How do changes in forest stand development affect frugivorous bird abundance and fruit removal in warm-temperate forests of western Japan?

Abstract

To better understand how plant–frugivore interactions change in relation to forest stand development, we investigated the abundance and species composition of frugivorous bird assemblages, and their fruit removal over several years in three warm-temperate forest stands. All stands were located in an urban forested area of western Japan, and each was at a different developmental stage: a secondary deciduous broad-leaved forest stand (DB), an evergreen broad-leaved forest stand where an evergreen oak became dominant recently (EB-I), and a well-developed evergreen broad-leaved forest stand (EB-II). Although the species richness and Shannon diversity indices of frugivorous birds did not vary among the three stands, the abundance of birds was almost always greatest in the EB-II stand, lowest in the DB stand, and intermediate in the EB-I stand. Generalized linear mixed model analyses using data on monthly fruit availability and the number of frugivorous birds observed in censuses showed that forest stand differences had a significant effect on the abundance of frugivorous birds, whereas fruit availability had no significant positive influence on bird abundance. Fruit removal was most intensive in the EB-II stand, where both fruit and bird abundance were highest, followed by the EB-I stand, where bird abundance was intermediate but fruit abundance was lowest. Our study demonstrates that increasing structural complexity during forest stand development likely had positive effects on the abundance of frugivorous birds, which in turn influenced fruit removal in this urban forested area. Hence, a positive feedback mechanism of plant–bird interactions would exist for the development of forest stands.