Spread of exotic plants in the landscape: the role of time, growth habit, and history of invasiveness

We investigated the relative contribution of minimum residence time, growth habit, and history of invasiveness to the spread of exotic plants in Michigan and California. Our data include minimum residence time as estimated by earliest herbarium collection records, growth habit, and history of invasiveness for over 2000 records from two herbaria (MI = 943, CA = 1131). Our data support the hypothesis that minimum residence time is highly associated with landscape spread, explaining 39-44% of variation in the number of counties invaded. In contrast, growth habit and history of invasiveness explained a small fraction of variation in spread in California but not Michigan. Over the past 30 years exotic plant species frequently became established in Michigan and California (a parts per thousand yen50 species per decade), suggesting that many more species will become invasive over time. There is an urgent need to develop effective policies for exotic plant management. In both states we found significant positive correlations between minimum residence time and species occurrence on state invasive plant lists. Further, we found historical information on the pest status of a plant species introduced into a similar environment to be relevant in determining landscape spread of exotic plants. We conclude that efforts to predict exotic species spread based on biological characteristics may have limited success, and instead endorse pest risk analysis for proposed new imports coupled with rapid detection and early response for unintended and unwanted introductions.