Do not come late to the party: Initial success of nonnative species is contingent on timing of arrival of co-occurring nonnatives

Agostina Torres, Mariano A. Rodriguez-Cabal and Martin A. Nuñez, Grupo de Ecología de Invasiones, Instituto de Investigaciones en Biodiversidad y Medioambiente, CONICET, Bariloche, Argentina, Mariano A. Rodriguez-Cabal and Martin A. Nuñez, Senior author, Mariano A. Rodriguez-Cabal, Rubenstein School of Environment and Natural Resources, University of Vermont, Burlington, VT

Background/Question/Methods
Multiple nonnative species commonly co-occur in communities worldwide, threatening biodiversity and ecosystems. We know that interactions among them influence their success. However, we know relatively less about the influence of community assembly history on the interactions within nonnative species and thus invasion success. To investigate this we transplanted seedlings of two highly invasive shrubs, the sweetbriar rose (Rosa rubiginosa) and the Scotch broom (Cytisus scoparius), at two different times into mesocosm communities of native and nonnative neighbor species. The invasive shrubs arrived, alone and simultaneously, earlier and later than the rest of the community. We predicted that if assembly history were important, the invasive shrubs will benefit from early-arriving and will be disadvantaged by late-arriving and that arriving before the co-invader shrub will be more beneficial than arriving before the rest of the community. We also predicted that assembly history treatments that give an advantage to the target species will negatively affect the presumably less competitive native plant neighbors while favoring other nonnative neighbors, as proposed by the invasional meltdown hypothesis.
Results/Conclusions
Neither the rose nor the broom showed an advantage when given a three weeks priority in our mesocosm communities, but both of them were negatively affected when the co-invader was given priority. As expected, the target species’ biomass was significantly lower when given three weeks delay in arrival (i.e. when arriving after the nontarget neighbor species). Contrary to our predictions, natives payed a lower cost than nonnatives by arriving late. Indeed, we found that early arrival of the invasive target species generally inhibited the performance of nonnative neighbor. However, neighbor species responses to the assembly history were species-specific. Overall, our mesocosm experiment showed that invasive species’ success depended on not being later than other invaders rather on arriving early to the community. We suggest that community assembly history modulates the sign and strength of nonnative species interactions and that nonnative species may be competing strongly rather than facilitating each other. Our research is a contribution to the emerging body of literature of the importance of the timing of species arrival in conservation biology and can specifically shade light on the process of nonnative species accumulation and restoration of invaded communities.