Why are leaf-cutting ants more common in early secondary forests than in old-growth tropical forests? An evaluation of the palatable forage hypothesis

I evaluated the hypothesis that leaf-cutting ants are more common in early successional forests than in old-growth forests because pioneer species, which dominate in early successional habitats, appear more susceptible to leafcutters than shade-tolerant species, which dominate primary forests (palatable forage hypothesis). The relative importance of pioneer and shade-tolerant species as plant resources for leaf-cutting ant colonies was evaluated (1) by literature review of leaf-cutting ants’ diet, and (2) experimentally, using field assays to determine leafcutter’s selectivity. Pioneer species were harvested three times more frequently than shade-tolerant species and made up the largest component of the diet in all the studies reviewed. The amount harvested was not correlated with the plant species abundance. In addition, leaves from pioneer plants were selected eight times more than leaves from shade-tolerant species in the field assays. These results support the palatable forage hypothesis. Leafcutters probably select pioneer leaves because of their low level of chemical defenses and high nutrient content. The high availability of pioneer species in early successional forest probably decreases the cost to locate palatable resources. Therefore, early successional habitats support more ant colonies than old-growth forests. On the other hand, the effective defense mechanisms of mature plant species and the high dispersion of palatable plants could explain the low density of leaf-cutting ant colonies in old-growth forests. The palatable forage hypothesis is compared with other hypotheses that explain leaf-cutting ant density. The preference of foundress queens for forest clearings, the dependence of small colonies on herbs, and the importance of pioneer plant species for mature colonies (palatable forage hypothesis) can be considered complementary, because they focus on different stages of the colony’s life history. Consequently, the availability of pioneer plants appears to be one of the most influential factors determining mature leaf-cutting ant nest densities in Neotropical forests.