We reexamined geographic factors explaining the number of plant species on islands in the Galapagos Archipelago. We hypothesized that plant species richness (S) was related to the number of source pools and that plant species dispersal preferentially followed direct, oceanic pathways. To test different dispersal pathways from multiple source pools, the total number of islands within a given dispersal radius (i) was posed as the sum of the number of line-of-sight islands (C-i) and of the number of islands without line-of-sight connection (B-i). In partial regression analyses, controlling for nearest island area (A(2)) and for recipient island elevation (E) and area (InA), C-i and C-i x E were found to be positively correlated with S in the Galapagos for nearly all dispersal ranges from 10 km to 419 km (maximum inter-island separation). In contrast, B-i x E was negatively correlated with S at the longest dispersal ranges. The connectivity index, C-i, multiplied by elevation, E, explained more variation in S in the Galapagos than prior regression models using additive forms of E, InA, A(2), and isolation from the central island. Using the variables C-i x E and InA, multiple-regression models explained \u3e 90% of the variance in both endemic and total plant species richness in the Galapagos Archipelago
