Résumé
P>1. Island floras are generally characterized by disharmony due to uneven representations of plant groups. This has usually been explained by the unequal capacity of plants to disperse and colonize islands, but it has been suggested that the ecology of the recipient islands may also be important.
2. We investigated the phylogenetic structure of the flora of New Caledonia, a biodiversity hotspot in the South-West Pacific, to examine the phylogenetic pattern of a disharmonic flora and to test two competing hypotheses: disharmony due to uneven dispersability versus disharmony due to uneven ability to adapt to the ultramafic soils that are widespread on the archipelago and constrain plant growth.
3. Dispersal did not seem to be a significant factor as plant families with low levels of endemism and thus high effective dispersal capacities tended to be under-represented.
4. We identified several clusters of over-represented families, such as the COM clade (Celastrales, Oxalidales and Malpighiales), in which nickel hyperaccumulation (a syndrome associated with ultramafic soils) was particularly common. We indeed found a positive association between hyperaccumulation and over-representation. Therefore, ultramafic soils may have been a major force for species sorting in the flora of New Caledonia.
5. Because we think that most colonizers were naive to ultramafic soils before they reached the archipelago, the over-represented groups in New Caledonia may have possessed an exaptation to ultramafic soils, which has facilitated their settlement and diversification.
6. Synthesis. This study supports that beyond dispersal, ecology is an important factor in determining the success of different plant groups to colonize an island. In the case of New Caledonia, exaptation to ultramafic soils is likely a primary cause of ecological filtering.