Hybridization has been associated with increased invasiveness in plants. In North America, the hybrid aquatic plant Myriophyllum spicatum × Myriophyllum sibiricum (hybrid watermilfoil, hereafter HWM) is a cross between non-native invasive Eurasian watermilfoil (M. spicatum, EWM) and native northern watermilfoil (M. sibiricum, NWM). Lab-based trials have demonstrated higher growth rates in HWM compared to EWM and NWM, but these patterns have not been systematically examined in the field. In this study, we compared the invasiveness of HWM to its parental taxa, EWM and NWM, by examining the amount and timing of: (1) flowering, (2) surface cover, and (3) biomass (using stem counts as a proxy). We conducted repeat surveys of Myriophyllum beds at eight lakes (2–3 lakes/taxon) in the Minneapolis–St. Paul Metropolitan area (Minnesota, USA) between June 2017 and November 2018. HWM produced more flower spikes earlier and overall, and maintained consistently more flower spikes throughout the growing season than EWM and NWM. In addition, surface cover reached greater annual peaks and was higher for longer throughout the growing season for HWM than for both parental taxa. We did not observe a significant difference in stem counts among the three taxa, but HWM did reach a higher maximum number of stems than either parental taxon. This study provides field-based evidence of increased invasiveness associated with hybridization between EWM and NWM; specifically, greater reproductive potential via flowering and greater surface cover may increase HWM spread, have greater impacts on native species, and pose more of a nuisance to lake users.
Bibliographical noteFunding Information:
Funding was provided by the U.S. Army Corps of Engineers Research and Development Center (Cooperative Agreement No. W912HZ-16-2-0011), with additional support from the Minnesota Aquatic Invasive Species Research Center. Michael Netherland provided vital support and guidance for this research. We thank Rachael Crabb and Rob Brown of Minneapolis Parks and Recreation, Adam Robbins of Saint Paul Parks and Recreation, and Eric MacBeth of the City of Eagan, MN, for access to study lakes. Michael Verhoeven, Carli Wagner, Carolyn Kalinowski, Noah Berg, Rafael Contreras-Rangel, Ranjan Muthukrishnan, James Dickson, and Stephanie DeMay assisted with fieldwork and/or collection of data from videos. We thank the associate editor and two anonymous reviewers for comments that greatly improved the manuscript.
© 2021, The Author(s).
- Aquatic invasive species
- Freshwater ecosystems
- Hybrid vigor
- Invasive traits