Native and alien species suffer from late arrival, while negative effects of multiple alien species on natives vary

Ongoing globalisation and climate change are causing plant species to invade new habitats and thereby alter biodiversity and ecosystem functioning. Since numbers of plant invasions continue to increase globally, it is crucial to investigate the effects of multiple co-occurring alien species on native communities. Furthermore, priority effects due to the earlier emergence of certain species affecting fitness of later arriving species can shape community structure and affect native species performance. We investigate in a common garden pot experiment the interactions among five alien-native species pairs. First we focus on the effect of growing with either one or two alien neighbour species on a native plant, second we alter the arrival time of the alien or native neighbour by 3 weeks. Generally, native species performance decreased when surrounded by two alien species compared to only one, although the magnitude of this effect varied depending on species, with one species even performing better with alien neighbours than in monoculture. Species performance greatly decreased when arriving second in the pot, for both native and alien species. In contrast, alien species tended to benefit more from arriving early. Given that we studied annual ruderal species, their potentially lower competitive ability might explain why we detected negative effects of late arrival. We highlight the need to further elucidate underlying mechanisms of small-scale invasion dynamics to achieve generalisations concerning the response of multiple alien and native plants given their species-specific differences in response to neighbour species and arrival time.