Subtle interplay of competition and facilitation among small herbivores in coastal grasslands

1. Overlap in habitat use between herbivores can result in facilitative interactions, through enhancement of forage quality, as well as competitive interactions. The latter result from either interference or indirectly from resource depletion.2. We investigated competitive and facilitative interactions between wild Barnacle and Brent Geese and European Brown Hares on a salt marsh in the Dutch Wadden Sea. In a multifactorial experimental design, we manipulated biomass and quality of grass swards and determined foraging preferences of the wild herbivores.3. We found that both Barnacle and Brent Geese select plots with plants that have a higher nitrogen content. Barnacle Geese avoid plots with high plant biomass.4. Hares prefer the combination of high biomass with high plant quality, when geese are absent. However, in the natural situation with geese present, hares select high biomass swards.5. Grazing increases the quality of the vegetation within one season. Geese mainly select plots that have been previously grazed by either geese or hares within the same season.6. We conclude that indirect competition through forage depletion by large numbers of geese in spring plays a significant role determining the foraging choices of hares, while Barnacle Geese profit from grazing facilitation by other small herbivores which prevents the maturation of forage tissues.