Passive Recovery Risks Non-native Vegetation Invasion Following Intensive Herbivory by Canada Geese in Two Salish Sea Estuaries

Abstract

Tidal marsh plant communities in the Pacific Northwest are characterized by tall, perennial graminoids (TPGs), which provide forage for herbivores such as Canada geese. Excessive grazing by Canada geese leads to loss of marsh habitat, and removal of grazing pressure is required for the vegetation to recover. Grazing exclosures (fences) are used to allow time and space for vegetation to recover following intensive herbivory; however, their effects on native plant community recovery has not been tested. Generalized linear models were used to compare TPG abundance in aboveground vegetation and surface seed banks in 1-year-old and 10-year-old exclosures at Nanaimo River Estuary (NRE) and Little Qualicum River Estuary (LQRE), respectively, to areas of the marshes that had no known history of grazing (undisturbed) and areas still actively grazed (grubbed). Compared to undisturbed sites, grubbed sites had 187.3% less mean TPG vegetation cover and 190.7% lower proportion of TPG seeds. The 1-year-old exclosures at NRE had 105.0% less mean TPG vegetation cover and 193.2% lower proportion of TPG seeds. The 10-year-old exclosures at LQRE had 7.0% greater mean TPG cover and 55.7% greater proportion of TPG seed than all undisturbed sites; however, these exclosures had 110.0% greater mean relative abundance of non-native TPGs than undisturbed sites. These results indicate vegetation may not recover towards comparable historic conditions through grazing exclusion alone, and that active restoration methods may be required following intensive grazing, especially in estuaries where the vegetation community and surface seed bank has a high abundance of non-native, invasive species.