How do islands become green?

Abstract

Aim Four long-distance dispersal (LDD) modes have generally been considered to play central roles in the colonization of islands by plants: anemochory (dispersal by wind), thalassochory (dispersal by oceanic currents), endozoochory (internal dispersal by animals) and epizoochory (external dispersal by animals). However, seeds can also be transported by vectors different from those to which they are best suited (non-standard dispersal), meaning that the actual vector of colonization cannot be inferred based on diaspore traits alone. We propose an alternative approach to explore the relative contribution of LDD syndromes to island colonization. Location Europe and the Azores. Methods We scored the presence of syndromes relevant for LDD in the native flora of Europe (c. 10,000 species) and the Azores (148 species). We then contrasted the importance of each syndrome in the recipient flora (Azores) and the source floras (Europe and mainland Portugal) to estimate which, if any, syndrome was particularly successful for overseas colonization. We further investigated whether particular LDD syndromes increased plant distribution within the Azores archipelago. Results Most native species in Europe (63%), mainland Portugal (67%) and the Azores (63%) produce unspecialized diaspores. Only species adapted to sea dispersal were overrepresented in the Azores, while those adapted to wind dispersal were underrepresented. The presence of LDD syndromes did not significantly improve the distribution of plant species across the archipelago, except for the moderate advantage of endozoochorous diaspores. Differences in the importance of LDD syndromes across plant families at least partially explain the floristic disharmony of the Azorean flora. Main conclusions Only thalassochory appeared to have significantly favoured the colonization of the Azores. The high proportion of unspecialized diaspores, the unexceptional representation of most specialized LDD syndromes and the dissociation between syndromes and inter-island plant distribution in the Azores suggest that non-standard events are more common than previously believed in the colonization of islands by plants.