Why is habitat breadth correlated strongly with range size? Trends amongst the alien and native floras of Mediterranean islands

Aim To identify the factors determining the relationship between habitat breadth and range size in the Mediterranean alien flora. Strong relationships have previously been uncovered between the two variables, which raises the possibility that habitat breadth could be used as a powerful predictor of invasiveness. However, previous investigations have yet to explain the phenomenon adequately, and greater understanding is required to establish its importance. Location Ninety-one islands in the Mediterranean basin. Methods I examined a data base of Mediterranean naturalized plant species created for the EU project EPIDEMIE, and containing information on island distributions, habitat occupancy and other descriptive parameters compiled from published floras. As a null hypothesis, I assumed that the relationship between habitat breadth and range size was a spatial scale artefact: if species colonize habitats at random, with the likelihood determined by a weighted probability in each habitat, then perceived habitat breadth will increase with the number of populations examined. I modelled this effect, and also considered circumstantial evidence (via indirect indicator variables) for six hypotheses to determine whether both habitat breadth and range size may be jointly driven by a major third factor (introduction frequency, residence time, dispersal ability, generation time, dependence on mutualisms, release from herbivory). The explanatory power of the hypotheses was compared using correlative statistics. Results When habitat breadth was modelled according to the null hypothesis, the form of its relationship with range size was very similar to that from the observed data, and the predicted range size estimates explained almost half the observed variance. The unexplained component may have been due to the same mechanism operating at finer spatial scales. From the direction of the residual effect, increases in habitat breadth were probably driven by range size rather than vice versa. None of the alternative hypotheses were found to be significant predictors of habitat breadth and range size in invasive species. Main conclusions Although a number of minor explanations cannot be ruled out (e.g. that widespread species have greater regional genetic diversity enabling them to adapt to local conditions), it seems likely that the habitat breadth effect is largely artefactual and does not have particularly great value as a predictor of invasiveness.