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.