High levels of seed dispersal by a declining wintering population of migratory geese

Abstract

Ducks are known to be important seed dispersers, but the role of geese in plant dispersal is less clear. Wintering populations of migratory geese are undergoing rapid changes in distribution and habitat use in response to climate change and changes in land use, and the implications for seed dispersal have not previously been studied. At the southern end of Eurasian goose flyways, numbers are declining through short‐stopping. The Doñana wetlands, or ‘marismas del Guadalquivir’, in south‐west Spain were formerly Europe's most important wintering grounds for the greylag goose Anser anser. There, we collected 151 faecal samples to compare seed dispersal by endozoochory in early and late winter in natural marshes and ricefields. We also tested seed germinability and simulated potential seed dispersal distances to assess the importance of this decreasing wintering population for dispersal of dry‐fruited plants previously assumed to rely on abiotic seed dispersal over short distances. We retrieved 1196 intact seeds belonging to 24 different taxa, including eight species not previously reported from waterfowl (Anatidae) vectors. Seeds were present in 47% of samples, with a peak of 90% in natural marshes in November, compared to only 27% in ricefields at the same time, or in the same marsh site in February. Seed abundance and richness per sample were significantly higher in early than late winter, and in natural compared to in artificial wetlands. Major differences in plant species composition between sampling sites were partly related to habitat differences and moisture requirements of individual species. Germinability in aquatic plants was higher when gut passage was followed by cold storage for 2 months prior to germination tests. We simulated seed dispersal events to and from our sampling sites using GPS tracking of three geese. This suggested that seeds can be dispersed up to 25 km during daily movements, with a median of 0.2–5 km, depending on the sampling site. The most frequent dispersal syndrome assigned to the plant species dispersed by geese was barochory (gravity), and geese disperse plants much farther than the mechanisms predicted by their syndromes. Geese likely have an important role in the dispersal and connectivity of plant populations within and beyond Mediterranean wetlands, providing an ecosystem service which is threatened by changes in migratory behaviour. Our results highlight the importance of studying seed dispersal interactions involving migratory waterbirds in the current context of species loss and distribution shifts, as many crucial interactions may be disappearing even before they are discovered.