Global patterns and drivers of phylogenetic structure in island floras

Abstract

Islands are ideal for investigating processes that shape species assemblages because they are isolated and have discrete boundaries. Quantifying phylogenetic assemblage structure allows inferences about these processes, in particular dispersal, environmental filtering and in-situ speciation. Here, we link phylogenetic assemblage structure to island characteristics across 393 islands worldwide and 37,041 vascular plant species (representing angiosperms overall, palms and ferns). Physical and bioclimatic factors, especially those impeding colonization and promoting speciation, explained more variation in phylogenetic structure of angiosperms overall (49%) and palms (52%) than of ferns (18%). The relationships showed different or contrasting trends among these major plant groups, consistent with their dispersal- and speciation-related traits and climatic adaptations. Phylogenetic diversity was negatively related to isolation for palms, but unexpectedly it was positively related to isolation for angiosperms overall. This indicates strong dispersal filtering for the predominantly large-seeded, animal-dispersed palm family whereas colonization from biogeographically distinct source pools on remote islands likely drives the phylogenetic structure of angiosperm floras. We show that signatures of dispersal limitation, environmental filtering and in-situ speciation differ markedly among taxonomic groups on islands, which sheds light on the origin of insular plant diversity.

Figure 1. General framework for testing hypothesized effects of dispersal filters, environmental filters and in-situ speciation on the phylogenetic structure of island assemblages.

Clade 1 represents good dispersers, clade 2 weak dispersers. Clades a and d share adaptations to environmental conditions on islands, clades b and c do not. If dispersal-related traits and environmental adaptations are not randomly distributed over the phylogeny, then dispersal and environmental filters should increase the probability of island colonization in certain clades and increase phylogenetic clustering of the island assemblage. Although such phylogenetic clustering on young islands is initially mostly observed relative to the mainland species pool, radiations within island lineages and extinction due to environmental changes should further increase phylogenetic clustering in some islands relative to a pool of island species. The strength of dispersal and environmental filters and the probability of in-situ speciation on islands should be related to the listed physical, geologic and bioclimatic island characteristics. Symbols beside environmental variables (not indicated for categorical variables) indicate the hypothesized relationships with the standardized effect sizes of phylogenetic diversity (PD_es) and mean pairwise phylogenetic distance (MPD_es): – negative, + positive, U U-shaped.

Figure 2. Global patterns of (a) species richness and (b) Faith’s phylogenetic diversity (PD) for all angiosperms, palms and ferns on islands.

PD was calculated as the sum of all branch lengths representing the species of an island in a clade’s phylogeny excluding the root, based on a dated family-level phylogeny for angiosperms and on dated genus-level phylogenies for palms and ferns. PD is shown only for islands with at least two species of the focal group (363 of 375 islands for all angiosperms, 71 of 386 for palms and 234 of 328 for ferns). Species richness is given in numbers of species, PD in billion years. Numbers in legends indicate category borders. Maps were created using the statistical programming language R.

Figure 3. Phylogenetic structure of island floras.

Phylogenetic structure is illustrated as deviations of (a) phylogenetic diversity (PD) and (b) mean pairwise phylogenetic distance (MPD) from null expectations based on insular species richness and a global species pool. Maps show results based on a dated family-level phylogeny for angiosperms and dated genus-level phylogenies for palms and ferns. The standardized effect sizes of PD and MPD (PD_es and MPD_es) were based on null models randomly shuffling all included species at the tips of the trees. Negative values indicate phylogenetic clustering, positive values overdispersion. Only islands with at least two species of the focal groups are included (363 islands for angiosperms, 71 islands for palms and 234 islands for ferns). Embedded histograms give the frequency distributions of the mapped metrics. Numbers in legends indicate category borders. Maps were created using the statistical programming language R.

Figure 4. Environmental predictors of phylogenetic structure in island floras.

Partial residual plots from averaged Generalized Additive Models illustrate the standardized effect size of phylogenetic diversity (PD_es) of angiosperms, palms and ferns as a function of environmental predictors. Models included spatial eigenvectors to account for spatial autocorrelation. Regression lines are shown if the variable was significant in the averaged model. In (a), PD_es was based on dated family-level phylogenies of angiosperms (orange) and ferns (blue). In (b), PD_es was based on dated genus-level phylogenies of palms (red) and ferns (blue). Only islands with at least two species of the focal group are shown (363 islands for all angiosperms, 71 for palms and 234 for ferns). MLSR = Mainland species richness, SLMP = surrounding landmass proportion, CCVT = Late Quaternary climate change velocity; Geologic island types: FR = continental fragment, OC = oceanic island, SH = continental shelf islands.