Comparing intra- and inter-specific variation in mutualist effectiveness in ant-plant interactions: An experimental and meta-analytical approach

Differences in mutualism effectiveness (ME), a measure of the fitness benefit provided by one partner to another, is generally considered in terms of different species coexisting in mutualist guilds. Nonetheless, there is likely extensive intraspecific variation in ME, but it is often overlooked. Understanding intraspecific variation is important since it may shape adaptive processes by modulating the strength of within-species mutualistic interactions. We compared intra- and inter-specific variation in partner quality in ant-plant mutualisms. We analyzed data on herbivory, ant bodyguard recruitment to herbivores in behavioral assays, during the interaction between the Amazonian ant-plant Cordia nodosa Lam, and its multiple species of mutualistic ant partners. In parallel, we conducted a meta-analysis by assembling data from published studies that measured intra- and/or inter-specific mutualistic trait variation in ant-plant interactions. We compared coefficients of variance within and among mutualist species. Our preliminary data set consisted of 7 traits from 30 publications on 108 ant and 37 plant species. Overall, we found that in most cases, intraspecific variation is equal to or greater than interspecific variation in mutualisms. Intraspecific levels of mutualistic variation resembled interspecific ones in all traits measured experimentally, and in the 91% of the associations examined in the meta-analysis. These results suggest that mutualistic redundancy might be common in ant-plant associations. As the promotion of interspecific cooperation is important for evolutionary stability, intra-specific variation in mutualistic partners is to be also beneficial. The latter may promote ant-plant coexistence through increasing evolutionary flexibility in mutualistic trait distribution.