Diverse outcomes of species interactions in an invaded annual plant community

Abstract

AimsDespite acknowledgement that interactions among native and exotic species are important for determining the structure and diversity of novel communities, directed experiments using multiple exotics from the same system are rare. Recent observational studies have highlighted distinct ways that exotic species interface with resident natives across invaded communities. The correlative nature of these studies, however, has provided few details about the mechanisms driving distinct interaction outcomes within the same communities. Our aim was to determine how three exotic annual plant species with distinct relationships with local plant diversity impact the performance of a co-occurring native annual in the York gum-jam woodlands of Western Australia.MethodsWe grew species in experimental communities in growth chambers at varying total planting densities to assess how interaction outcomes varied among natives and exotics across a gradient of competition intensity. We measured a variety of performance responses, including survival, biomass and population-level and individual-level reproductive investment.Important FindingsOverall, the effects of interspecific versus intraspecific competition on performance varied with the identity and density of exotic competitors. The exploitative exotic grass Bromus madritensis was dominant in polyculture, whereas the diminutive grass Pentameris airoides conferred weak intraspecific competition and interspecific facilitation on native Waitzia nitida. The exotic broadleaf forb, Hypochaeris glabra, suppressed growth and survival of W. nitida, while W. nitida had weakly negative, neutral or positive effects on all exotics. These outcomes highlight the complexity of interactions impacting the diversity, stability and structure of novel plant communities. As few of these communities contain a single exotic species, understanding the competitive dynamics occurring in diverse novel communities is critical for their conservation and restoration.