Explaining intricate morphometric variability with environmental predictors: the case of Globularia cordifolia species complex

Globularia is a genus of small evergreen and perennial shrubs that are widespread in Europe. Globularia section Empetron includes a group of three species, G. cordifolia, G. meridionalis, and G. neapolitana, that have been taxonomically disputed for more than 150 years. Many morphological features have been proposed to discriminate these species. Nevertheless, evidence from both past and recent literature suggest that these differences among species are not consistent. In order to shed new light in this long-disputed group, we investigated 10 populations of the G. cordifolia species complex with both classical and geometric morphometrics and used environmental predictors in multivariate regression to explain patterns of variation. Our results showed that bract area and calyx teeth length are correlated with solar radiation and annual precipitation, whereas leaf dry mass per unit area can be explained by temperature seasonality. Leaf shape can be explained by temperature seasonality as well, although with a lower amount of explanatory and predictive power. Despite a comparatively low sample size in terms of populations, our results were based on a large number of individuals and were supported by a robust statistical approach. We can conclude that differences among the three species of Globularia could be related to the combined effects of several ecological variables and might not have taxonomical value. Our novel approach provided an ecological interpretation on a species complex that makes up a continuum of forms within the environmental framework of the Mediterranean basin.