Cyclic Winter Grazing Patterns in Brent Geese and the Regrowth of Salt- Marsh Grass

Abstract

This paper describes the large-scale spatial and temporal patterns of grazing on salt-marsh by Dark-Bellied Brent Geese Branta bernicla bernicla in winter and examines them in relation to grazing optimization theory. The numbers and spatial distribution of geese feeding on salt-marsh were recorded, on average, every 2.5 days from October to April 1991/92, and the data analysed for regular cyclic patterns of grazing within areas of the marsh. The mean percentage ground cover of the main food plant, Puccinellia maritima (Poaceae), was estimated in all areas and the proportion of each area composed of short vegetation was measured. The effects of grazing on the age structure of leaves of P. maritima, and on the patterns of regrowth following grazing, were monitored in swards of high, medium and low mean P. maritima cover. Eleven of 23 defined areas of salt-marsh were grazed by geese infrequently (fewer than five records) or never. These areas were clearly distinguished from those used frequently for at least part of the winter by their lower coverage of short vegetation (average 9%, compared with 40% for frequently used areas). Of the 12 frequently grazed areas, six showed evidence of cyclic patterns of grazing, with the interval between grazing bouts ranging from 7 to 15 days. The remaining six areas showed only weak evidence or no evidence of cyclic grazing patterns. The available evidence suggested that disturbance, position in relation to roost and inland feeding areas, and the occurrence of night-time feeding may influence whether or not grazing cycles occur. The time required for the complete regrowth of grazed leaf tissue was greater than 50 days but on cyclically grazed areas, regrazing intervals were much shorter than this. The geese were therefore grazing partially depleted swards. The lengths of cycles were unrelated to the intensity of grazing (measured in goose min m -2 ) but positively related to the percentage cover of P. maritima in the sward. Grazing resulted in a younger leaf age structure on tillers of P. maritima, particularly in swards of low cover. Because geese showed no selection for younger leaves or unbitten tillers, it is suggested that the shorter cycle lengths on lower biomass swards represent a preference for the higher nutrient quality provided by the younger leaf age structure following grazing.