Interactive effects of multiple seed dispersers on plant fitness: An empirical study of the Japanese walnut and two rodent species

Abstract

The effects of seed dispersers on plant fitness (seed dispersal effectiveness, SDE) have been evaluated based on the number (quantity) and recruitment probability (quality) of dispersed seeds. Although seeds of most zoochore species are dispersed by two or more animal species, which may interact with each other, SDE has been studied assuming a one-plant and one-animal species system. We compared the SDE of Japanese walnut (Juglans ailantifolia) between squirrel-only and squirrel-mouse sites in natural forests of Hokkaido, Japan, and found that the SDE from red squirrels (Sciurus vulgaris), considered a primary seed disperser, was altered by an alternative seed disperser species, the Japanese wood mouse (Apodemus speciosus). Seed removal rates at the squirrel-mouse site were significantly higher than those at the squirrel-only site, and both dispersed seeds and seedlings were less aggregated with the strongly repulsive relationship with adult conspecific trees at the squirrel-mouse site. Seedlings established themselves at a location with fewer medium-sized trees (< 10 cm DBH) at the squirrel-mouse site. These results suggest that the interactive effect of the rodent species affects the SDE of the Japanese walnut.