Description of the variables in the dataset associated to the following work: Pulido F, B Castagneyrol, F Rodríguez-Sánchez, E Moracho, Y Cáceres, A Pardo, J Kollmann, F Valladares, J Ehrlén, AS Jump, J-C Svenning & A Hampe. Asymmetry in marginal population performance foreshadows widespread species range shifts. - "study.id" Unique identifier for each study. - "study.sp.id" Unique identifier for each species in each study. - "study.sp.vble.id" Unique identifier for each variable and species in each study. - "study.sp.vble.reg.id" Unique identifier for region/margin, variable species, and study. - "case.id" Unique identifier for comparisons margin vs centre. - "kingdom" Animals or Plants. - "habitat" Terrestrial or Marine. - "species" Species name - "variable" Performance variable measured. - "fitness" Category of performance variable. Either 'Survival', 'Reproduction' or 'Lifetime fitness'. - "margin" Either HLM (High-Latitude Margin) or LLM (Low-Latitude Margin). - "m.center" Average performance in central populations. - "sd.center" Standard deviation of average performance in central populations. - "n.center" Number of central populations studied. - "m.margin" Average performance in marginal populations. - "sd.margin" Standard deviation of average performance in marginal populations. - "n.margin" Number of marginal populations studied. - "mult.comp" Indicator variable for specifying appropriate comparisons in the meta-analysis. - "yi" Overall difference of performance in marginal compared to central populations (effect size, Hedges' d). - "vi" Variance of the difference of performance in marginal compared to central populations (effect size, Hedges' d). - "n.uncorrected" Total number of populations in the study. - "ni" Number of independent populations in the study. - "Tmean" Average temperature in marginal populations (1990-2013). - "Tmean.centre" Average temperature in central populations (1990-2013). - "Tmean.dif" Difference in the average temperature between marginal and central populations (1990-2013). - "ref" Reference of the study. - "year" Year of publication. - "journal" Journal of publication. - "citation" Full citation.