Avian seed ingestion changes germination patterns of bilberry, Vaccinium myrtillus

Frugivores functioning as seed dispersers may have fundamental effects on the reproductive success of fruiting plants. Studies on this interaction have, however, mostly ignored possible within-season and between-year variations in seed germination behaviour. The aim of this study was to assess how berry ingestion by thrushes (Turdus spp.) affects seed germination of bilberry (Vaccinium myrtillus). Specifically, we studied if the timing of dispersal within the fruiting season affects germination success of seeds and whether these effects vary between years. The germination rate of intact seeds declined in the course of the fruiting season both in 1997 and in 1999. Ingestion by thrushes accelerated the germination rate of passed seeds and the final germination percentages of passed seeds were higher than or similar to those of intact seeds. These effects varied within season and between years. Thus, when effects of frugivores on seed germination success are studied, experiments should not rely on the data collected at a single time in the season and in a single year.