Uncoupling the Effects of Seed Predation and Seed Dispersal by Granivorous Ants on Plant Population Dynamics

Frederick R Adler,Xavier Arnan,Roberto Molowny-Horas,Javier Retana,Anselm Rodrigo

doi:10.1371/journal.pone.0042869

Frederick R Adler, Xavier Arnan + Show 3 more

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https://doi.org/10.1371/journal.pone.0042869

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Abstract

Secondary seed dispersal is an important plant-animal interaction, which is central to understanding plant population and community dynamics. Very little information is still available on the effects of dispersal on plant demography and, particularly, for ant-seed dispersal interactions. As many other interactions, seed dispersal by animals involves costs (seed predation) and benefits (seed dispersal), the balance of which determines the outcome of the interaction. Separate quantification of each of them is essential in order to understand the effects of this interaction. To address this issue, we have successfully separated and analyzed the costs and benefits of seed dispersal by seed-harvesting ants on the plant population dynamics of three shrub species with different traits. To that aim a stochastic, spatially-explicit individually-based simulation model has been implemented based on actual data sets. The results from our simulation model agree with theoretical models of plant response dependent on seed dispersal, for one plant species, and ant-mediated seed predation, for another one. In these cases, model predictions were close to the observed values at field. Nonetheless, these ecological processes did not affect in anyway a third species, for which the model predictions were far from the observed values. This indicates that the balance between costs and benefits associated to secondary seed dispersal is clearly related to specific traits. This study is one of the first works that analyze tradeoffs of secondary seed dispersal on plant population dynamics, by disentangling the effects of related costs and benefits. We suggest analyzing the effects of interactions on population dynamics as opposed to merely analyzing the partners and their interaction strength.

Highlights

Seed dispersal is one of the most ecologically significant plantanimal mutualisms [1,2], and it is central for understanding plant population and community structure and dynamics [2,3]
There is a need of studies examining the plant-disperser interaction within the context of plant population dynamics in general [2,16,17], and in the case of plant-ant interactions
By comparing four different scenarios which are the outcome of the cross combination of the two sides of the interaction, our simulation model allowed to analyze which of the two ecological processes may affect the population dynamics of three shrubby plant species with different biological attributes

Summary

Introduction

Seed dispersal is one of the most ecologically significant plantanimal mutualisms [1,2], and it is central for understanding plant population and community structure and dynamics [2,3]. As with many other interspecific interactions [4,5], seed dispersal by animals involves both costs and benefits [2]. Their balance determines whether the net outcome falls between mutualism or antagonism [2,6]. Throughout this study we use the term ‘seed dispersal’ and ‘seed predation’ to refer to the benefits and costs of secondary seed dispersal, respectively. The net outcome might be context-dependent [1,2,5,10] and may depend on the composition of the assemblage, environmental conditions and fruiting neighborhoods [2], and on diaspore traits and crop sizes [2,11,12]

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