Twenty year contrast of non-native parrotfeather distribution and abundance in an unregulated river

Efficient management of invasive species benefits from understanding patterns of persistence and change over time. In this study, we compare distribution and abundance of the invasive macrophyte parrotfeather (Myriophyllum aquaticum) in an unregulated river system between the time near its presumed introduction and 20 years later. Initial surveys were conducted in 1996–1997, and were repeated in 2015–2016 using similar methodology. Moderate increases in the proportion of river kilometers with parrotfeather between the two periods were observed, but the distribution of sites with low, medium, and high abundance remained consistent, with small numbers of sites in either period having well-established extents of parrotfeather. The distributional extent has moved downstream, with the most upstream and downstream presences shifted by 17 and 28 river kilometers, respectively; however, parrotfeather remains sparse in the lower reaches below the historical downstream extent. Sites with high abundance and stable presence over time are in the intermediate segment of the river, indicating longitudinal and hydrologic controls on distribution. In contemporary sites, area of parrotfeather cover was associated with larger, deeper habitats, whereas dominance of parrotfeather occurred in smaller sites having uniformly shallow depth and low bank slopes. Sites where both abundance and dominance were low had dense canopy cover. Our results demonstrate that landscape and site-level characteristics restrict establishment and persistence of parrotfeather, and that hydrologic alterations to stabilize flow regimes and land use changes should be considered for their potential to increase presence of parrotfeather and other invasive aquatic plants in dynamic floodplain habitats.