Restoration of a prairie ecosystem: grass-specific herbicide facilitates plant-animal interaction

Leland Dwyth Bennion

Invasive species alter ecosystem structure, impact biodiversity through extirpation and extinction, and have significant economic costs. Restoration of ecosystems that have been invaded by unwanted exotic species is a challenging and complex task. In the Willamette Valley in Oregon, the invasive grasses tall oatgrass (Poaceae Arrhenatherum elatius) and tall fescue (Poaceae Schedonorus arundinaceus) alter the dynamics of phenologically paired interaction between an endangered butterfly, Lycaenidae Plebejus icarioides fenderi and its larval hostplant, Leguminosae Lupinus oreganus. To restore this interaction, we established a three-year experiment where post-emergent grass-specific herbicide, Fusilade DX®, was applied to known P. i. fenderi habitat. Plant community data were recorded at regular intervals during the growing season at eight paired plots for one year prior to treatment and three years during treatment. We asked if the annual application of herbicide could reduce the height of invasive grasses to levels at or beneath the height of the flowering stalks of L. oreganus throughout the P. i. fenderi flight season. We hypothesized that following the release from the competitive dominance of the invasive grasses, native non-target species such as L. oreganus and other important nectar sources for P. i. fenderi would increase in cover and frequency. Grass-specific herbicide was found to be an effective tool for reducing grass height during the flight season of P. i. fenderi. We found no significant change in native nectar resources in plots that had been treated with herbicide. Following three years of treatments, we found control plots had slightly more lupine cover than herbicide plots, but still noted an overall increase across the study area. Ordinations revealed significant changes in the plant community between control and herbicide plots. It is notable that significant amount of secondary invasion by forbs occurred during the three years of treatments; there is not enough information to determine the long-term impact of these new invaders. We recommend that herbicide can be effectively used to reduce grass height. We suggest that herbicide application could play an important role in a whole-system approach to restoration.

Restoration of a prairie ecosystem: grass-specific herbicide facilitates plant-animal interaction

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