Undetected but Widespread: the Cryptic Invasion of Non-Native Cattail (Typha) in a Pacific Northwest Estuary

Abstract

The early detection of invasive species is an important predictor of management success. Non-native narrow-leaved cattail (Typha angustifolia) has been detected in the Fraser River Estuary (FRE) in recent decades, but questions around their degree of establishment, and the potential emergence of hybrid cattail (Typha × glauca), remain unanswered. This study models the current and potential future distribution of non-native cattail in the FRE using a combination of spectral imagery analysis and species distribution modeling. Contrary to our expectation, we find that non-native cattails are widespread, currently occupying approximately 4 or 50 ha of FRE tidal marshes. Though never formally recorded in the estuary previously, T. × glauca appears to be the more abundant taxon, suggesting heterosis may be facilitating this invasion. We describe these taxa as cryptic invasive species, as their resemblance to native cattail (Typha latifolia) likely inhibited their detection. In our species distribution model, we distinguish between site suitability (ability to establish and persist) and susceptibility (risk of colonization when suitable). Our model predicts the scale of this invasion may increase over time, as 29% and 20% of the estuary has moderate or high suitability and susceptibility probabilities, respectively, while 16% and 24% of these habitats are currently occupied. Estuary-wide containment and eradication are unlikely given the extent of this invasion. Consequently, we recommend management prioritize monitoring and early eradication in areas of high conservation and cultural value. This study highlights the vulnerability of estuaries to cryptic invasions and the invasibility of Pacific Northwest estuaries by non-native cattail.