Cheats can boost the success of a cooperative invader

Successful biological invasions are dependent on the invader being able to grow and reproduce in the new environment. One way that microbial invaders may facilitate this is to use cooperative public goods, such as metal-binding siderophores. However, siderophore production can be exploited by non-producing cheats who benefit from production without paying any associated costs. Here, we test the importance of cooperation for the success of Pseudomonas aeruginosa invading a 5-species microbial community. We do this by comparing the success of a siderophore-producing strain, a siderophore-deficient mutant strain and a 50:50 mixed population, both in environments with weak (copper absent) and strong (copper present) siderophore requirement. We found no effect of invader type on success when siderophores were less essential for growth, but large differences when they were selectively favoured. Here the producer-cheat mix had the greatest success, with both strains having near equal fitness and reaching high densities, whilst in isolation producers had intermediate success and cheats the lowest. Similarly, resident diversity only differed across invader treatments when copper was present. In conclusion, we show that the presence of cheats can provide a larger benefit for invasion success than pure cooperator populations, but only when public goods are particularly beneficial.