Assessing herbivory rates of leaf-cutting ant (Atta colombica) colonies through short-term refuse deposition counts

Leaf-cutting ants (genera Atta and Acromyrmex) are considered dominant herbivores of Neotropical forests. However, so far quantitative, long-term, and large-scale assessments of their impact on these ecosystems are rare, because the available assessment methods were laborious and/or destructive. We describe a rapid, nondestructive, and inexpensive method to estimate the long-term harvest of Atta colombica colonies. Workers of A. colombica dump the colony refuse (exhausted fungal substrate) outside the nest. A single trail connects the refuse pile and the nest. In contrast to the foraging activity, the refuse deposition rate (the number of deposited refuse particles per minute) is diurnally constant and varies little on subsequent days. The number of refuse particles deposited per day was tightly correlated with the number of harvested fragments in nests of differing sizes (R² = 0.77, P < 0.0001). Therefore, the daily harvest of a particular colony can be calculated from short-term counts (5 min) of the refuse deposition rate at any time of the day. Combining these data with information on average fragment size (weight and/or area) allows the calculation of the total daily amount of biomass and/or foliage area harvested by the colony. This new method facilitates quantifying A. colombica herbivory on scales of populations and ecosystems, or over long-term scales.