Canadian Forest Service Publications
Compensatory indirect effects of an herbicide on wetland communities. 2020. Edge, C.B.; Baker, L.F.; Lanctot, C.M.; Melvin, S.D.; Gahl, M.K.; Kurban, M.; Kidd, K.A.; Mudge, J.F.; Navarro-Martin, L.; Pauli, B.D.; Robertson, C.; Thompson, D.G.; Trudeau, V.L.; and Houlahan, J.E. Science of the Total Environment 718
Issued by: Atlantic Forestry Centre
Catalog ID: 40552
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The direct effects of large-scale disturbances are readily studied because their effects are often apparent and result in large changes to ecosystems. Direct effects can cascade through the ecosystem, leading to indirect effects that are often subtle and difficult to detect. Managing anthropogenic disturbances, such as chemical contamination, requires an understanding of both direct and indirect effects to predict, measure, and characterize the impact. Using a replicated whole-ecosystem experiment and path analyses (assesses the effects of a set of variables on a specified outcome, similar to multiple regression), we examined the direct and indirect effects of a glyphosate-based herbicide and nutrient enrichment on wetland communities. The latter did not impact any measured endpoints. The strongest drivers of macrophyte, benthic invertebrate, and amphibian assemblages were the ephemerality and the size of wetlands, factors which were not altered by herbicide applications. The herbicide had a direct negative effect on macrophyte cover, amphibian larval abundance, and the proportion of predatory benthic invertebrates. However, both amphibians and invertebrates were positively affected by the reduction in the macrophyte cover caused by the herbicide applications. The opposing directions of the direct and indirect effects lead to no net change in either group. The compensatory dynamics observed herein highlight the need for a better understanding of indirect effects pathways to determine whether common anthropogenic disturbances alter the ecological communities in small wetland ecosystems.
Plain Language Summary
Glyphosate-based herbicides are the most commonly used herbicides in the world. Considerable past work has investigated the environmental risk of using these herbicides from the perspective of direct effects of the herbicide on different ecosystem components. In the present paper, I and a large team of colleagues synthesized data we collected 10 years ago on the effects of glyphosate application on wetland communities. The present paper reviews our past work on direct effects and then explores indirect effects that could be caused by the removal of vegetation. Results demonstrate that glyphosate has a direct negative effect on plant cover, and this effect can be traced through the ecosystem to small, short-lived, indirect effects on invertebrate and amphibian communities. Overall, the paper demonstrates that wetland communities are more strongly influenced by ecosystem structure (e.g., hydroperiod, size) than the removal of vegetation by herbicide use. Wetland communities appear relatively robust to the short-term perturbation caused by 2 years of herbicide application. However, more dramatic effects are expected if vegetation is removed for longer than 2 years. Implications for forestry are not discussed directly in the paper because the research focused on an agricultural exposure scenario. That being said, forestry applications typically occur once or twice every 40–60 years making it extremely unlikely that dramatic ecosystem shifts would occur.