Effect of Birds and Bears on Seed Germination of Fleshy-Fruited Plants in Temperate Rainforests of Southeast Alaska

Abstract

The effects of avian and mammal seed dispersers on seed germination were investigated for six common plant species (five shrubs and one herb) in the temperate rainforests of southeast Alaska. Patterns of germination behaviour were compared between defecated seeds and control (unpassed) seeds for a period of two years. Seed passage through vertebrate frugivores enhanced germination in two species, Rubus spectabilis and Sambucus racemosa, and a similar trend was observed in Ribes bracteosum. In contrast, it did not affect seed germination in Oplopanax horridus, Streptopus amplexifolius and Vaccinium ovalifolium/alaskaense. No differences in germination behaviour were found between bear- and bird-treated seeds, suggesting that seed retention time (much greater in bears than in birds) in the guts does not relevantly affect germination. Differences among frugivore effects were detected only for Sambucus, with varied thrushes enhancing germination more than American robins and black bears. The two colour morphs of Rubus spectabilis showed similar germination patterns, suggesting that seed dispersers do not exert an important selective pressure contributing to the maintenance of the polymorphism, at least through the mechanism of a differential effect on germination. The results of this study support the idea that the advantages of animal seed dispersal lie more in seed movement away from the parent plant than in seed treatment within the disperser's guts.