological mechanisms.
