Local Insect Availability Partly Explains Geographical Differences in Floral Visitor Assemblages of Arum maculatum L. (Araceae).

Danae Laina,Valerie Scheurecker,Anja C Hörger,Roman Fuchs,Stefan Dötterl,Rüdiger Wagner,Eva Gfrerer,Hans Peter Comes,Marc Gibernau,Carina Zittra,Marielle Schleifer

doi:10.3389/fpls.2022.838391

Abstract

Geographical variation in abundance and composition of pollinator assemblages may result in variable selection pressures among plant populations and drive plant diversification. However, there is limited knowledge on whether differences in local visitor and pollinator assemblages are the result of site-specific strategies of plants to interact with their pollinators and/or merely reflect the pollinator availability at a given locality. To address this question, we compared locally available insect communities obtained by light-trapping with assemblages of floral visitors in populations of Arum maculatum (Araceae) from north vs. south of the Alps. We further investigated whether and how the abundance of different visitors affects plants’ female reproductive success and examined the pollen loads of abundant visitors. Local insect availability explained inter-regional differences in total visitor abundance, but only partly the composition of visitor assemblages. Northern populations predominantly attracted females of Psychoda phalaenoides (Psychodidae, Diptera), reflecting the high availability of this moth fly in this region. More generalized visitor assemblages, including other psychodid and non-psychodid groups, were observed in the south, where the availability of P. phalaenoides/Psychodidae was limited. Fruit set was higher in the north than in the south but correlated positively in both regions with the abundance of total visitors and psychodids; in the north, however, this relationship disappeared when visitor abundances were too high. High pollen loads were recorded on both psychodids and other Diptera. We demonstrate for the first time that the quantitative assessment of floral visitor assemblages in relation to locally available insect communities is helpful to understand patterns of geographical variation in plant–pollinator interactions. This combined approach revealed that geographical differences in floral visitors of A. maculatum are only partly shaped by the local insect availability. Potential other factors that may contribute to the geographical pattern of visitor assemblages include the region-specific attractiveness of this plant species to flower visitors and the population-specific behavior of pollinators.

Highlights

It is widely recognized that geographical variation in abundance and composition of pollinator assemblages can impose variable selective pressures on floral phenotypes within and among plant species (Waser et al, 1996; Dodd et al, 1999; Johnson, 2010; Van der Niet and Johnson, 2012; Hernández-Hernández and Wiens, 2020)
Floral Chambers (FC) In the northern populations, insect visitors collected from the floral chambers were mainly Psychodidae, while besides those, other dipteran families (e.g., Sphaeroceridae and Chironomidae) were the predominant visitors in the south
Chironomidae exhibited similar numbers in the two regions, both in floral chambers and light traps with high abundances in the light traps and low abundances in floral chambers (Figure 3). These results indicate that differences in visitor assemblages within and between the two regions can only partly be explained by the light trap catches

Summary

Introduction

It is widely recognized that geographical variation in abundance and composition of pollinator assemblages can impose variable selective pressures on floral phenotypes within and among plant species (Waser et al, 1996; Dodd et al, 1999; Johnson, 2010; Van der Niet and Johnson, 2012; Hernández-Hernández and Wiens, 2020). There is limited knowledge about the relative importance of local pollinator availability vs site-specific pollination strategies in shaping the local abundance and composition of pollinator assemblages. This is mainly because local pollinator availability is usually estimated from presence/absence data of particular pollinators (Diaz and Kite, 2002; Moeller, 2006; Johnson and Raguso, 2016 but see Sayers et al, 2020) or by comparing pollinator visitation rates among populations, assuming that visitation rates positively correlate with local pollinator abundance (e.g., Gómez et al, 2009b, 2014; Rech et al, 2018). Up to now, studies that quantify local insect availability in relation to visitor assemblages are scarce (Sayers et al, 2020)

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Conclusion