Additive partitioning as a tool for investigating the flora diversity in oceanic archipelagos

This paper introduces the integration of additive partitioning with species-area relationships to island biogeography in order to address the question "How are the pteridophyte and spermatophyte native and endemic flora of different oceanic archipelagos partitioned across islands?". Species richness data of all endemic species and all native species of pteridophytes and spermatophytes were obtained for the Azores, Canaries and Cape Verde in the Atlantic Ocean and Galápagos, Hawaii and Marquesas in the Pacific Ocean. Additive partitioning of species diversity was used to quantify how much of the total diversity of an oceanic archipelago flora (γ-diversity) is due to (i) the mean species richness of the flora of each island (α-diversity), (ii) the variability in species richness of the floras across islands (βNestedness) and (iii) the complementarity in species composition of the floras of different islands (βReplacement). The analysis was separately performed for the native and endemic pteridophyte and spermatophyte floras. The diversity partitioning of the six archipelagos showed large differences in how the flora of each archipelago is partitioned among the α, βNestedness and βReplacement components, for pteridophytes and spermatophytes and for all endemic species and all native species. The α-diversity was more important for all native species than for endemic species and more important for pteridophytes than for spermatophytes, with the Azores showing outstanding high values of α-diversity. The βNestedness was higher for pteridophytes than for spermatophytes and higher for endemic species than for all native species in both pteridophytes and spermatophytes. The values of βReplacement suggested that: (i) the spermatophyte native flora is more differentiated across islands than the pteridophyte native flora and (ii) the pteridophyte endemic flora and, especially, the spermatophyte endemic flora are more differentiated across islands than the corresponding native flora. An outstanding value of βReplacement for endemic and all native spermatophytes was found in Hawaii, confirming the biogeographical island differentiation in this archipelago.