The topological differences between visitation and pollen transport networks: a comparison in species rich communities of the Himalaya–Hengduan Mountains

Abstract

Pollination networks are usually constructed and assessed by direct field observations which commonly assume that all flower visitors are true pollinators. However, this assumption is often invalid and the use of data based on mere visitors to flowers may lead to a misunderstanding of intrinsic pollination networks. Here, using a large dataset by both sampling floral visitors and analyzing their pollen loads, we constructed 32 networks pairs (visitation versus pollen transport) across one flowering season at four elevation sites in the Himalaya–Hengduan Mountains region. Pollen analysis was conducted to determine which flower visitors acted as potential pollinators (pollen vectors) or as cheaters (those not carrying pollen of the visited plants). We tested whether there were topological differences between visitation and pollen transport networks and whether different taxonomic groups of insect visitors differed in their ability to carry pollen of the visited plants. Our results indicated that there was a significantly higher degree of specialization at both the network and species levels in the pollen transport networks in contrast to the visitation networks. Modularity was lower but nestedness was higher in the visitation networks compared to the pollen transport networks. All the cheaters were identified as peripheral species and most of them contributed positively to the nested structure. This may explain in part the differences in modularity and nestedness between the two network types. Bees carried the highest proportion of pollen of the visited plants. This was followed by Coleoptera, other Hymenoptera and Diptera. Lepidoptera carried the lowest proportion of pollen of the visited plants. Our study shows that the construction of pollen transport networks could provide a more in‐depth understanding of plant–pollinator interactions. Moreover, it suggests that detecting and removing cheater interactions when studying the topology of other mutualistic networks might be also important.

Promiscuous pollinators—Evidence from an Afromontane sunbird–plant pollen transport network

“…However, FVN includes all visitors, including those that rob flower resources (Genini, Morellato, Guimarae & Olessen, ), so that including them in a pollination network may result in misleading perceptions of community structure and stability. In contrast, PTNs, constructed from flower visitors which collect pollen, result in a more realistic network (Alarcón, ; Zhao et al., ). Because each of the two network types contain unique information on the ecology of the system, creating both should improve our overall understanding of how species contribute to ecosystem function (Bosch, Gonzalez, Rodrigo & Navarro, ; Vizentin‐Bugoni et al., ) and may help identify species traits such as morphology (Maglianesi, Blüthgen, Bohning‐Gaese & Scheulning, ) and/or phenology which influence species interactions.…”

Plant-centred sampling estimates higher dissimilarity of interactions than pollinator-based sampling across habitats

“…Although pollen data do not always lead to higher estimates of interaction partitioning in pollination networks (Ramirez-Burbano et al 2017;Manincor et al 2020), there is a trend that subsequently calibrating interaction data with "better" estimates of potential pollination, i.e. pollen loads, pollen deposition on stigma and fruit set, will increasingly render only the subsets of interactions constrained by specific coadaptations and higher estimates of interaction specialization (Bosch et al 2009;Santiago-Hernandez et al 2019;Zhao et al 2019). Considering only the visitors that contact the reproductive structures of flowers is a first step in including only potential pollination interactions, but not all such visitors actually carry and deposit pollen (e.g.…”

“…Plant-pollinator interactions may be sampled either using a plant-centred (phytocentric) approach, which consists in identifying and quantifying interactions observing focal flowering plants, or an animal-centred (zoocentric) one, in which interactions are inferred through the identification of pollen grains attached to pollinators bodies (Bosch et al 2009;Jordano 2016;Zhao et al 2019). The plant-centred approach is by far the most common one in the existing literature on pollination networks (Vizentin-Bugoni et al 2018) probably because it is the most efficacious method and pollen sampling from example vertebrates is not straightforward.…”

“…The plant-centred approach is by far the most common one in the existing literature on pollination networks (Vizentin-Bugoni et al 2018) probably because it is the most efficacious method and pollen sampling from example vertebrates is not straightforward. This overrepresentation of one method in relation to another may be especially relevant, as indices derived from the two methods sometimes lead to distinct estimates of network structure (Bosch et al 2009;Ramirez-Burbano et al 2017;Zhao et al 2019). For instance, the addition of pollen load to visitation data recovered higher species degree and connectance in a Mediterranean pollination network (Bosch et al 2009).…”

“…We expect to find higher specialization and modularity in animal-than plant-centred networks due to the greater refinement of interactions obtained through the analysis of pollen grains attached to the pollinators' body (Bosch et al 2009;Zhao et al 2019). Furthermore, pollen grains may adhere more or less to the bodies of these animals, or pollinators may have different behaviours during the visit to the flowers, affecting estimates of specialization (Ballantyne et al 2015;Zhao et al 2019). In addition, plants with nonspecialized morphology and generalist pollination system will possibly be associated to higher differences on species level metrics between the two sampling methods.…”