Long-Term Dynamics of Bird Dispersal Network Structures Associated with the Functions in Bird Migration and Fruit Abundance

Abstract

In temperate zones, the complex network of seed dispersal by migrant birds is formed and the structure is dynamic on long time scale. Over 12 years, we examined interannual variability of structures of bird dispersal networks and factors affecting them by observing the characteristics of fruit abundance, bird migration and bird dispersal interactions in central Japan. The fruit abundance exhibited a remarkable fluctuation across years, with the number of fruiting trees and matured fruits fluctuating repeatedly every other year, leading to the periodic fluctuations. The abundance of migrants was also fluctuated. According to the abundance of fruits and migrants, the 12 years as study period was classified into three types. The seed transporting frequency and the dispersal networks were investigated by collecting faeces of migrants. Of the 6652 samples collected from 15 bird species, 1671 (25.1%) included seeds from 60 plant species. Main dispersers were composed of Turdus pallidus, T. obscurus and Zosterops japonicus. The structures of bird dispersal networks were highly nested over 12 years, suggesting the networks are stable. Specifically, the nested structure developed in years when fruit abundance was low. GLM analyses showed the abundance of migrants, particularly T. pallidus and T. obscurus, had strong positive effects on construction of nested structure. The development of nested structure may be caused by the fact the two Turdus species were more frequently functioning as generalist dispersers when fruit abundance was lower. Our study revealed one of the mechanisms determining the structure of bird dispersal network on long time scale.