ON FACILITATION BETWEEN HERBIVORES: HOW BRENT GEESE PROFIT FROM BROWN HARES

Abstract

Brown hares (Lepus europaeus) are shown to facilitate grazing by Brent Geese (Branta bernicla) in a temperate salt marsh in the Netherlands by retarding vegetation succession for >25 yr. Winter grazing by hares prevented the shrub Atriplex portulacoides from spreading in younger parts of the salt marsh. Clipping experiments showed that Atriplex had poor recovery after removal of aboveground tissue, which makes Atriplex vulnerable for hare attack. Once Atriplex swards were cut to the ground to mimic hare grazing, Brent Geese visited those sites more than twice as frequently than untouched control plots. Goose visitation was reduced when bushes of Atriplex were planted. Large parts of the core feeding area of Brent Geese would be unsuitable for goose grazing if hares were not present. A reduction of at least 44% in the carrying capacity of the marsh for Brent Geese was calculated in the absence of hares. Vulnerability of Atriplex to hare grazing and the high food intake of geese are key elements to this facilitative pattern. Additionally, hares reduced the number of dead Artemisia maritima stems in grassy swards, which otherwise might have hampered grazing by geese. Facilitation by herbivores such as hare, rabbit, cattle, and sheep is likely to be a prominent factor enhancing feeding conditions for Brent Geese all along the northwest European coast.