Use of Geographic Profiling for tracing a biological invasion: the case of Caulerpa taxifolia in the Mediterranean

Geographic profiling is an analytical method used for investigating a series of crimes in order to identify the origin (the home of the offender) of them (1). This method uses data given by the position (x, y) of the investigated facts on a map. The analysis is based on a function that reduces the probability of the fact increasing the distance from the origin and another function that introduces an exclusion zone around the origin where the probability of the fact would be very low. The attempts of using the method to find the origin of biological events on a map is related to the presence and the meaning of the exclusion zone, that should be evaluated case by case. The use of this method for the identification of the origin of a biological invasion is justified by the hypothesis that the exclusion zone may be related to the increase in efficiency of the dispersal mechanisms of the organisms spreading in the territory and, as a matter of fact, the method proved to be useful in various cases of biological invasions, for instance for the spreading of two species of Caulerpa (Chlorophyta) in the Mediterranean (1) and of a pest (2). Enhancements of the method are based on the use of other analytical tools that can deal with 1) multiple origins for an invasion (Fig. 1); 2) non-centrifugal patterns of dispersal; 3) very different quantitative meaning of each observations on the territory; 4) the proposal of robustness indexes for the obtained results. The first point was faced with the use of Kmeans, Voronoi tessellation and Silhouette for finding the most probable number of subclusters (3); of the Dbscan method for not centrifugal dispersal (4); of a weighted method for cases in which the observations have a very different importance point by point (5), while the robustness of the results may be assessed with Jackknife (6).