Symbiotic relationships between plants and other organisms are hugely important mediators of invasion success for introduced plants. Seed dispersal and pollination by animal agents and relationships between plants and micro organisms in the soil often determine whether a species will invade, and/or the extent to which it will spread in a new environment. The importance of such relationships was poorly appreciated until very recently, but research in the last decade has lead to a marked improvement in understanding this area.
Biological invasions also have a profound effect on naturally occurring mutualisms in many ecosystems. Some such impacts are widely known, but the mechanisms that produce such impacts are poorly understood. Many types of invasive species have huge potential to alter evolutionary trajectories by causing many types of changes to prevailing mutualisms. This introduces a new layer of complexity to programmes aiming to reduce or mitigate the effects of biological invasions. These findings have important implications outside the field of invasion ecology - for understanding the importance of mutualistic interactions in population dynamics. As with the role of mutualisms as facilitators of invasions, understanding of the dimensions and magnitude of disruptions to mutualisms caused by invasions is a very recent field of study. This chapter reviews emerging insights in these areas and proposes challenges for the future.