Topography affects microclimate, spatial distribution of species, and thus species composition. In turn, it may affect the network between plants and pollinators. Here, we studied the effects of different combinations of topography factors and seasonality on plant-pollinator networks, to address the question: how do plant-pollinator interactions and network structures change between ridge and valley site across different seasons? This study was conducted at a stem-mapping forest plot in southern China established in 2011. This area has a complex topography and monsoon climate with distinct rainy season (April to September) and dry season (October to March). We examined four 2 ha plots in ridge and valley sites with different ratios of dioecious and non-dioecious plants. We observed 1776 interactions between 84 pollinator morphospecies and 28 flowering plants (9 dioecious plants) in the understory throughout the February to November flowering season in 2013 and 2014. For dioecious plants, we treated male and female as different species, due to their sex-specific differences in floral display and phenology. From these data, we created plant-pollinator networks across four sites and four seasons and calculated three common network indices for each: interaction evenness, complementary specialization and weighted nestedness.
We found that plant-pollinator communities had a high turnover between seasons, while the network structure properties remained unchanged. The diversity of plant sexual phenotypes and pollinator functional groups did not show difference between rainy and dry seasons. The compositions of plant-pollinator communities showed a high turnover between ridge and valley sites. Floral abundance and pollinator visiting frequencies were higher in ridge site than in valley site. The diversity of plant sexual phenotypes were higher in ridge site than in valley site. In ridge site, our networks displayed significantly lower complementary specialization. The other two network properties did not differ between the two sites. In networks with higher plant sexual diversity, our network indices indicated higher diversity of pollinator functional groups, with most of the pollinators being generalists of the dioecious plants. This suggests that dioecious plants can provide quality resources for generalist pollinators and communities with high plant sexual diversity can support and sustain plant-pollinator networks.