From Competition to Facilitation: Mechanisms of Species Interactions in a Novel World

Invasive species outcompete and displace native species through competition and are an enormous threat to biodiversity. Theories designed to explain mechanisms of plant species competition have been in development over the last ninety years. Nonetheless, understanding why invasive species are competitively superior remains elusive. This is likely due to the fact that invasive species do not conform to traditional assumptions made in competition models. In this study, we focus on three species found in competition in grassland ecosystems of much of the US Great Plains. The species include the non-indigenous, invasive, C4, perennial grass King Ranch bluestem (Bothiochloa ischaemum (L.) Keng var. songarica) and the native, C4 grass species sideoats grams (Bouteloua curtipendula) and little bluestem (Schizachyrium scoparium). In order to improve understanding of the competitive exclusion of native species by invasive species, I first review the last six decades in the development of competition theory and models of competition as well as their application to recent theory on invasive species competitive dynamics. I then describe a study I conducted to modify a mechanistic competition model, originallydeveloped by Carroll, Cardinale and Nisbet, to better apply to a competitive dynamic found in our system. In this particular interaction between our focal invasive King Ranch bluestem and the native sideoats grams (Bouteloua curtipendula), the invasive species is facilitated by the presence of the native species. Through this study we demonstrate the limitations of the model and make suggestions for improvements for the interactions found in our system. The last chapter of the thesis is dedicated to a study designed to apply and test the modified Carroll et al. model to a real world scenario of grassland competition. In this study, I aimed to understand mechanisms underlying the success of King Ranch bluestem with either sideoats grams and little bluestem using a growth chamber competition study. The work is ultimately designed to further theoretical understanding of the competition dynamics of exotic, invasive species competition and provide insight into how to manage natives to enable their use in restoration and biocontrol