SYMP 7-2
Interactions among co-occurring invaders: The importance of facilitation and competition

Tuesday, August 11, 2015: 2:00 PM
307, Baltimore Convention Center
J. Hall Cushman, Department of Biology, Sonoma State University, Rohnert Park, CA
Caroline E. Christian, Department of Environmental Studies and Planning, Sonoma State University, Rohnert Park, CA

Most ecological systems are invaded by multiple exotic species that have the potential to interact with resident species in various direct and indirect ways.  Understanding how these invaders affect native species is critical and has been studied extensively.  Co-occurring invaders may also interact with each other, either through facilitation or competition.  Focusing on such interactions is essential for developing a comprehensive understanding of invasion biology and the consequences of efforts to reduce the dominance of exotic species.  Here, we synthesize field experiments from four systems in northern California to address the following questions: 1) To what extent do multiple exotic plant species co-occur in these systems? 2) How does removal or reduction of a dominant invader influence the remaining exotic plants in the community?, and 3) Do exotic neighbors indirectly facilitate other invaders by protecting them from their herbivores?  


Our four study systems have an abundance of co-occurring exotic species and they frequently alter each other’s success.  When we reduced the abundance of Cape ivy (Delairea odorata), a perennial vine from South Africa, species richness and seedling abundance of exotic plants increased.  Similarly, when we removed ice plant (Carpobrotus edulis), a succulent perennial from South Africa, the abundance, biomass and species richness of exotic annual forbs increased.  These results suggest that both invaders were competitively suppressing other exotics in the community.  However, such effects were not universal.  For example, exotic annual grasses as a whole were unaffected by iceplant removal.  In addition, reduction in the abundance of giant reed (Arundo donax), a perennial grass from Asia, led to large increases in native abundance and species richness but had no effect on exotic taxa. 

Our studies have also shown that facilitation from co-occurring exotic plants can indirectly affect the success of a problematic invader.  Veldt grass (Ehrharta calycina), a perennial grass from South Africa, was grazed heavily by native jackrabbits (Lepus californicus) but experienced reduced herbivory by associating with two exotic neighbors, ice plant (Carpobrotus edulis) and European beach grass (Ammophila arenaria). 

In conclusion, our experiments indicate that exotic plant species can have both positive and negative effects on each other.  These results suggest that it will be difficult to predict the extended consequences of controlling problematic invaders.  On the encouraging side, control efforts may eliminate positive interactions that have previously promoted the success of particular invaders.  However, these actions may also lead to the proliferation of other, non-target invaders.