Negative effects of ant-plant interaction on pollination: costs of a mutualism

Rodrigo R Nogueira,Eduardo S Calixto,Helena Maura Torezan-Silingardi,Danilo Ferreira Borges Santos,Kleber Del-Claro

doi:10.13102/sociobiology.v68i4.7259

Abstract

The mutualism of ants and extrafloral nectary (EFN)-bearing plants is known to reduce rates of herbivory. However, ants may have negative impacts on other mutualisms such as pollination, constituting an indirect cost of a facultative mutualism. For instance, when foraging on or close to reproductive plant parts ants might attack pollinators or inhibit their visits. We tested the hypothesis that ants on EFN-bearing plants may negatively influence pollinator behavior, ultimately reducing plant fitness (fruit set). The study was done in a reserve at Brazilian savannah using the EFN-bearing plant Banisteriopsis malifolia (Malpighiaceae). The experimental manipulation was carried out with four groups: control (free visitation of ants), without ants (ant-free branches), artificial ants (isolated branches with artificial ants on flowers) and plastic circles (isolated branches with plastic circles on flowers). We made observations on flower visitors and their interactions, and measured fruit formation as a proxy for plant fitness. Our results showed that pollinators hesitated to visit flowers with artificial ants, negatively affecting pollination, but did not hesitate to visit flowers with plastic circles, suggesting that they recognize the specific morphology of the ants. Pollinators spent more time per flower on the ant-free branches, and the fruiting rate was lower in the group with artificial ants. Our results confirm an indirect cost in this facultative mutualism, where the balance between these negative and positive effects of ants on EFN-bearing plants are not well known.

Highlights

In tropical areas, 94% of plants require pollinators for reproduction and only a small number of species can reproduce by spontaneous self-pollination or agamospermy (Ollerton et al, 2011)
The aim of this study was to investigate the impact of ants on floral visitor behavior and fruit production of an extrafloral nectaries (EFNs)-bearing plant, B. malifolia
This study showed that artificial ants present on flowers have negative impacts on flower visitors’ behavior of B. malifolia, resulting in reduced fruit production; with no significant difference for the control group

Summary

Introduction

94% of plants require pollinators for reproduction and only a small number of species can reproduce by spontaneous self-pollination or agamospermy (Ollerton et al, 2011). Many plants are engaged in mutualisms with ants (Kuriakose et al, 2018). The main role these mutualistic ants play on plants is the protection against herbivore attack; in turn plants provide different types of resources, such as places for nesting and food. One of the most common food resources offered by plants to ants is the extrafloral nectar produced by specialized glands known as extrafloral nectaries (EFNs) (Calixto et al, 2018; Del-Claro et al, 2016; Heil, 2015). Some studies have shown that ants that hunt herbivores present in EFN-bearing plants decrease leaf herbivory (Rosumek et al, 2009; Trager et al, 2010) and/or increase fruit production (Del-Claro and Marquis, 2015; Nascimento and Del-Claro, 2010)

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