This study concerns relationships between gamma diversity and beta diversity (species turnover) along a resource gradient. Model sites are the Hawaiian Islands, where the regional floristic species pool of tropical rain forests is much impoverished compared with Borneo, where the regional species pool is two orders of magnitude greater than in Hawai'i. I examined the relationship between changes in percentage floristic similarity (PS) and changes in the soil P pool among pairs of plots located in these two regions. PS sharply decreased with increasing difference of soil P pool in Borneo (Mount Kinabalu), whereas PS did not significantly change with increasing difference of soil P pool in Hawai'i. I suggest that a greater number of tree species that have high P-use efficiencies occur in Borneo due to its high gamma diversity. Stronger competitive exclusion because of the high gamma diversity will lead to displacements by species of progressively greater P-use efficiencies along a soil P gradient. Consequently, niche divisions along the soil P gradient will be formed, causing a greater beta diversity in Borneo. By contrast, a single species, Metrosideros polymorpha, monodominates the entire soil P gradient in Hawai'i, leading to nonexistence of beta diversity along the soil P gradient. There are important implications of beta diversity for ecosystem maintenance. I suggest that a greater beta diversity can maintain forest biomass and productivity against soil P decline by providing more P-use efficient species along the gradient. © 2012 by University of Hawai'i Press.