Frequency and extent of aquatic plant management with herbicides determines long-term effects on native plants

Aquatic plant management can help mitigate the ecological, social, and economic harms caused by nuisance plants but can also have unintentional negative effects on native aquatic plants. The characteristics of management regimes, such as frequency and extent, may modify the impacts on native plant communities. Long-term and large-scale monitoring can span a range of management frequencies and extents, as well as habitat conditions, providing insights on native plant responses to management. We used a dataset of aquatic plant management and plant surveys collected from 373 waterbodies over 22 years in Florida, USA, to assess the effects of management on native plant communities. Because plant surveys were collected through nonstandardized approaches (i.e., meanders), we first evaluated the surveys relative to point-intercept sampling. We then evaluated the effects of management frequency (percentage of years managed) and extent (percentage of waterbody area managed), with respect to different targets and methods, on native plants. Management targets included Hydrilla verticillata (L.f) Royle subsp. verticillata, the floating plants Pontederia crassipes and Pistia stratiotes (grouped together), and all other managed plants (both native and non-native) grouped together (e.g., Cyperus blepharoleptos, Typha spp.). The meander approach estimated higher native plant richness and occurrences than point-intercept sampling, but there were large discrepancies in abundance estimates. Based on meander surveys and complimentary management records, native plant richness and occurrences were positively associated with more frequent management of H. verticillata and other plants. Native plant richness and occurrences were negatively associated with more frequent floating plant management and more extensive management with herbicides. Overall, we demonstrated that frequency and extent are important characteristics of management to consider when evaluating the effects of management on native plant taxa. Adjustments to management frequency and extent that maintain effective control of aquatic plants may also allow managers to maintain or even promote native plant diversity.