Benefits of extrafloral nectar to ants in a facultative ant-plant mutualism

Background/Question/Methods

By definition, in a mutualism both partner species must benefit.  In the interaction between extrafloral nectary bearing (EFN) plants and the ants that visit them, benefit to plants is well-documented in many species, but benefit to ants has been little studied; the ants receive a food source so they are assumed to benefit. I asked whether sugars provided by extrafloral nectar affect colony growth, survival, and foraging behavior in an omnivorous neotropical ant, Ectatomma ruidum.  All work was conducted in lowland rainforest at La Selva Biological Station, Costa Rica, where E. ruidum frequently visits EFN plants.  I collected 18 live colonies and housed them in artificial nest boxes in the laboratory.  All colonies were fed termites ad libitum, and half were also given artificial extrafloral nectar (15% sucrose solution).  After 4 months I counted and weighed all colonies to look for differences in colony size and growth rates with artificial extrafloral nectar.  I also conducted behavioral trials with laboratory colonies to determine whether foraging for protein and sugar baits changed with diet.  In field experiments I manipulated nectar resources in 5x5 m plots and mapped all E. ruidum nests in the plots three times over two years of manipulations.

Results/Conclusions

Laboratory Ectatomma ruidum colonies fed artificial extrafloral nectar (15% sucrose solution) for 4 months were significantly larger than control colonies not given artificial nectar, as measured by total colony biomass and total number of individuals in a colony (workers, larvae, and pupae, n=18 colonies).  Colonies given artificial extrafloral nectar also had a significantly lower ratio of adult workers to juveniles, indicating a higher colony growth rate.  In foraging trials, both sucrose-supplemented and control colonies preferred sugar to protein baits, and the strength of this preference did not change with diet.  Although laboratory E. ruidum colonies showed evidence of increased growth and survival with artificial nectar, in field experiments nest densities of E. ruidum did not increase with increasing nectar levels.  The highest nest densities were observed in unmanipulated plots, while reduced-EFN plots and plots supplemented with artificial nectar both had slightly lower nest densities.  Competition with other ant species might be limiting E. ruidum in supplemental nectar plots; abundance of other ants also increased with nectar addition.  This study found evidence that supplementing sugar increases colony growth and survival in an omnivorous ant; true EFN contains amino acids as well as sugars, and thus might provide even greater benefit.