FRUIT REMOVAL IN VIBURNUM OPULUS - COPIOUS SEED PREDATION AND SPORADIC MASSIVE SEED DISPERSAL IN A TEMPERATE SHRUB

The fruit removal in Viburnum opulus (Caprifoliaceae), an animal-dispersed shrub with fleshy fruits, was studied during two fruiting seasons in central Sweden. The fruit crops were ripe in September, but mainly removed during November-December. Large infructescences were located in the upper part of shrubs and received the slowest removal rate. The removal agents were a diverse assemblage of seed predators and dispersers. At two field study-sites the most important removal agents were seed predators: bullfinches (Pyrrhula pyrrhula) (73% removal) and rodents, mainly bank voles (Clethrionomys glareolus) (26% removal). Fallow deer (Dama dama) infrequently contributed to fruit removal. Legitimate seed dispersers, waxwing (Bombycilla garrulus) and thrush species (Turdus spp.), removed only a small fraction of the fruit crop, 0.3%. Thus seed predation (ca 99%) copiously exceeded seed dispersal. The removal agents preferred other fruits to V. opulus, e.g. Sorbus aucuparia; when other fruit resources had been depleted they switched to V. opulus. At a garden-site, however, waxwings in at least four out of seven years removed all the fruits in only a few days. It is concluded that the frequently low dispersal potential of V. opulus because of heavy seed predation is compensated by sporadic massive dispersal by waxwings in certain favourable years. The plant apparently exhibits a costly bet-hedging seed-dispersal strategy.