Highly mobile seed predators contribute to interisland seed dispersal within an oceanic archipelago

Long-distance dispersal (LDD) is an essential event for species
colonization and expansion in oceanic island ecosystems. Endozoochory by
birds is an important factor promoting the LDD of plant seeds, but its
contribution to interisland seed dispersal is still unclear. Here, we show
possible seed dispersal by a seed predator pigeon, the Japanese wood
pigeon Columba janthina, among oceanic islands in the Izu archipelago,
Japan. Although some previous studies showed that most seeds swallowed by
this pigeon are crushed, intact seeds were found in 44.5% of pigeon feces,
indicating the contribution of these birds to seed dispersal. Seasonal
population fluctuations and movements by pigeons between neighbouring
islands which are 4 km apart by pigeons can promote interisland seed
dispersal. We found seeds from fruiting trees not located on the islands
where the pigeon feces were collected, which could have been transported
from another island. Although many tree species overlap their fruiting
phenology with pigeon movement between islands, the seeds of only six
species were dispersed by pigeons, and most of the dispersed seeds were
from species with small seeds less than 1.5 mm in diameter. Seed
consumption and frequent interisland movement of Japanese wood pigeons
might homogenize the distribution of plants with small seeds among
neighboring islands. In contrast, their intensive seed consumption might
disturb the recruitment of plants with large seeds and make their
expansion to other islands difficult. Such effects of mutualistic
(dispersal) and antagonistic (predation) interactions with highly mobile
seed predators may modify plant distributions and gene flow in oceanic
archipelagos.