Investment in reward by ant-dispersed plants consistently selects for better partners along a geographic gradient.

Gilad Ben-Zvi,Itamar Giladi,Merav Seifan,Nataly Levine

doi:10.1093/aobpla/plz027

Abstract

Seed dispersal by ants (myrmecochory) is an asymmetric, presumably mutualistic interaction, where a few ant species benefit many plants. Myrmecochorous plants express specialized adaptations, most notably a large elaiosome, which promote interactions with efficient seed dispersers while decreasing interactions with poor dispersers, resulting in de facto partner choice. However, because variation in plants’ investment in reward and ant response to them may vary spatially and temporally, it is unclear whether such specialization is consistent along geographic gradients; especially towards myrmecochory’s range margin. To answer this question on context-dependent partner choice, we first estimated variation in reward investment by co-occurring myrmecochores along a steep environmental gradient in a Mediterranean region. Second, we tested whether variation in plant investment in reward was positively and consistently correlated with the quality of dispersal plant received along the same gradient. Using in situ cafeteria experiments, we simultaneously presented diaspores of locally co-occurring myrmecochorous species to ants of two guilds representing high- and low-quality dispersers. We then recorded ant-seed behaviour, seed preference and seed removal rates for each ant guild. We found both overall and within-site high variation among plant species in the total and relative investment in elaiosomes. Both ant guilds removed substantial proportion of the seeds. However, scavenging ants (high-quality dispersers) clearly preferred diaspores with larger elaiosomes, whereas granivorous ants (low-quality dispersers) exhibited no preference. Furthermore, both the variation in plant traits and the corresponding response of different ant guilds were consistent along the studied geographic gradient. This consistency holds even when granivores, which removed seeds in a non-selective fashion and provided apparently low-quality seed dispersal services, were, at least numerically, the dominant ant guild. This dominance and the consistency of the partner choice shed light on the functionality of elaiosomes at the margins of myrmecochory’s distribution.

Highlights

Seed dispersal by animals is often portrayed as a network of mutualistic interactions, involving many plant and animal species that exchange resource for a service
We addressed the following questions: (i) Do co-occurring myrmecochore species exhibit variation in reward investment and, in particular, in elaiosome mass and in elaiosome to seed mass ratio?; (ii) Is the investment in reward correlated with higher dispersal quality? and (iii) If there is a relationship between reward investment and dispersal quality, is it consistent along a geographic gradient? We hypothesized that the seeds of species that invest more in the elaiosome will be consistently removed (a) at higher rates, (b) more so by scavenging ants and (c) that the effect of reward size on seed removal rate will decrease towards the species range limit
We found that the ant species that are most likely to interact with seeds of myrmecochores are large granviorous of the Messor genus (M. ebeninus, M. arenarius and M. semirufus) and medium to large scavenging ants of the Cataglyphis genus (C. israelensis, C. savignyi and C. albicans)

Summary

Introduction

Seed dispersal by animals is often portrayed as a network of mutualistic interactions, involving many plant and animal species that exchange resource (e.g. fleshy fruit) for a service (dispersal). In Australia, myrmecochores are well represented in arid zones, in the Palaearctic biogeographical region, this dispersal syndrome is common in mesic habitats and becomes rare towards drier climates (Lengyel et al 2009, 2010) Across their range, myrmecochorous plants interact with a variety of seed-collecting ants that differ in their diet preferences, activity time, foraging mode, nest location and nest dynamics (Giladi 2006; Bas et al 2007; Ness et al 2009), which results in differences in the efficiency of seed dispersal service they provide (Hughes and Westoby 1990; Boulay et al 2007b)

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Conclusion