Undetected but widespread : the cryptic invasion of non-native cattail (Typha) in the Fraser River Estuary

Invasive species represent a significant and growing threat to biodiversity in ecosystems around the world. Research that can address key knowledge gaps is invaluable, particularly as managers grapple with diminishing time, resources, and data to deal with species invasions. Non-native narrow-leaved cattail (Typha angustifolia) is a wetland invader that has been detected in western Canada’s Fraser River Estuary (FRE) in recent decades, but questions around their degree of establishment, impact, manageability, and the potential emergence of invasive hybrid cattail (Typha x glauca), remain unanswered. This research aimed to address these knowledge gaps, investigating the threat potential of these taxa. Using a spectral analysis of aerial imagery, I found that invasive cattails are widespread, currently occupying approximately 4% of FRE marshes. Though never formally recorded in the FRE, T. x glauca is more abundant than T. angustifolia, and likely went undetected due to its cryptic nature. A species distribution model for invasive cattail predicted that 28% and 21% of the FRE has suitability (establishment and persistence) and susceptibility (risk of colonization when suitable) probabilities of > 50% respectively, indicating this invasion is likely to continue. Restoration projects were invasion hotspots, with proportionally more cattail, susceptible habitat, and suitable habitat than the overall estuary. Vegetation sampling demonstrated that cattail-invaded marshes contained lower richness and diversity than uninvaded habitats. Cattail leaf litter had a significant negative effect on richness and diversity, while ramet density and foliar cover did not, suggesting litter may be an important dominance mechanism behind this invasion. Results from a two-year management experiment suggest these impacts may be counteracted, but not without expending considerable resources. Belowground energy reserves declined in response to cutting, however cattail ramets remained unchanged or increased in abundance. Native plant communities have yet to respond significantly to cutting and litter removal, suggesting that more time may be required for their recovery. I conclude that the extent of this invasion, likelihood of further invasion, and management challenges presented by invasive cattail require a strategic shift towards preventative management approaches, such as surveillance and early eradication in uninvaded high-value habitats, along with restoration designs that inhibit litter accumulation.Forestry, Faculty ofGraduat