Biogeographic, climatic and spatial drivers differentially affect α-, β- and γ-diversities on oceanic archipelagos

Abstract

Island biogeographic studies traditionally treat single islands as units of analysis. This ignores the fact that most islands are spatially nested within archipelagos. Here, we took a fundamentally different approach and focused on entire archipelagos using species richness of vascular plants on 23 archipelagos worldwide and their 174 constituent islands. We assessed differential effects of biogeographic factors (area, isolation, age, elevation), current and past climate (temperature, precipitation, seasonality, climate change velocity) and intra-archipelagic spatial structure (archipelago area, number of islands, area range, connectivity, environmental volume, inter-island distance) on plant diversity. Species diversity of each archipelago (γ) was additively partitioned into α, β, nestedness and replacement β-components to investigate the relative importance of environmental and spatial drivers. Multiple regressions revealed strong effects of biogeography and climate on α and γ, whereas spatial factors, particularly number of islands, inter-island distance and area range, were key to explain β. Structural equation models additionally suggested that γ is predominantly determined by indirect abiotic effects via its components, particularly β. This highlights that β and the spatial arrangement of islands are essential to understand insular ecology and evolution. Our methodological framework can be applied more widely to other taxa and archipelago-like systems, allowing new insights into biodiversity origin and maintenance.

Keywords: archipelago; spatial variables; species richness; α-diversity; β-diversity; γ-diversity.

Figure 1.

Archipelagic plant richness (γ) and α, nestedness (β_nst) and replacement (β_rpl) components. The vascular plant floras of the 23 archipelagos analysed are highlighted in green: 1, Phoenix Islands; 2, Cook Islands; 3, Hawai'i; 4, Society Islands; 5, Marquesas; 6, Pitcairn Islands; 7, Northern Californian Channel Islands; 8, Revillagigedo Islands; 9, Galapagos Islands; 10, Juan Fernandez Islands; 11, Dutch Caribbean; 12, Azores; 13, Cape Verde; 14, Madeira; 15, Canary Islands; 16, Tristan da Cunha; 17, Balearic Islands; 18, Prince Edward Islands; 19, Aldabra; 20, Crozet Islands; 21, Inner Seychelles; 22, Marianas; 23, Kuriles.

Figure 2.

Relative importance of abiotic variables affecting vascular plant diversity components. Relative importance was obtained from AICc-weighted standardized coefficients of linear regression models including all possible factor combinations (N = 23 archipelagos). Note the different importance of biogeographic variables to α, γ and β_rpl/β_nst, and of intra-archipelagic variables to all β-components and ratios. Variables: AREA, island area; SLMP, surrounding landmass proportion; AGE, age of the oldest island; ELEV, maximum island elevation; TEMP, annual mean temperature; PREC, annual precipitation; VART, variation in annual temperature (range); VARP, coefficient of variation of precipitation; CCVT, climate change velocity of temperature; VARA, variation in island area (range); ENV, environmental volume; N, number of islands; HULL, archipelago hull; CONN, connectivity; DIST, mean inter-island distance.

Figure 3.

Direct and indirect effects of biogeographic, climatic, and intra-archipelagic spatial factors on γ-richness. Structural equation models are based on principal component analysis (PCA) axes for (a) α and β_A influencing γ; (b) ratio β_A/α influencing γ; (c) β_A replacement (β_rpl) and β_A nestedness (β_nst) influencing γ and (d) ratio β_rpl/β_nst influencing γ. (e) illustrates PCA axes and the individual contributions of single variables to PCA axes. PCA axes refer to biogeographic (BioGeo 1–2), climatic (Clim 1 and 3) and intra-archipelagic (IntraArch 1–2) variables, with the amount of variation explained by the axes given in parentheses (see legend of figure 2 for abbreviations). Blue indicates positive and red negative relationships. Explained variance (r²) is given on top of the response variables. Standardized coefficients are given for each path. p-values: *0.05 > p > 0.01, **0.01 > p > 0.001 and ***p < 0.001. Arrow width is proportional to the effect size given by the coefficient. Note that to assess the relationship between response and abiotic variables, it is necessary to consider the signals of SEM paths and PCA axis loadings.

Figure 4.

Generalized results and framework for addressing determinants of multiple components of species diversity in archipelago-like systems. The relationship strength between variables is indicated with dashed (weak effects), thin (intermediate effects) and thick arrows (strong effects). The framework is derived from vascular plant richness on oceanic archipelagos, but could be tested more widely in other archipelago-like systems (e.g. alpine communities, coral reefs, freshwater lakes, isolated seamounts, or isolated deserts).