Understanding the role of birds in sustaining indigenous turf communities in a lacustrine wetland in New Zealand

Since human settlement, wetland ecosystems in New Zealand have been severely modified by fire and reduced by drainage for agricultural development. Those remaining are problematic to manage, with modified water regimes, invasive weeds and grazing by livestock. Before settlement, wetland habitats supported diverse avian herbivores, the majority of which are now extinct. However, introduced birds are increasing in wetlands. We sought to understand the role of grazing birds in the maintenance of local turf communities in a lacustrine environment in Otago, New Zealand. To determine the causes of the vegetation patterns, we investigated the influence of abiotic (water table and soil nutrients) and biotic (direct via grazing and indirect via faeces deposition) effects of the birds on the vegetation. Four plant communities were distinguished, two dominated by Leptinella and two by Carex species. The occurrence of the communities was correlated with the distance from the nearest permanent water, soil phosphorus levels, and amount of faecal deposition. The results indicate that avian grazing decreased the proportion of dominant exotic plant species by biomass removal, but not through enhanced nutrient inputs via faecal deposition. Results from this site suggest that naturalised geese facilitate the maintenance of indigenous plant species in the face of exotic invasion, and have a conservation role in highly modified wetland systems, perhaps restoring a herbivore function lost with the extinction of native avian grazers. The long-term conservation management of wetlands in New Zealand may require the utilisation of both native and introduced avian grazers to facilitate the dominance of indigenous plants.