Predicting the invasion success of Mediterranean alien plants from their introduction characteristics

Abstract

We describe a model to predict the invasion success of alien plants on Mediterranean islands, according to their mode, frequency and time since introduction and simple species traits. The model was developed using canonical discriminant analysis to identify categories of low‐ and high‐risk species. Whilst classifying ca 80% of the data set correctly, the type II error rate remained moderately high, which limits its value in practical terms. This level of accuracy is typical of similar models, and critical analysis of the weaknesses is necessary to develop long‐term improvements in methodology. Sensitivity analysis indicates that, overall, mode of introduction had an important influence on invasiveness, with species introduced for public amenity being more likely to attain high‐risk status than species introduced privately to gardens. Frequency and time since introduction had a relatively minor influence on the model outcome, but remained highly confounded with other variables. Although invasion success, as measured by the number of islands colonized, is expected to increase over time, it was not possible to estimate the rates of expansion from historical trends, which are complicated by changes in import fashions over the centuries. Many older introductions were crops, whereas there has been a recent tendency towards exotic ornamentals and weedy species. Despite a fundamental influence on the results, similar historical influences are seldom taken into account adequately during trait analyses. Unravelling such inter‐correlations between predictors is therefore an important challenge for the future of screening protocols.