Background/Question/Methods:
The potential importance of functional responses in determining the net effect of engaging in mutualism is well understood in conceptual terms.  However, the prevalence of particular patterns in nature has not been explored.  Many plant and insect species engage in food-for-protection mutualisms with ants, and I use a meta-analysis approach to describe the functional forms of the relationships between 1) reward production and ant density, 2) reward production and ant fitness, and 3) ant density and the provision of services to the partner.  I also contrast the forms of these relationships in generalized facultative interactions versus specialized, obligate relationships (e.g., those involving ants and extrafloral nectary-bearing plants and myrmecophytic ant-plants, respectively).   
Results/Conclusions:
A majority of studies demonstrate a linear relationship between reward production and ant density, suggesting that a) prospective mutualists can alter ant abundance and b) the costs of attracting ants remain constant on a per-unit ant basis, irrespective of ant abundance.  This pattern was consistent among generalized and highly specialized mutualisms. I also found new evidence of linear relationships between ant colony growth rates and access to mutualist-produced rewards.  However, the benefits of supporting greater densities of ants often saturated (i.e., decreased on a per-unit ant basis as ant density increased).  This saturation was more prevalent in myrmecophytic systems than in generalized interactions.  As a consequence of this similarity in the functional form of costs of ants and difference in the functional form of benefit received from ants, the mutualistic phenotype (e.g., reward production rate) that confers the greatest net benefit to the ant’s partner is more predictable in specialized, obligate systems.