EFFECTS OF WINTERING GREYLAG GEESE ANSER ANSER ON THEIR SCIRPUS FOOD PLANTS

Herbivory of Scirpus litoralis and S. maritimus tubers by wintering greylag geese, Anser anser, was studied at the marshes of the Guadalquivir in southwestern Spain. Exclosures were constructed to compare plant growth in grazed and ungrazed sites. Scirpus litoralis tubers are clustered, with clusters being interconnected by rhizomes. Scirpus maritimus tubers develop individually, and are connected by rhizomes. The geese forage on individual S. maritimus tubers but take entire clusters of S. litoralis tubers, from which only one tuber is normally consumed. The reproductive mode of S. maritimus, the main food plant of the wintering geese, was dependent on tuber size. The proportion of tubers whose shoots bear inflorescences decreases with increasing tuber size, and the larger tubers reproduce vegetatively. The geese feed on the smaller S. maritimus tubers and the smaller S. litoralis clusters. Reasons for this preference may be that these tubers and clusters are easier to extract and also that small tubers require less time to break down in the gizzard. The number of shoots of S. litoralis and S. maritimus in grazed sites was 88% and 32%, respectively, compared with that in ungrazed sites. Nevertheless this difference reflects in part a decrease of S. litoralis inside some exclosures during a five years period. The length of shoots of both species was shorter in grazed than in ungrazed sites. The size-related selection of tubers by geese may have a profound effect on the structure and dynamics of these plants populations, and the trade-off between sexual and asexual reproduction. However, due to the clonal nature of the plants, interpretation of plant-goose interactions from an evolutionary perspective is difficult because of the elusive unit of selection involved.