UBC Theses and Dissertations
Mechanisms of coexistence between native and exotic plant species Johnson, Jens
One of the primary goals of ecology is to understand the processes that maintain biological diversity. The development of a modern coexistence theory has helped to advance this understanding by proposing a set of specific mechanisms that enable coexistence, specifically, species can coexist when niche differences between species are large enough to overcome fitness differences between species. Recent advances have used fully parameterized demographic studies to explain how traits, phenology, and evolutionary history contribute to niche and fitness differences, but there is a lack of empirical evidence of how competition and the environment interactively influence coexistence outcomes. Moreover, there is little empirical evidence of how indirect interactions between species mediate coexistence outcomes. Using an outdoor pot experiment and observational data from natural plant communities I explored the interactions between a pair of native (Plectritis congesta) and exotic (Valerianella locusta) co-occuring, annual plant species. With this system I answered the following four questions: (1) Can Plectritis and Valerianella coexist over the long run? (2) Does environmental variation change the intrinsic interaction between these species? (3) Does environmental variation enable coexistence by providing each species with an opportunity for positive low density growth rate in certain spaces or at certain times? (4) Do indirect interactions with pollinators destabilize or promote coexistence? The pot experiment predicted that Plectritis will exclude Valerianella over the long run. Although the coexistence outcome did not change between environmental treatments, the parameters used to calculate niche and fitness differences experienced significant changes. I did not find any evidence that Valerianella maintains abundances through variation in the environment in natural communities. Additionally, I did not find any evidence that niche differences are decreased through indirect interactions with pollinating insects. These species are still observed to co-occur at the site level and thus Plectritis may limit, but not totally eliminate, the abundance of Valerianella. Moreover, my experiment showed how vital rates and interaction coefficients depend on the environmental context, emphasizing that abundances are driven not only by competition and environment, but also through the interaction between competition and environment.
Item Citations and Data
Attribution-NonCommercial-NoDerivatives 4.0 International