Native exotic relationships in plant communities: the role of exotic dominance in framing community composition

In tropical urban areas, prone to invasions by multiple exotic species, there is a need for studies to understand the precise nature of the relationship between native and exotic species. This observational study was conducted in a rapidly urbanizing Indian metropolis. It examined the native and exotic species separately as a function of richness and abundance of two focal exotic species, namely Mikania micrantha and Alternanthera philoxeroides, to find support for the hypothesis that an overall native-exotic relationship does not provide a true picture of the community composition. The richness of exotic species did not turn out to be a good predictor for the native-exotic richness relationship even after focal exotics were considered. However, when the richness components were analysed separately, Poisson log-linear models identified M. micrantha as the community-dominant, by virtue of its extensive cover. Neither soil resource availability nor the presence of other exotics had any influence on native species richness. The negative relationship of M. micrantha cover with other exotic species could be associated with a lower risk of native species loss in a community invaded by multiple exotic species. M. micrantha appeared to be a 'passenger' of habitat alteration, but was likely to become a 'driver' once it attains high covers due to its reported superior competitive abilities. Therefore, in tropical areas there is a need to prioritize management initiatives in order to identify the dominant invader species in a community and effectively manage the dominant-homogenized plots.