Seed Dispersal Patterns Of Castanopsis Sclerophylla In A Fragmented Landscape

The granivore-mediated seed dispersal plays a decisive role on the recruitment and the long-term persistence of plant populations. Habitat fragmentation is widely recognized as one of the greatest threats for the sustainability of the local ecosystems, and thereby is expected to impact seed dispersal. However, it is still unclear that how habitat fragmentation affects the process of seed dispersal. Based on the theory of food chain, we hypothesized that seed dispersal patterns will vary across remnant habitats suffering different degrees of fragmentation. In the habitats with medium size, the abundance of granivores can be extremely high in the absence of top-down regulation (i.e. the local extinction of top predators), leading to catastrophic pressure of seed predation, while in the seriously fragmented habitats, granivores will be mainly controlled by bottom-up regulation (restricted resources, such as seed production), causing varying seed dispersal patterns over time. In this study, we delineated the seed dispersal processes in habitats with different extents of fragmentation in the southeastern region of Qiandao Lake for two consecutive years (2010and2011) of Castanopsis sclerophylla, a nut-bearing tree species, to test our hypothesis. Consistent with our expectation, most serious seed predation appeared in the habitats with medium sizes, supported by the highest seed removal rate, granivore abundance and the lowest seed survival rate. Simultaneously, seriously fragmented habitats also suffered limited seed dispersal relative to slightly fragmented and unfragmented habitats with high seed removal rate and seed predation pressure. Furthermore, the seed dispersal patterns in small-size habitats were significantly different between years, also confirming our hypothesis. Generally, the seed remaining days was significantly negatively related with granivore density but positively correlated with both the densities of seed production and top predators, indicating both top-down and bottom-up regulation influenced the seed dispersal. However, the negative relationship between granivore abundance and top predators density was significant only when the data from the most fragmented habitat were removed, providing an evidence that bottom-up rather than top-down regulation dominated in this habitat type. In conclusion, different degrees of fragmentation can result in heterogeneous and limited seed dispersal through affecting the food chain, which is valuable for the biological conservation and restoration.