Extending the suite of bioclim variables: a proposed registry system and case study using principal components analysis

The so-called BIOCLIM variables have played a central role in the advancement of ecologicalmodellingmethods for correlative species distribution modelling. We propose to establish a register of the BIOCLIMvariables, allowing for the extension of the suite of described, readily available covariate data tosupport bioclimatic modelling. The registry will provide a simple means for researchers to add usefuldata sets in a transparent, documented manner. As a case study, we introduce and describe the set offivemost significant principal components of the first 35 BIOCLIM variables (Bio01 to Bio35) and proposethey be assigned as Bio36 to Bio40.Together, these five PCA variables capture more than 90%of thevariability in the full suite of 35 BIOCLIM variables. The PCA variables may provide a means to explore the climaticlimits of poorly known species, with a reduced risk of overfitting the models. The BIOCLIM variable registry should facilitate the expansion of the suite of variables commonly used in species distribution modelling.