Effects of the spatial scale and the type of predictor on cross-taxon congruence

A major challenge in conservation planning and in biodiversity surveys is the identification of reliable surrogate taxa as effective shortcuts for the preservation of the biodiversity. The effectiveness of the surrogate taxa depends on different factors, like the spatial scale of analysis and the type of predictor variable. In this work, the effect of the grain size and the choice of predictor variables was explored on the strength of the community congruence relationships among vascular plants, bryophytes and lichens. Data for these three taxa collected by a multiscale design based on a restricted random sampling applied in the “Bosco di S. Agnese” Nature Reserve(Tuscany, Italy). CoHcorrespondence analysis (CoHCA) was performed on (i) two different response taxa (e.g. bryophytes and lichens presence/absence data) considering (ii)three different matrices of predictor variables (presence/absence of vascular plant species, abundance of vascular plants and a structureHbased dataset) for (iii) the three grain sizes available (1m2, 100m2, 1000m2). Results obtained from the CoHCA were analyzed using threeHway ANOVA. From a general point of view, our results revealed weak explanatory power of vascular plant composition with respect to bryophyte and lichen communities. Noteworthy, the effects of the spatial grain and the type of predictor variable on the strength of congruence among the considered taxonomic groups provided two main results: i) the amount of explained variance varies with the grain size; ii) the sign of observed relationships mainly depends on the analyzed taxon and on the choice of predictor variables. Concluding, the choice of grain size, taxon and type of predictor variables are so essential that could be the cause for the inconsistent results observed in previous studies examining the congruence among taxonomic groups and potential effectiveness of surrogate taxa.