Flowering plant community composition affects plant-pollinator networks
Global change drivers are likely to impact pollination via their effect on biodiversity and ecological interactions. Understanding these impacts is especially important when considering plant communities in restoration, where pollinator function is crucial for long term success. We studied the interactive effects of different combinations of native and non-native flowering plants and simulated nitrogen deposition on plant-pollinator networks. We asked: how do plant-pollinator networks’ structures change with removal of non-native plants and nitrogen addition? This study was conducted at a large-scale restoration experiment, in south-western Australia, which was revegetated in 2010. This area is a known global biodiversity hotspot and home to many undescribed insect pollinators. We examined four planting assemblages with different ratios of native and non-native plants, and with or without nitrogen addition. We observed 3557 insect pollinator specimens of 60 morphospecies throughout the August to November flowering season in 2013. From these data, we created plant-pollinator networks across our treatments and calculated four common network indices for each: connectance, nestedness, linkage density and complementary specialisation.
We found that floral abundance decreased with the removal of non-native plants, but did not differ with nitrogen addition. As floral abundance increased the number of pollinator visits increased, but not pollinator richness. In plant communities with higher native floral abundance and decreased non-native floral abundance, our networks displayed significantly lower network complementary specialisation scores. The other three network properties did not differ across our four plant communities. In networks with the greatest floral abundance our network indices indicated high level of pollinator diversity, with most of the pollinators being generalists of the few flowering plants. This suggests that non-native flowers can provide quality resources for generalist pollinators in the early stages of restoration. In general, a diverse pollinator assemblage with generalised pollinators contributes to the structure and stability of networks. This demonstrates that a simplified restoration community can support and sustain plant-pollinator networks. Even though nitrogen had no effect on our plant-pollinator networks, there is a need for further research to establish the indirect effects of nitrogen deposition on other aspects of plant reproduction.