Downscaling pollen–transport networks to the level of individuals

Abstract

Summary1. Most plant-pollinator network studies are conducted at species level, whereas little is known about network patterns at the individual level. In fact, nodes in traditional speciesbased interaction networks are aggregates of individuals establishing the actual links observed in nature. Thus, emergent properties of interaction networks might be the result of mechanisms acting at the individual level. 2. Pollen loads carried by insect flower visitors from two mountain communities were studied to construct pollen-transport networks. For the first time, these community-wide pollentransport networks were downscaled from species-species (sp-sp) to individuals-species (i-sp) in order to explore specialization, network patterns and niche variation at both interacting levels. We used a null model approach to account for network size differences inherent to the downscaling process. Specifically, our objectives were (i) to investigate whether network structure changes with downscaling, (ii) to evaluate the incidence and magnitude of individual specialization in pollen use and (iii) to identify potential ecological factors influencing the observed degree of individual specialization. 3. Network downscaling revealed a high specialization of pollinator individuals, which was masked and unexplored in sp-sp networks. The average number of interactions per node, connectance, interaction diversity and degree of nestedness decreased in i-sp networks, because generalized pollinator species were composed of specialized and idiosyncratic conspecific individuals. An analysis with 21 pollinator species representative of two communities showed that mean individual pollen resource niche was only c. 46% of the total species niche. 4. The degree of individual specialization was associated with inter-and intraspecific overlap in pollen use, and it was higher for abundant than for rare species. Such niche heterogeneity depends on individual differences in foraging behaviour and likely has implications for community dynamics and species stability. 5. Our findings highlight the importance of taking interindividual variation into account when studying higher-order structures such as interaction networks. We argue that exploring individual-based networks will improve our understanding of species-based networks and will enhance the link between network analysis, foraging theory and evolutionary biology.

Geographical variation in mutualistic networks: similarity, turnover and partner fidelity

“…Others have also distinguished between species-driven interaction turnover and rewiring [9,15], but because species-driven turnover was the main contributor, we further partitioned it into pollinator-, plant-and pollinator þ plant-driven (figures 1 and 3). Along the entire gradient, pollinator-driven turnover accounted for a larger fraction of the overall species-driven turnover than that of plants, which probably is an effect of (i) the initial selection of sites based on their vegetational similarity, (ii) the plant-centred sampling protocol where we observed plants for pollinators and not vice versa [42,43], (iii) the higher diversity of pollinators compared with plant species, and (iv) the perennial lifestyle of plants versus the annual lifestyle of the pollinators. Thus, pollinator species probably fluctuated more in abundance and diversity, and consequently accounted for more of the observed interaction turnover.…”

“Hummingbird” floral traits interact synergistically to discourage visitation by bumble bee foragers

“…We found that bees readily visited M. cardinalis flowers in monospecific greenhouse populations but mostly avoided them Foraging rate (no. Recent studies on plant-pollinator interaction networks have shown that individual specialization, although a common behavioral attribute in generalist pollinators, is often overlooked by traditional species-level indices of specialization (e.g., visitation rates to a focal plant) due to inter-individual variability in floral resource use (Tur et al 2014(Tur et al , 2015. Despite suggestions to the contrary Schemske 2003, Lunau et al 2011), bumble bees clearly have the capacity to detect bird syndrome flowers and learn to associate them with a nectar reward, a finding well supported by previous work (Chittka and Waser 1997, Mayfield et al 2001, Martinez-Harms et al 2010, but choose to avoid them in mixed floral environments when it makes economic sense to do so.…”

Upscaling the niche variation hypothesis from the intra- to the inter-specific level

“…Those results are in agreement with some experimental studies performed with fishes for which inter-individual variation increased with population density. Abundance also decreased intraspecific overlap in plant-pollinator networks (Tur et al 2014). In contrast, solitary species such as roe deer could be less prone to intra-specific competition, as each individual should be able to feed on their preferred resources without pressure from congeners in their close vicinity.…”