Male Visitors May Decrease Modularity in Flower–Visitor Networks

Shigeki Kishi,Takehiko Kakutani

doi:10.3389/fevo.2020.00124

Abstract

An ecological community network generally has a clustering structure formed by evolutionary and ecological processes. Because females and males of a single visitor species often differ in their evolutionary and ecological relationship with flowering plants, visitor sex should affect the clustering structure of flower-visitor networks. Two related metrics are used to evaluate clustering structures: compartmentation and modularity. Compartmentation refers to the number of clearly separate subgroups, whereas modularity describes subgroups according to their number of aggregating links. Thus, compartmentation is a measure of network fragmentation and modularity is a measure of network heterogeneity. Because male visitors tend to give priority to search for mates, we hypothesized that male visitors increased compartmentation and decreased modularity compared with female visitors. By using museum specimens of flower-visitor insects together with the plant species that they were visiting, recorded at the time of collection, we constructed 11 networks for each research site and collection year, separated each into male and female subnetworks, and then compared the two metrics between them. Results showed that compartmentation was not different between them, while male subnetworks had lower modularity than female subnetworks, and strengthened modularity of species networks together with female subnetworks. These structural characteristics of male subnetworks likely reflect less choosy and less mutualistic visits of males compared with females. This might imply that the role of male visitors in maintaining flower diversity has been overlooked.

Highlights

Ecological community networks can be considered to be assemblies of multiple species, and they are generally composed of several subgroups (Dicks et al, 2002; Olesen et al, 2007; Fortuna et al, 2010; Guimerà et al, 2010; Thébault and Fontaine, 2010)
Flower–visitor networks in particular commonly have a clustering structure (Olesen et al, 2007), and it has been shown theoretically that the clustering structure of mutualistic flower–visitor networks are more likely to cause chained extinction, because extinction of a single visitor species is likely to be followed by the second extinction of Pollination Networks the plant species within the same subgroup that depends on the visitor species for pollination (Thébault and Fontaine, 2010)
Maruyama et al (2014) studied a network composed of hummingbirds and the flowers that they visited in Brazil and demonstrated that the critical factors leading to the formation of subgroups were flower phenology, variation in habitat environments, and traitmatching between flower corolla length and hummingbird bill length

Summary

INTRODUCTION

Ecological community networks can be considered to be assemblies of multiple species, and they are generally composed of several subgroups (Dicks et al, 2002; Olesen et al, 2007; Fortuna et al, 2010; Guimerà et al, 2010; Thébault and Fontaine, 2010). Intraspecific variations in the temporal and spatial distributions and the phenological and physiological traits of individuals of a single visitor species can affect clustering and other network structures (Bolnick et al, 2011) Such intraspecific variations can differ depending on the sex of the flower visitor. In other species as well, male visitors generally make a far larger effort to mate with female partners than to visit flowers and to collect flower rewards, and tend to visit flowers by chance (Smith et al, 2019) This male flower-visiting pattern suggests a weaker preference for a few certain flowers and more randomized and less mutualistic flower–visitor interactions compared the female flower-visiting pattern. Our hypothesis predicts that male subnetworks have lower modularity and higher compartmentation than female subnetworks

MATERIALS AND METHODS

RESULTS

DISCUSSION