Biogeographical determinants of pteridophytes and spermatophytes on oceanic archipelagos

Abstract

Using the data from six oceanic archipelagos, we investigated the species richness patterns on islands for all natives, archipelagic endemics and single-island endemics of pteridophytes and spermatophytes. We tested the hypothesis that the descriptive ability of biogeographical factors for species is reduced as we move from native species, to archipelagic endemics and to single island endemics, because of the increasing importance of island ‘idiosyncrasies’ (i.e. unique features of each island shaping its biota, such as catastrophic volcanic eruptions, random colonization events, mega-landslides) in controlling the species richness of endemic species. This hypothesis was addressed using two approaches: (1) the island species–area relationships (ISARs), and (2) a multiple regression approach with variable selection based on permutation, to test the combined effects of island area with other biogeographical factors. Area was an effective predictor of species richness for all native species (R2 = 0.568 and R2 = 0.624 for pteridophytes and spermatophytes respectively), but its predictive capacity decreased for archipelagic endemics (R2 = 0.261 and R2 = 0.531) and single-island endemics (R2 = 0.084 and R2 = 0.438). The reduction of R2 from all native species, to archipelagic endemics and to single-island endemics was attributed to the increasing effects of the ‘idiosyncrasies’ of each island. The predictive capacity of multiple regression models increased with respect to ISARs, ranging from 27.3% (for single-island endemic pteridophytes) to 83.3% (all native pteridophytes), and included three to five predictors. Island area remained the most important variable for spermatophytes but was less important for pteridophytes. For pteridophytes, elevation was the most important predictor for native species, while isolation-related variables were the most important predictors for archipelagic endemics and single-island endemics. Our results support the hypothesis that as we move from native, to archipelagic endemic and to single-island endemic species the predictive ability of models is reduced, indicating an increased effect of the ‘idiosyncratic’ character of islands.