Long-term, indirect consequences of herbicide treatment to control the invasive saltmarsh grass, Spartina anglica

Invasive plants acting as habitat modifiers or ecosystem engineers in coastal wetlands can have extensive impacts on local species assemblages, food-web dynamics, geomorphology, and biogeochemistry. Control of these invasive... [ view full abstract ]

Invasive plants acting as habitat modifiers or ecosystem engineers in coastal wetlands can have extensive impacts on local species assemblages, food-web dynamics, geomorphology, and biogeochemistry. Control of these invasive plants often relies on herbicide use, although little is known about subsequent environmental impacts. Studying effects of herbicides on non-target species and long-term cascading consequences is crucial for the protection of biodiversity and informed management practices, and may yield insights into the ecology of invasive species by revealing direct and indirect interactions with native species. We conducted a long-term (>1.5 years) field experiment measuring effects of treating the invasive saltmarsh grass, Spartina anglica, with the herbicide Fusilade Forte® in temperate Australia. S. anglica is a problematic invasive species on several continents and is commonly managed with herbicides. No changes in sedimentary macrofaunal abundances or species richness, diversity, or assemblages were detected one or two months after spraying, despite known toxicity of Fusilade Forte® to fauna. This lack of impact may have been due to low exposure, since the herbicide was taken up primarily by the plant leaves, with the small amount that reached the sediment hydrolyzing rapidly. Six months after spraying, however, total macrofauna in treated plots was more than four times more abundant than in control plots, due to a fifteen-fold increase in annelids. This population growth correlated with increased organic matter content of the sediment in treated plots, most likely due to the decomposition of dead S. anglica leaves that served as a food source for annelids. After another year, no differences in macrofauna or organic matter remained between treated and control plots. The temporary, indirect effect on annelid populations from herbicide treatment could benefit management efforts by providing greater food resources for wading birds, in addition to improving birds' access to sediments by reducing plant cover. This study reveals an ecologically significant interaction between an invasive grass and resident fauna that is mediated by herbicide usage and should be an important consideration in management practices for wetlands.