Plant palatability to leaf-cutter ants (Atta laevigata) and litter decomposability in a Neotropical woodland savanna

Abstract

Although leaf-cutter ants have been recognized as the dominant herbivore in many Neotropical ecosystems, their role in nutrient cycling remains poorly understood. Here we evaluated the relationship between plant palatability to leaf-cutter ants and litter decomposability. Our rationale was that if preference and decomposability are related, and if ant consumption changes the abundance of litter with different quality, then ant herbivory could affect litter decomposition by affecting the quality of litter entering the soil. The study was conducted in a woodland savanna (cerrado denso) area in Minas Gerais, Brazil. We compared the decomposition rate of litter produced by trees whose fresh leaves have different degrees of palatability to the leaf-cutter ant Atta laevigata. Our experiments did not indicate the existence of a significant relationship between leaf palatability to A. laevigata and leaf-litter decomposability. Although the litter mixture composed of highly palatable plant species showed, initially, a faster decay rate than the mixture of poorly palatable species, this difference was no longer visible after about 6 months. Results were consistent regardless of whether litter invertebrates were excluded or not from litter bags. Similarly, experiments comparing the decomposition rate of litter from pairs of related plant species also showed no association between plant palatability and decomposition. Decomposition rate of the more palatable species was faster, slower or similar to that of the less palatable species depending upon the particular pair of species being compared. We suggest that the traits that mostly influence the decomposition rate of litter produced by cerrado trees may not be the same as those that influence plant palatability to leaf-cutter ants. Atta laevigata select leaves of different species based – at least in part – on their nitrogen content, but N content was a poor predictor of the decomposition rates of the species we studied.