Correlations between phylogenetic and functional diversity: mathematical artefacts or true ecological and evolutionary processes?

Abstract

AbstractQuestionsIs phylogenetic diversity (PD) an accurate surrogate for functional diversity (FD)? How areFD:PDcorrelations affected by the diversity index used, covarying factors and/or the strength of the phylogenetic signal in ecological traits?LocationField study, coastal marsh plain Mekhada, Algeria, complemented by simulated data.MethodsFDandPDindices might correlate simply because variation in species richness and evenness (referred to as co‐factors) influences bothFDandPDvalues. We partitionFDandPDindices into components influenced by species richness, evenness and species' (functional and phylogenetic) characteristics. When a simple partition was not found, comparison to null models was used to remove the effects of co‐factors. We examined correlations between tenFDandPDindices, among which several were shown to be connected using our mathematical partitioning approach and several were transformed by comparison with null models to control for effects of co‐factors. In doing this,FDvalues were calculated using simulated trait values with varying phylogenetic signal. We then selected a subset of complementaryFDandPDindices in exploring the influence of environmental variables on diversity across 75 plant assemblages in Mekhada.ResultsAltogether, mathematical partitioning and the comparison to null models successfully removed the effects of co‐factors when comparingFDandPD. For all indices affected by species richness,FD:PDcorrelations approached 1, irrespective of the trait evolution model used. In contrast, simulations showed thatFD:PDcorrelations measured with indices unaffected by co‐factors decreased when the phylogenetic signal in traits decreased. Applied to plant assemblages in Mekhada, complementary diversity indices showed that, despite significant (but moderate)FD:PDcorrelation,FDbut notPDwas significantly correlated with the main stress gradient (salinity).ConclusionsFrom both our simulations and analysis of plant community diversity,PDwas a poor surrogate forFD. In Mekhada,PDwas also less correlated with environmental variables thanFD. Species richness was found to be a better surrogate forFDthanPDin identifying the ecological processes that distribute species along the salinity gradient.