Seed dispersal by the omnivorous ant Tetramorium tsushimae Emery (Formicidae) in three common weed species

Abstract

The Japanese pavement ant Tetramorium tsushimae Emery (Formicidae) is a species of seed-collecting ant found in Japan. However, its role in seed dispersal has not yet been satisfactorily documented. We investigated the seed-carrying behaviour of T. tsushimae both in the laboratory and in the field. In the laboratory, seeds of three common weeds [evening primrose Oenothera biennis (Onagraceae), purslane Portulaca oleracea (Portulacaceae), and dropseed Sporobolus fertilis (Poaceae)] were examined. More than 65% of each kind of seed was carried by the ants, but some were later discarded outside the nest. Among the three, S. fertilis seeds were collected most frequently. The germination rate of the seeds was higher than 80% for both discarded and stored O. biennis and S. fertilis seeds but lower than half for discarded P. oleracea seeds. In the field, the relationship between the ants and the growing locations of O. biennis or P. oleracea was investigated. The distribution of O. biennis plants considerably overlapped with the distribution of T. tsushimae nest holes at all survey sites. The distribution of P. oleracea also overlapped with that of ant nest holes at some sites. In areas where O. biennis and P. oleracea were absent, a high proportion (> 65%) of artificially placed seeds were carried by T. tsushimae. A few months later, O. biennis seedlings were observed near many ant nest holes. The mean distance between the seedlings and seed-set point was 7.2 m. Although P. oleracea seeds were observed near nest holes, their seedlings were not detected. The present results suggest that O. biennis plants benefit from seed dispersal by T. tsushimae and that the distribution of P. oleracea plants might also be influenced by ant foraging behaviour.