U.S. Department of Agriculture: Forest Service -- National Agroforestry Center


Date of this Version



New Phytologist (2013) 197: 9–10.


Forest insect and pathogen species are expanding their geographical ranges through international trade at a rate that most pest specialists and ecologists find alarming. While many invaders are relatively innocuous, several species have damaging impacts on agricultural and natural resources. Furthermore, some of these non-native pests have had catastrophic impacts on ecosystem functions when they invade native communities in which they have no prior evolutionary history. Examples include the demise of chestnut trees in North America, major losses of elms in Europe and North America, Jarrah dieback in Australia, and the devastating effects of pine wood nematode in Asia. Predicting which insects or pathogens will become most problematic and devising mitigation measures to reduce the risk of their arrival and establishment has become the ‘holy grail’ for many entomologists and plant pathologists worldwide. Analysis of historical data is an essential tool for identifying important invasion pathways and weak links in the chain of biosecurity measures that must be strengthened to protect local economies and ecosystem stability. In this issue of New Phytologist, Santini et al. (pp. 238–250), provide a comprehensive and insightful analysis of historical forest pathogen establishment and spread records from Europe. A product of a collaborative (20 nations) European Union-funded project, the paper identifies dominant plant pathogen invasion pathways and key factors predicting pathogen species invasiveness, as well as habitat characteristics that render certain regions more vulnerable to pathogen invasion. All of these associations have implications for preventing future invasions.

In their paper, Santini et al. observed dramatic changes over time in the sources of forest pathogens establishing in Europe. The earliest known pathogen species establishments were largely a result of intra-European spread. North American species first trickled in during the late nineteenth century, and became even more important during the twentieth century. More recently, Asia has become a major source of pathogens. These changes reflect Europe’s expanding sphere of trading partners, particularly for live plant imports, the primary invasion pathway identified in this work and others (Kenis et al., 2007; Liebhold et al., 2012). These observations also suggest a potential ‘invasive species source depletion’ in which a long history of trade with a given source country results in early establishment of many species followed by fewer invasions as the stock of aggressive invaders becomes depleted. However, the authors correctly observe that there is danger associated with new intra- European invasions originating from previously trade-isolated member states, particularly given the lack of trade barriers currently in place within the European Union.

A surprising wealth of exotic pathogen species established in Europe is documented, compared with recent reports from the United States. Aukema et al. (2010) reported only 17 non-native forest tree pathogens established in the United States, whereas Santini et al. found 60 in Europe (only 60 of the 91 pathogens alien to Europe infect forest trees, a criterion used by Aukema et al., 2010). This discrepancy suggests that either taxonomists are more diligent at describing new pathogens in Europe, or perhaps Europe’s long history of colonialism and more extensive planting of non-native trees may contribute to a heavier burden of introduced pathogens in Europe.