Plant populations can be limited by their natural enemies; escape from these enemies often is invoked to explain the success of exotic species in new regions (the Enemy Release Hypothesis). We hypothesize that escape from natural enemies is not limited to the introduced range of a species but may operate within the native range as well. Here I report on a series of experiments in Ontario, Canada, testing whether common ragweed (Ambrosia artemisiifolia) escapes aboveground and belowground enemies within its native range. This disturbance-dependent plant is native to North America but is widespread throughout Europe, where it has escaped many of its insect herbivores. In Ontario, experimentally planted ragweed experienced greater levels of herbivory in field plots located within pre-existing populations as compared to isolated plots. The same significant trend was also found for seeds damaged by the ground beetle Harpalus rufipes. In a greenhouse experiment and contrary to our expectations, we found that pots containing inocula from different habitats (disturbed sites with ragweed vs. undisturbed old-field sites) had no difference in growth. However, ragweed does grow significantly better in pots inoculated with field-collected soil as compared to sterile controls. Current experiments are investigating seed bank dynamics of ragweed and comparing different types of herbivore damage. Overall, these results suggest local escape from insect enemies, but perhaps not from soil pathogens. Understanding the population dynamics of a species within the native habitat will help explain its success both at home and abroad.
