The comparative analysis of historical alien introductions

The comparative analysis of past introductions has become a major approach in investigating the rules governing invasions, yet their utility to understand the invasion process is not exempt of problems. The relevance of some of these problems has not yet been fully appreciated, but it has now become clear that not taking them into account may lead to invalid conclusions. Taking examples of the plants' and birds' literature, this paper reviews these difficulties by discussing the comparative analysis of region invasibility. The difficulties include biased information toward successful introductions, confounded effects of many explanatory variables, statistical non-independence of introduction events and taxonomic levels, and inappropriate definition of the units of study. Provided that there is good information on introduction events at the appropriate spatial scale, reliable results may be obtained by using modelling techniques that control for the effects of introduction effort and species properties while dealing with spatial and phylogenetic non-independence of introduction events. In conclusion, although important progress can be made in understanding the factors behind invasibility of regions by the comparative analysis of the past introductions, this will only be possible by acknowledging the existence of biases and confounding effects in historical introductions and by using appropriate methods to deal with them. © Springer Science+Business Media B.V. 2007.