Few immigrant phytophagous insects on woody plants in Europe: legacy of the European crucible?

Abstract

Exotic phytophagous insects are invading forest ecosystems worldwide. So far, 109 invasive insects on woody plants, 57 from North American (NA), and 52 from Asia (A) have established populations in European forests. Four orders account for about 84% of the immigrants: Homoptera 39%, Lepidoptera 13%, Coleoptera 19%, and Hymenoptera 13%. The majority of these invasive species (63% of NA and 77% of A) live on deciduous trees, of which 36% have been introduced from NA and Asia. The remaining insect species (37% NA and 25% A) live on various conifers, of which 53% have also been introduced. Most (57%) of the NA insects feeding on coniferous plants live upon their introduced, native host plants. These data suggest that many NA immigrant phytophagous species in Europe have been successful in estab- lishing permanent populations because their native hosts preceded or accompanied them into Europe and/or were asexually reproducing spe- cies. We propose that fewer invasive phytopha- gous insects have become established in European compared to North American wood- lands because of the unique legacy of the Euro- pean Pleistocene/Holocene crucible (i.e. endless cycles of populations contracting into highly disparate, dispersed metapopulation refugia and eventually expanding out of them) on European species and ecosystems that caused highly dimin- ished heterogeneity. This translates to fewer and less penetrable tri-trophic niches in Europe due to fewer and less available host plants, but greater zootic resistance per niche derived from more competition-hardened competitors and possibly natural enemies. Moreover, many European spe- cies are probably superior invasion specialists because the crucible favored traits that are conducive to success in highly subdivided, and extinction-prone metapopulations: asexual repro- duction, polyploidy, and other traits especially conducive to persistence under stress, and explo- sive growth/spread under amelioration.