Pollinator-Mediated Indirect Effects on Plant Fecundity Revealed by Network Indices

Abstract

Indirect effects arise when one species influences how another species interacts with a third. Pollinator-mediated indirect effects are widespread in many plant communities and are often not restricted to plant species pairs. An analytical framework does not exist yet that allows for the evaluation of indirect effects through shared pollinators in a community context as well as their consequences for plant fitness. We used network indices describing pollinator sharing to assess the extent to which plant species affect and are affected by others in a pollination network from a species-rich dune community. For 23 plant species, we explore how these indices relate to plant fecundity (seeds/flower) over two years. We further linked plant traits and indices to uncover functional aspects of pollinator-mediated indirect interactions. Species frequently visited by shared pollinators showed higher fecundity and exhibited traits that increase pollinator attraction and generalization. Conversely, species whose shared pollinators frequently visited other plants had lower fecundity and more specialized traits. Thus, pollinator sharing benefited some species while others suffered reproductive disadvantages, consistent with competition. The framework developed here uses network tools to advance our understanding of how pollinator-mediated indirect interactions influence a species' relative reproductive success at the community level.

The non‐random assembly of network motifs in plant–pollinator networks

“…Understanding the global patterns of network structure is key to unravelling the processes that govern community dynamics (Bascompte & Jordano, 2007; Bastolla et al, 2009; Guimarães, 2020). However, common analytical approaches at the node or the global network level are unable to capture the full complexity of ecological systems (Bergamo et al, 2021; Simmons, Cirtwill, et al, 2019). Because network subgraphs can capture both direct and indirect interactions, by analysing their over‐ and under‐representation we were able to explore in great detail the structure of interactions between animal pollinators and the plants they visit.…”

“…Although some of the most studied indirect interactions are the ones caused by trophic cascades with consumer–resource interactions (Abrams et al, 1996; Finke & Denno, 2004; Schmitz & Suttle, 2001), there is also a growing body of work that examines the consequences of indirect effects in mutualistic networks (Bastolla et al, 2009; Guimarães et al, 2017; Knight et al, 2005; Vieira & Almeida‐Neto, 2015). Traditionally, the study of animal–plant mutualistic networks has commonly focused on direct interactions and overlooked indirect effects such as competitive or facilitative processes between plants for pollinators (but see, Bergamo et al, 2021; Carvalheiro et al, 2014; Moeller, 2004; Sargent & Ackerly, 2008). In part, because conventional analytical tools in bipartite networks often discard relevant information about indirect interactions by condensing the information into single indices either at species or network level (Cirtwill, Roslin, et al, 2018; Simmons, Cirtwill, et al, 2019).…”

The non-random assembly of functional motifs in plant-pollinator networks

“…Understanding the global patterns of network structure is key to unravel the processes that govern community dynamics (Bascompte & Jordano, 2007; Bastolla et al, 2009; Guimarães, 2020). However, common analytical approaches at the node or at the full network level are unable to capture the full complexity of ecological systems (Bergamo et al, 2021; Simmons, Cirtwill, et al, 2019). Because network motifs can capture both direct and indirect interactions, by analysing over- and under-representation of motifs we were able to explore in great detail the structure of interactions of plant-pollinator networks.…”

The role of intra-guild indirect interactions in assembling plant-pollinator networks

“…While the assembly of mutualistic communities is often driven by direct interactions between well-connected species [17][18][19] , few studies have investigated the role of indirect interactions 11,20,21 . Indirect effects are difficult to detect empirically 13,22,23 because they require increasing the ecological and temporal scales of study in order to consider more species and interactions 24,25 . Nevertheless, indirect interactions are as influential as direct interactions to plant-pollinator dynamics 12 .…”

“…Nevertheless, indirect interactions are as influential as direct interactions to plant-pollinator dynamics 12 . Particularly in communities where plant-pollinator mutualisms are obligate, species within a guild sharing mutualistic resources will boost or hinder each other's reproductive success 13,23,[26][27][28][29] .…”