Abstract: Exotic invasions by a single plant species have been well researched, but multi-species invasions are also common and are studied much less. In a greenhouse experiment, we studied co-invasions with two native species and three invasive species which co-occur in grasslands in the northern Rocky Mountains in a greenhouse experiment. Without competition from native neighbors and for all species combined, the biomass of two-invader mixtures was greater than the monoculture means when different species pairs were combined, generating a synergistic non-additive positive effect of co-invasions on total biomass. However, this increase in biomass with co-invasion did not correspond with a greater cumulative competitive effect, across all species combined, on native species or a cumulative competitive response to native species. However, when explored as separate species combinations, two pairs of the invasive species increased their competitive effects relative to their monocultures and one pair increased biomass and also increased their competitive response, or tolerance, to natives. Different combinations of co-invading species had strikingly different effects. Potentilla recta facilitated the growth of both of the other invasive species it was paired with, whereas Centaurea stoebe suppressed the growth of both species it was paired with. Our results suggest that even weak invaders might amplify synergistic effects when paired with stronger invasive species. This indicates that early management of emerging invaders prior to intense co-invasion scenarios may be important.

Key words: Invasional meltdown, co-invasions, non-additive effects, competitive effects, competitive responses