Estuaries in western Canada are threatened by invasive, non-native resident Canada geese (Branta canadensis, “CAGO”). After their introduction to the west coast of British Columbia in the 1960’s, populations expanded and now pose significant threats to sensitive ecosystems including estuaries. Excessive grazing pressure by CAGO shift herbaceous marsh habitat to mudflat, and grubbing can lead to erosion of the mudflat and marsh recession. Excluding CAGO may allow marsh habitat to recover before mudflats erode completely, however the degree to which they can recover remains unknown.
This study examines marsh recovery following CAGO exclusion. We compare the recovery trajectory of exclosures that are 1 versus 10 years old and ask the questions 1) How does CAGO grazing impact species and Plant Functional Groups (PFGs)? 2) How do species and PFGs respond over time since grazing exclusion? 3) How do seed banks respond over time since grazing exclusion? 4) Is seed bank composition similar to above-ground species composition?
We used mixed effects models to test time since grazing exclusion on plant species cover, seed bank richness, seed bank abundance, and abundance of native and non-native species following grazing exclusion across seasons. We used NMDS to illustrate seed bank dissimilarity from above-ground vegetation.
Results/Conclusions
We show that 1) sites recently impacted by excessive grazing had lost cover of native perennial graminoids, were dominated by annual species, and had higher proportion of unvegetated ground compared to ungrazed sites. 2) Perennial graminoid cover increased between 1 and 10 years at sites protected by exclosure. Sites exclosed for 10 years had perennial graminoid PFG cover similar to ungrazed sites, but had higher cover of non-native graminoid species. 3) Seed abundance was highest in recently grazed sites dominated by annual species; however, seed species richness did not vary between grazing conditions. 4) Seed bank species composition was most similar to above-ground vegetation in grazed sites dominated by annual species.
Excluding geese from habitat is an effective method of protecting remnant vegetation; however, these sites may be prone to invasion during the recovery period. Understanding whether management actions facilitate resilience of above-ground vegetation and seed banks is important to inform conservation and restoration best practices. Anticipating community changes following disturbance will be of increasing importance in estuaries as these ecosystems experience greater environmental disturbance such as storm surges under sea level rise.
