Can below-ground interactions drive the spread of an invader?
The majority of research and management efforts for invasive plant species have been centred on above-ground herbivory. The importance of the below-ground biotic community on invader success remains understudied. For instance, do local or regional variations in below-ground impacts have the potential to help or hinder invasions? To address this question, the noxious weed Cirsium arvense (Canada thistle) was used as a study system. Soil was collected inside and outside established thistle populations at 8 locations, which spanned a 700 kilometre transect from agricultural southern Ontario to boreal northern Ontario, Canada. Seeds were grown in pots prepared with a 1:5 ratio of live field soil to sterilized potting soil, with sterile controls. Growth rate measurements were taken throughout the experiment, and forty days following seedling transplant the above- and below-ground biomass were harvested, dried, and weighed in order to provide a measure of overall plant performance.
Plants grown in soils sourced from northern Ontario or from outside thistle populations performed better than those grown in southern Ontario soils (p<0.0001) or those from inside thistle populations (p<0.0001). This suggests that subterranean herbivores and pathogens accumulate in pre-existing thistle populations, and that such accumulation is more likely at southern sites than in more isolated northern locations. As a result, those individuals who are able to establish outside of existing populations and in geographically marginal areas more likely to succeed. In this way, below-ground interactions may be driving the success and spread of C. arvense northwards.