Plant-Pollinator Networks
It is now 20 years since the first plant-pollinator network studies took the field by storm. On the one hand, these studies raised many objections, from the methodological tendency to assume that abundant floral visitors are effective pollinators to the emphasis placed on more generalized interactions, along with a new vocabulary (nestedness, connectedness, hub species) that needed to be scrutinized. On the other hand, network studies revealed some unexpected truths, including the fact that presumably specialized interactions (e.g. Cypripedium ladies’ slipper orchids pollinated only by queen Bombus bees) are highly asymmetric. That is, the bee species might also pollinate many members of the same plant community, and (given that the orchid is nectarless) would need to acquire nectar from a more common, co-blooming plant, like a cherry tree, which could effectively be thought of as subsidizing the orchid’s specialized pollination system.
It is now 20 years since the first plant-pollinator network studies took the field by storm. On the one hand, these studies raised many objections, from the methodological tendency to assume that abundant floral visitors are effective pollinators to the emphasis placed on more generalized interactions, along with a new vocabulary (nestedness, connectedness, hub species) that needed to be scrutinized. On the other hand, network studies revealed some unexpected truths, including the fact that presumably specialized interactions (e.g. Cypripedium ladies’ slipper orchids pollinated only by queen Bombus bees) are highly asymmetric. That is, the bee species might also pollinate many members of the same plant community, and (given that the orchid is nectarless) would need to acquire nectar from a more common, co-blooming plant, like a cherry tree, which could effectively be thought of as subsidizing the orchid’s specialized pollination system.
Most of the early plant-pollinator network studies focused on density-dependent aspects in space (floral abundance) or time (floral phenology) as drivers of network structure. When ecologists focused on actual floral traits, they tended to study aspects of trait matching (floral depth vs. pollinator mouthparts) as important factors in network structure. As usual, my students and I wondered if these studies might be overlooking floral scent (in both attractive and repellent roles) as an “invisible hand” that could further impact network structure. Pioneering studies by Robert Junker and collaborators demonstrated proof of concept, with floral scent attracting obligate floral visitors (bees and flies) but repelling facultative visitors (ants).
[Below, analysis of floral color-scent combinations in a complex plant-pollinator network from Lesvos, Greece. L to R: tanglegram showing 7 modules of plant species clustered by similarity in volatile composition; abundant hub species in module 3, Lavandula stoechas and Cistus creticus, sharing sesquiterpene volatiles; modules organized by Pearson correlation statistics with different volatile compound classes]
[Below, analysis of floral color-scent combinations in a complex plant-pollinator network from Lesvos, Greece. L to R: tanglegram showing 7 modules of plant species clustered by similarity in volatile composition; abundant hub species in module 3, Lavandula stoechas and Cistus creticus, sharing sesquiterpene volatiles; modules organized by Pearson correlation statistics with different volatile compound classes]
Afroditi Kantsa and her advisor at the University of the Aegean, Theodora Petanidou, initiated an intensive project in a well-studied phrygana community on the Greek island of Lesvos. Afroditi collected pollinator visitation, floral volatiles and reflectance spectral data in an unbiased fashion for 41 species of flowering plants in this habitat, then used modularity analysis to group plant species by shared patterns of scent composition. Remarkably, most of the modules appear to have been grouped by flower color rather than scent, indicating an unexpected coordination of floral scent and color across the phrygana. AIC models testing the relative contributions of different factors to network structure revealed that several elements of floral display (density, color modeled in pollinator perceptual space, emissions of specific volatile classes) all contributed significantly to network properties such as normalized degree, betweenness centrality and closeness centrality. Finally, when the total volatilome (351 compounds) was substituted for the plant community, a new network constructed between scent compounds and pollinators revealed surprising modules connecting taxonomic groups with compound classes, such as megachilid bees associated with sesquiterpenes (largely due to hub plants, Cistus and Lavandula) and apid bees associated with plants emitting benzenoid volatiles. These findings compel a closer look at how floral scent and color interact with pollinator richness in floral communities worldwide.
Kantsa, A, R.A. Raguso, A.G. Dyer, S.P. Sgardelis, J.M. Olesen, T. Petanidou. 2017. Community-wide integration of floral colour and scent in a Mediterranean
scrubland. Nature Ecol. & Evolution 1: 1502. https://www.nature.com/articles/s41559-017-0298-0
Kantsa, A, R.A. Raguso, A.G. Dyer, J.M. Olesen, T. Tscheulin, T. Petanidou. 2018. Disentangling the role of floral sensory diversity in pollination networks. Nature
Communications 9: 1-13. https://www.nature.com/articles/s41467-018-03448-w
Kantsa, A., R.A. Raguso, T. Lekkas, O-I. Kalantzi, T. Petanidou. 2019. Floral volatiles and visitors: a meta-network of associations in a natural community. Journal of
Ecology 107: 2574-2586. https://doi.org/10.1111/1365-2745.13197
Kantsa, A, R.A. Raguso, A.G. Dyer, S.P. Sgardelis, J.M. Olesen, T. Petanidou. 2017. Community-wide integration of floral colour and scent in a Mediterranean
scrubland. Nature Ecol. & Evolution 1: 1502. https://www.nature.com/articles/s41559-017-0298-0
Kantsa, A, R.A. Raguso, A.G. Dyer, J.M. Olesen, T. Tscheulin, T. Petanidou. 2018. Disentangling the role of floral sensory diversity in pollination networks. Nature
Communications 9: 1-13. https://www.nature.com/articles/s41467-018-03448-w
Kantsa, A., R.A. Raguso, T. Lekkas, O-I. Kalantzi, T. Petanidou. 2019. Floral volatiles and visitors: a meta-network of associations in a natural community. Journal of
Ecology 107: 2574-2586. https://doi.org/10.1111/1365-2745.13197