Patterns and mechanisms of plant invasions: cross-habitat insights from central Alberta

Biological invasions are a leading cause of biodiversity loss and ecosystem change globally, with non-native vascular plants altering nutrient cycling, disturbance regimes, and community structure. Non-native plant invasions are increasingly acknowledged as context-dependent processes. Yet relatively few studies have disentangled patterns and mechanisms of invasions across habitat types, although habitat context has been suggested to be a key factor underlying invasion dynamics. Research on habitat context has relied either on observational surveys to describe levels of invasion or manipulative experiments to test for habitat invasibility. When combined, these contrasting approaches could help us understand whether these two metrics align. This thesis examined the role of habitat type in shaping invasion processes in the Central Parkland Natural Subregion of Alberta, and its interaction with propagule pressure, disturbance, and species identity. I tested hypotheses using field surveys and field experiments in a landscape defined by natural gradients and a legacy of agricultural and industrial land use. In Chapter 2, I quantified the relative richness and cover of agronomic and noxious species across seven habitat types. My research was the first to show that agronomic species – those intentionally introduced for forage and reclamation – can be a dominant component of the non-native flora, surpassing noxious weeds in both frequency and cover. Additionally, levels of invasion varied significantly by habitat type, with ruderal habitat showing the highest levels of invasion and saline marsh the lowest. In Chapter 3, I explored how habitat type interacts with environmental and anthropogenic predictors and species identity (agronomic vs. noxious) in shaping of non-native richness and cover. Habitat type, native plant cover, and proxies for propagule pressure (e.g. road length, amount of nearby cultivated land) emerged as significant predictors, although effects depended on species identity. Agronomic species were strongly associated with anthropogenic land use, while noxious species showed weaker and more variable responses, possibly due to their smaller frequency and abundance, and thus a higher stochasticity. In Chapter 4, I experimentally manipulated propagule pressure and soil disturbance across three habitat types to disentangle intrinsic habitat invasibility from realized levels of invasion. Germination increased with propagule pressure across all study species and habitats, but responses varied by habitat and species identity. Notably, prairie grassland showed strong invasion resistance despite exhibiting high levels of invasion in field surveys. Together, these findings highlight that plant invasions are shaped by complex, context-dependent interactions among habitat characteristics, human land use, and species identity. Overall, my thesis shows that land use legacies, disturbance and propagule pressure can override intrinsic resistance in habitat types, leading to high levels of invasion. A novel finding is that agronomic species represent a uniquely challenging threat: they are widespread, unregulated, and strongly linked to economic activity, yet ecologically disruptive. I argue for a shift toward habitat-specific management strategies that integrate species identity, landscape context, and socio-economic trade-offs. These should include limiting propagule pressure from agronomic species, promoting native vegetation recovery, and re-evaluating practices to balance ecological resilience with human land use. By emphasizing the role of context in non-native plant invasion dynamics, this thesis contributes to a more nuanced and practical understanding of plant invasions.

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