Nutrient supplementation alters trophic dynamics within nests of the desert leafcutter ant Acromyrmex versicolor

Background/Question/Methods

Leafcutter ants function as herbivores, harvesting leaves to grow a fungus that serves as food for the colony.  The ants are choosy about the leaves they collect, tending to prefer young or wilted leaves that are less tough, higher in sugar and nitrogen, and lower in plant defense compounds, than surrounding vegetation.  While leaf selection has been intensively studied, its consequences for performance across the colony trophic system are poorly understood.  To test for the effects of nutrient limitation on growth and nutrient flow through colonies, we supplemented a standard laboratory diet with additional protein, carbohydrate, or potassium phosphate, and provided it to colonies of the desert leafcutter ant Acromyrmex versicolor over the course of four months.  We measured the effects of supplementation on foraging rates, fungus garden and worker population growth, and on the elemental composition of colonies’ waste material (refuse). 

Results/Conclusions

Supplementation with carbohydrate led to increased foraging and fungal growth, but no change in worker population growth rate or refuse composition.  In contrast, supplementation with protein caused a decrease in foraging and fungus garden growth, in some cases leading to complete fungus garden collapse, coupled with an increase in refuse nitrogen (N) content.  Addition of potassium phosphate had no discernible effect on colony growth rates or refuse phosphorus (P) content.  These findings partly explain leafcutter colony preference for young, high-carbohydrate leaves and suggest that variation in plant community composition and fertilization influences leafcutter ant colony population dynamics and the effects of leafcutter ant colonies on nutrient cycling.