Seasonal difference in the effects of fragmentation on seed dispersal by birds in Japanese temperate forests

Many studies have demonstrated that forest fragmentation reduces populations of animal species and causes local extinction, triggering many cascading effects. The effect of fragmentation on animals can be exerted through various processes, but such effects have been understudied. In this study, we posed the possibility of differences in the seasonal effects of fragmentation on frugivorous birds and their dispersal of seeds belonging to five tree species. We hypothesized that these effects may be caused by birds and their habitat selection for suitable breeding forests. We compared the abundance and species richness of frugivorous birds and the number of bird-removed fruits between a well-preserved and a fragmented temperate forest for two consecutive years. The abundance of birds was lower in the fragmented compared to the well-preserved forest during the breeding season, although no clear differences in species richness were observed. In contrast, similar decreases in bird abundance were not observed during the migratory season. After controlling for variation in crop size, the number of bird-removed fruits was lower in the fragmented forest compared to the well-preserved forest during the breeding season, but there was no such tendency during the migratory season. These results indicate that evaluations regarding the effects of fragmentation on seed dispersal that do not consider seasonal factors may lead to erroneous conclusions. This study suggests that the effects of fragmentation can be exerted though various processes, many of which remain poorly studied and warrant further examination.