Abstract
Costs and benefits for partners in mutualistic interactions can vary greatly, but surprisingly little is known about the factors that drive this variation across systems. We conducted a meta-analysis of ant-plant protective mutualisms to quantify the effects of ant defenders on plant reproductive output, to evaluate if reproductive effects were predicted from reductions in herbivory and to identify characteristics of the plants, ants and environment that explained variation in ant protection. We also compared our approach with two other recent meta-analyses on ant-plant mutualisms, emphasizing differences in our methodology (using a weighted linear mixed effects model) and our focus on plant reproduction rather than herbivore damage. Based on 59 ant and plant species pairs, ant presence increased plant reproductive output by 49% and reduced herbivory by 62%. The effects on herbivory and reproduction within systems were positively correlated, but the slope of this relationship (0.75) indicated that tolerance to foliar herbivory may be a common plant response to absence of ant guards. Furthermore, the relationship between foliar damage and reproduction varied substantially among systems, suggesting that herbivore damage is not a reliable surrogate for fitness consequences of ant protection. Studies that experimentally excluded ants reported a smaller effect of ant protection on plant reproduction than studies that relied upon natural variation in ant presence, suggesting that study methods can affect results in these systems. Of the ecological variables included in our analysis, only plant life history (i.e., annual or perennial) explained variation in the protective benefit of mutualistic ants: presence of ants benefitted reproduction of perennials significantly more than that of annuals. These results contrast with other quantitative reviews of these relationships that did not include plant life history as an explanatory factor and raise several questions to guide future research on ant-plant protection mutualisms.
Highlights
Ant-plant protection mutualisms are model systems for examining the evolution and maintenance of mutualistic relationships [1,2,3], plant defense strategies [4,5,6], species coexistence [7,8,9,10] and multitrophic interactions [11,12]
Our questions and methodological approach differed from previous meta-analyses on ant-plant protection systems in four key ways: 1) we included only studies examining ant effects on plant reproductive output, a better proxy measure of fitness consequences than ant effects on herbivory or herbivore abundance [35]; 2) we included more characteristics of the ant and plant species, including plant life history; 3) we explicitly examined correlations among predictor variables to aid in our interpretations and avoid spurious results; and 4) we applied a flexible and powerful hierarchical statistical approach to better deal with non-independence of multiple results from the same plant species and to allow simultaneous analysis of more than one predictor variable
The effect sizes varied substantially but, on average, ant presence increased plant reproductive output by 49% (r = 0.4060.074 SE) and decreased foliar herbivory by 62% (g = 0.9660.23 SE: Fig. 1)
Summary
Ant-plant protection mutualisms are model systems for examining the evolution and maintenance of mutualistic relationships [1,2,3], plant defense strategies [4,5,6], species coexistence [7,8,9,10] and multitrophic interactions [11,12]. These relationships most commonly involve an exchange of resources and services in which plants produce food rewards or housing for ants that defend the host plant against herbivores, pathogens and encroaching vegetation. Despite numerous studies testing various ant, plant and environmental characteristics that affect individual ant-plant protection mutualisms, we still know relatively little about the traits that drive patterns in the efficacy of ant defense across systems
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