Unexpected relationships and valuable mistakes: non-myrmecochorous Prosopis dispersed by messy leafcutter ants in harvesting their seeds

MILESI, F.A.; LOPEZ DE CASENAVE, J.

AUSTRAL ECOLOGY

WILEY-BLACKWELL PUBLISHING, INC

Año: 2004 vol. 29 p. 558 - 567

1442-9985

Ants generally disperse seeds while predating fruits or structures attached to the seed. Seed dispersal as a by-product of seed predation (dyszoochory) was recognised in specialised harvester ants, but not in ants predating seeds opportunistically. Leafcutting ants are the main herbivores in much of the Neotropics, and they were reported removing fruits and seeds, but their role as seed predators and dispersers has not been acknowledged. Prosopis flexuosa D.C. (Fabaceae, Mimosoideae) is the most abundant tree species in the central Monte desert, Argentina and it is likely to depend on secondary animal dispersal. Mammalian frugivores are usually considered its main dispersers, but the opportunity for dispersal may be small since the removal of fruits and seeds by seed predators is very intense. Our objective is to identify which ant species interact with P. flexuosa fruits and to evaluate their relative importance as seed predators and dispersers. We offered whole and segmented pods and identified several ant species exploiting the fruits. We examined all pod segments remaining around nests of the three species able to remove them (the leafcutters Acromyrmex lobicornis Emery and A. striatus Roger, and Pheidole bergi Mayr) during and after P. flexuosa primary dispersal season. We found up to 753 pod segments and 90 sound seeds accumulated in a 1 m-radius circle over nests of A. lobicornis, and even more in an examined trail. Acromyrmex striatus left a smaller proportion of sound seeds and P. bergi left a smaller number of pod segments. All tendencies were similar during shorter known periods of accumulation. Leafcutting ants are acting as important seed predators, and "by mistake" may be dispersing a key nonmyrmecochorous tree. This is an unexplored path in P. flexuosa seed dispersal cycle that challenges the tendency to predict interactions based on classifications made with other goals.