Plant-pollinator network assembly along the chronosequence of a glacier foreland

Forelands of retreating glaciers offer an ideal model system to study community assembly processes during primary succession. As plants colonize the area that is freed from ice they should be accompanied by their pollinators to successfully reproduce and spread. However, little is known about the assembly of plant-pollinator networks. We therefore used quantitative network analysis to study the structure of plant-pollinator interactions at seven sites representing a chronosequence from 8 to 130 years since deglaciation on the foreland of the Morteratsch glacier (southeastern Switzerland). At these sites, individual visits of plant flowers by insects were recorded throughout the flowering season. Species richness of insect-pollinated plants and plant-pollinating insects, together with measures of interaction diversity and evenness, increased along the chronosequence at least for the first 80 years after deglaciation. Bees were the most frequent flower visitors at the two youngest sites, whereas flies dominated in mature communities. Pollinator generalization (the number of visited plant species weighted by interaction strength), but not plant generalization, strongly increased during the primary succession. This was reflected in a pronounced decline in network level specialization (measured as Bluthgen's H-2') and interaction strength asymmetry during the first 60 years along the chronosequence, while nestedness increased along the chronosequence. Thus, our findings contradict niche-theoretical predictions of increasing specialization of pollination systems during succession, but are in agreement with expectations from optimal foraging theory, predicting an increase in pollinator generalization with higher plant diversity but similar flower abundance, and an increase in diet breadth at higher pollinator densities during primary succession.