Role of Plant Specialists in Fine‐Scale Diversity–Area Relationships (<scp>DARs</scp>) in Southern European Atlantic Coastal Dunes

ABSTRACT Questions Species–area relationships (SARs) have traditionally focused on species richness only. However, other diversity components, such as phylogenetic diversity (phylogenetic diversity–area relationships [PDARs]), can also be measured at different spatial scales, providing a more comprehensive picture of the importance of spatial scale on plant diversity. Here, we focus on coastal dune habitats along the sea‐inland gradient. We ask: (1) do fine‐scale SAR and PDAR patterns change along the dune gradient? (2) is the phylogenetic structure of plant communities scale‐dependent along this gradient? (3) does the rate of increase in taxonomic and phylogenetic diversity change along this gradient? (4) do specialist and/or generalist species influence these patterns? Location Atlantic coast of SW Europe. Methods A nested‐plot sampling design (0.0001–100 m 2 ) was applied in three dune habitats. Diversity–area relationships were calculated based on taxonomic (TD) and phylogenetic (PD) diversity. Null models were also applied to the latter to partial out the effect of TD on PD (PD SES ) and to analyse the phylogenetic structure. Changes in the rate of increase in taxonomic and phylogenetic diversity at a fine scale along the dune gradient were analysed by means of z ‐values. All analyses were performed for the whole species pool and for the subsets of dune specialists and generalists. Results TD and PD increased inland along the dune gradient with a significantly major contribution of specialist species. PD SES mean values in embryo and mobile dunes were positive at all grain sizes, suggesting overdispersion. However, when individual PD SES values were analysed at the different grain sizes, the prevailing phylogenetic structure in these habitats did not deviate from random expectations. In fixed dunes, PD SES suggested a decrease in phylogenetic clustering that was partly confirmed when the individual values of PD SES were analysed at different grain sizes. z ‐values for TD and PD increased inland along the dune gradient, while for PD SES these values were close to zero in all three habitats. Again, specialists contributed significantly to the patterns in z‐values. Conclusions Dune specialists played a major role in the observed taxonomic and phylogenetic diversity patterns in Atlantic dune habitats along the sea‐inland gradient, even in fixed dunes where the contribution of generalists was higher. Once the effect of TD on PD was removed, the effect of area on phylogenetic diversity was at most modest in the three dune habitats.