* Biological invasions
* Ecology of plant populations and communities
* Grazing by geese
* Invasive plants
* Natural history
* Plant-soil interactions
* Seed ecology

I study the ecology of biological invasions. My current research focusses on factors limiting invasions by non-native plants at high latitudes. Other recent research has centered on the question of whether escape from natural enemies (herbivores and pathogens) promotes the survival and spread of exotic plants, and whether escape from these enemies is more likely in marginal populations. I also have a longstanding interest in the effects of grazing by northern populations of geese.