The Drivers of Invasive Species Establishment and Impact on Ecological and Evolutionary Aspects of Resident Plant Communities

Abstract

Invasive species can have important ecological and evolutionary impacts on resident species. Yet, we still lack a broad understanding of the conditions that determine invasive species establishment, performance and impact on ecological and evolutionary aspects of communities. In this thesis, I first determined the drivers of the strength of biotic resistance to invasion by integrating a meta-analysis and remote-sensing data. I found that the magnitude of biotic resistance to invasion increased with temperature and precipitation, while productivity was a poor predictor. Second, I focused on the invader Bromus inermis (Leyss.) to determine the drivers of invasive species impact on different aspects of community structure and function, as well as whether an invasive species can lead to the establishment of alternative community states by altering and homogenizing communities where they invade. B. inermis is a perennial grass considered one of the most widespread and serious invasive species in North America. I performed a survey across eight sites and used space-for-time substitutions and time-series to quantify B. inermis’ impact on different aspects of the resident communities. I found that B. inermis impact on community structure and function was consistently negative within and across sites, yet stronger in warm, species-rich and productive sites. However, although B. inermis’ impact on native species richness was consistently negative, its impact on exotic species richness was highly variable. This can have important management consequences, potentially resulting in secondary invasions or an even greater impact on native species diversity. B. inermis was also found to rapidly alter resident communities, reducing native species diversity, changing species composition, altering ecosystem function and homogenizing communities. In doing so, B. inermis led to the establishment of a potentially persistent alternative community state. Due to its impact on resident species and ecosystem function, B. inermis has the potential to act as a selective agent on co-occurring individuals, through direct or soil-mediated effects. Further, the adaptive responses to the invader, such as increased tolerance or suppression, can be only against the invader or generalized strategies, which remains unknown. Therefore, I evaluated whether previous maternal experience and soil conditions alter individuals’ ability to tolerate or suppress B. inermis, as well as conspecifics, in three species. Results indicated no evidence of an adaptive response to coexist with B. inermis. Instead, I found that previous maternal experience interacting with B. inermis resulted in a decreased ability to suppress B. inermis performance, while tolerance was dependent upon soil conditions and not on previous maternal experience. Interestingly, these responses appear to be B. inermis-specific, as no effect of soil conditions or maternal experience were observed when interacting with conspecifics. These results highlight the need to better understand the conditions that facilitate or prevent species adaptation to invaders. Hence, I developed and proposed a theoretical model on how spatial patterns of invasion can be used to predict native species adaptation to invaders. If large, dense and well-connected invaded patches result in greater strength of selection and increased gene flow from other invaded areas (i.e. reinforcing gene flow), this would result in greater likelihood of an adaptive response to invaders. I highlight how this knowledge could be used to complement current management practices. Overall, this thesis’ results indicate that invaders can be important biotic forces structuring communities, altering ecological and evolutionary dynamics. Further, result from this thesis suggest that a better understanding of the conditions that determine the establishment, as well as the impact of invasive species on different aspects of resident communities, can help us design better management strategies to prevent further losses in biodiversity.