Flower‐visitor networks only partially predict the function of pollen transport by bees

Abstract

Mutualistic networks display distinct structural and organizational features such as nestedness, powerlaw degree distribution and asymmetric dependencies. Attention is now focused on how these structural properties influence network function. Most plant-pollinator networks are constructed using records of animals contacting flowers, which is based on the assumption that all visitors to flowers are pollinators; however, animals may visit flowers as nectar robbers, florivores, or to prey upon other visitors.To differentiate potential pollinator interactions from other interaction types, we examined individual bees that had visited flowers to detect if they carried pollen. Using these data, we constructed visitation and pollen-transport networks for a spinifex-dominated arid zone grassland. To determine how the structure of the visitation network reflects pollen transport, we compared the two networks using a null model approach to account for differences in network size. Differences in number of species, nestedness and connectance observed between the visitation and pollen-transport networks were within expected ranges generated under the null model.The pollen-transport network was more specialized, had lower interaction evenness, and fewer links compared to the visitation network. Almost half the number of species of the visitation network participated in the pollen-transport network, and one-third of unique visitation interactions resulted in pollen transport, highlighting that visitation does not always result in pollination. Floral visitor data indicate potential pollen transporters, but inferring pollination function from visitation networks needs to be performed cautiously as pollen transport resulted from both common and rare interactions, and depended on visitor identity. Although visitation and pollen-transport networks are structurally similar, the function of all species cannot be predicted from the visitation network alone. Considering pollen transport in visitation networks is a simple first step towards determining pollinators from non-pollinators. This is fundamental for understanding how network structure relates to network function.

Flower-visitor and pollen transport networks in a large city: structure and properties

“…Body pollen loads are one of the necessary prerequisites of pollination (Johnson and Steiner 2000;Pellmyr 2002), but they still do not provide much information on the real importance of visitor taxa (see e.g., Zych et al 2013) and hence do not offer the ultimate solution to our efforts of studying real pollination networks, as opposed to flower-visitor networks. Pollen transport networks seem, however, an important step towards better understanding the complex relationships that exist between plants and their pollinators on an ecosystemic scale (Popic et al 2013). …”

“…Quite a high proportion of ''cheating'' floral visitors is also observed in ecosystem-wide analyses, ranging from 18 % in British hay meadows (Forup and Memmott 2005) to 25-34 % in montane communities in California (Alarcon 2010). In such cases, pollen analyses may add a valuable perspective to our knowledge of plantanimal networks (Forup and Memmott 2005;Gibson et al 2006;Forup et al 2008;Bosch et al 2009;Alarcon 2010;Devoto et al 2011;Popic et al 2013). Also, some traditionally used metrics for studying network properties, especially connectance, are said to be strongly dependent on sampling effort (Nielsen and Bascompte 2007;Vazquez et al 2009;Blüthgen et al 2006Blüthgen et al , 2008Blüthgen 2010;Dorado et al 2011), and adequate sampling of interaction diversity is labour intensive, so networks published to date may be largely undersampled (Chacoff et al 2012) and may not necessarily be ecologically meaningful (Ulrich 2009;Blüthgen 2010).…”

“…Studies of pollination networks in particular, carried out in different parts of the world, revealed some common properties of such assemblages, namely: (1) low connectance, as there is rather a small fraction of realized links in the interaction matrix when compared with all the possible links that can be established between interacting partners (plants and animal pollinators), (2) the probability distribution of link number (node degree) follows power law or truncated power law, (3) the matrices of interactions are nested, which means that there are few highly connected species (generalists) that interact with other generalists and also with specialists (weakly connected species) and, conversely, there are many specialists that interact solely with generalists and (4) most of the studied networks have a modular structure, with some species acting as system hubs binding the whole network together (for general review of biological network properties see, e.g., Jordano et al 2006;Bascompte and Jordano 2007;Vazquez et al 2009;Olesen et al 2010Olesen et al , 2012Tylianakis et al 2010). However, most of the network analyses conducted so far rely on insect visits as a measure of mutualism (Alarcon 2010;Popic et al 2013), whereas visitation patterns are, in fact, only one of the components of animal quality as pollinators (Johnson and Steiner 2000;Pellmyr 2002;Willmer 2011), and sometimes a misleading one, since plants seem to be pollinated by fewer pollinators than assumed from visit observations (Ollerton 1996). This is especially pronounced of generalist plants.…”

The secret service – analysis of the available knowledge on moths as pollinators in New Zealand

“…To be an effective pollinator, a visitor must remove and carry viable pollen and deposit it on a conspecific stigma when the stigma is receptive (King et al 2013;Popic et al 2013). However, the ability of an animal to act as a pollinator is often inferred from various indirect lines of evidence rather than proven, resulting in a range of evidence types offering different amounts of certainty.…”

“…The New Zealand literature also contained reports of moths visibly contacting the reproductive organs of flowers. Whether these moths removed or deposited pollen after visiting flowers was not determined, so this evidence cannot be accepted as proof of pollination, as not all visitors to flowers act as pollinators (Genini et al 2010;Hegland et al 2010;King et al 2013;Popic et al 2013). Furthermore, visitation can be detrimental to a plant because the visitor can remove resources such as nectar and pollen that are better used by other more effective pollinators that offer higher fitness returns (Waddington 1983;Thomas 2003;Newstrom & Robertson 2005).…”