Why does plant diversity generally decline eastward across the Pacific Islands?

The decline in plant diversity across the Pacific Islands as one moves eastward is primarily attributed to the increasing distance from major source areas of vegetation, such as Indonesia and New Guinea.1

This pattern is consistent with the theory of island biogeography, which posits that the number of species on an island is inversely proportional to its distance from a mainland or other source of colonization. As islands become more remote, the probability of successful dispersal and establishment for various plant species decreases. While some plants possess mechanisms for long-distance dispersal—such as buoyant seeds or fruits that can travel across oceans—the overall generic variety of flora diminishes the further an island is located from the western Pacific margin. Additionally, the smaller size and limited habitat variety of many eastern oceanic islands further contribute to lower species richness compared to the larger, more diverse continental islands found in the west.2314