Landscape factors driving the spread of the invasive grass, Hymenachne amplexicaulis, among wetlands in a Florida subtropical grazing land

Wetlands embedded in agroecosystems provide vital ecosystem services (i.e., freeze protection, water retention, nutrient cycling, biodiversity support). However, they are particularly susceptible to invasion by nonnative species. West Indian marsh grass [Hymenachne amplexicaulis (Rudge) Nees] is a major wetland invader in Florida. Despite the documented consequences of H. amplexicaulis invasions, the landscape factors influencing the spread of this species are poorly understood. In this study, we asked whether landscape factors associated with wetland isolation, connectivity, and land management influence the presence of H. amplexicaulis among wetlands embedded in pastures. We recorded the presence or absence of H. amplexicaulis in 158 seasonal wetlands embedded in different pasture types (semi-natural vs. intensively managed). Wetland area, isolation from neighboring wetlands, isolation from the nearest source ditch, and connectivity were determined using a geographic information system (GIS). We related landscape factors to H. amplexicaulis using generalized linear models and model selection based on the second-order Akaike information criterion. Hymenachne amplexicaulis was first detected at the study site in the early 2000s. By 2018, we observed this species in 66% of the surveyed wetlands. The likelihood of observing H. amplexicaulis was higher in wetlands embedded in semi-natural pastures and higher in less isolated wetlands, especially when connected to a ditch. These results indicate that H. amplexicaulis spreads both overland (during seasonal flooding) and via the ditch network. Future work is needed to understand whether seeds or stolons are the primary invasion propagule and whether the species forms a persistent seed bank that could slow down restoration efforts. Additionally, further research is required to understand the ecological impact of this highly invasive plant in Florida wetlands.