Functional complementarity of seed dispersal services provided by birds and mammals in an alpine ecosystem

Endozoochory is an important ecosystem function that, in temperate and boreal regions, is carried out mainly by birds and mammals. Due to their different quantitative and qualitative contributions to seed dispersal, these animals usually differ in their effectiveness as seed dispersers. However, there is still little information about how spatio-temporal differences in frugivory between birds and mammals may create complementarity in their seed dispersal services. We investigated the complementarity of seed dispersal services provided by avian and mammalian seed dispersers of bilberry Vaccinium myrtillus in an alpine ecosystem, the Tatra Mountains (Poland). We collected bird droppings and mammal scats containing bilberry seeds in coniferous forests and alpine meadows. Birds were identified by DNA barcoding techniques while mammals were visually assigned in the field. We analysed the effects of habitat, microhabitat and timing of seed dispersal on the quantitative contributions of each species to the total seed rain and conducted ex situ germination experiments to assess the quality of the seed dispersal services. At least 13 bird and three mammal species dispersed bilberry seeds in the Tatra Mountains. Two species of thrushes-song thrushes Turdus philomelos and fieldfares T. pilaris, brown bears Ursus arctos and red foxes Vulpes vulpes were the most effective dispersers. Quantitative and qualitative contributions to seed dispersal differed between birds and mammals. Mammals, particularly brown bears, dispersed the majority of bilberry seeds in both habitats. The total seed rain provided by birds and mammals differed according to the habitat, microhabitat and timing of seed dispersal. Birds dispersed most seeds in the middle of the fruiting season and within the forest, while seed dispersal by mammals peaked in the upper meadows and at the end of the season. Seeds defecated by birds had higher probability to germinate as their droppings contained less seeds and were defecated in more suitable microhabitats than seeds defecated by mammals. Synthesis. Despite differences in the quantity and quality of seed dispersal, both bird and mammal species are effective bilberry dispersers. Complementary seed dispersal by these two groups guarantees the arrival of bilberry seeds to different microhabitats suitable for germination and during the entire fruiting season, maximizing the chances of bilberry seedling recruitment.