Interaction between ants and seeds of exotic and native species in the Patagonian steppe

PIRK, G.I.; FARJI-BRENER, A.; LOPEZ DE CASENAVE, J.

Congreso; XX Simposio de Mirmecologia, I Encuentro de Mirmecologistas de las Americas; 2011

Universidade Federal Rural do Rio de Janeiro

Plant invasions are a worldwide conservation issue. Herbivores may influence the performanceof invasive plants. In particular, granivores may reduce seed abundance of exotic species, whichcould in turn affect the density of adult plants. A high number of exotic plants, commonly associated with roadsides, are established in the Patagonian steppe. We studied the interaction between ants and seeds of native and exotic species in areas with different levels of plant invasion in the Patagonian steppe aiming at: (a) identifying ant species which interact with seeds, (b) comparing species richness and composition of the ant assemblage which interacts with native and exotic seeds in areas of high (H) and low (L) invasion levels, and (c) analyzing if the level of ant-seed interactions depends on the species of seeds offered and on the invasion level. We selected 8 sites along route 40, at 40-80 km from Bariloche. Two grids were placed at each site, one by the roadside (H) and the other one at least 100 m away from the route (L). Each grid had 20 sampling stations where seeds of two abundant species, Carduus thoermeri (Ct, exotic) and Pappostipa speciosa (Ps, native), were offered on different days. We identified ants which manipulated, carried and/or removed seeds from the stations at four different times during each day. A total of 9 species (3 subfamilies) interacted with the seeds. Species richness did not differ between species of seeds offered or invasion level. The composition of the ant assemblage interacting with Ps was similar between H and L sites and characterized by Pogonomyrmex carbonarius. Species composition of ants interacting with Ct was variable and characterized in H sites by Dorymyrmex wolfhuehelii and Pheidole spininodis, and in L sites by D. antacticus, D. tener and Lasiophanes valdiviensis. Interaction levels (# stations with interacting ants) of ants with Ct seeds in L sites were the highest, with D. tener and D. antarcticus being responsible for most interactions. Ps seeds presented fewer interactions and P. carbonarius and D. tener were the species most frequently involved. In conclusion, the composition of the ant assemblage interacting with native seeds did not differ with invasion level. In contrast, it did differ for exotic seeds, which also presented higher interaction levels in less invaded areas. These results suggest that ants could be holding back the arrival of exotic seeds in areas with low invasion levels.  Carduus thoermeri (Ct, exotic) and Pappostipa speciosa (Ps, native), were offered on different days. We identified ants which manipulated, carried and/or removed seeds from the stations at four different times during each day. A total of 9 species (3 subfamilies) interacted with the seeds. Species richness did not differ between species of seeds offered or invasion level. The composition of the ant assemblage interacting with Ps was similar between H and L sites and characterized by Pogonomyrmex carbonarius. Species composition of ants interacting with Ct was variable and characterized in H sites by Dorymyrmex wolfhuehelii and Pheidole spininodis, and in L sites by D. antacticus, D. tener and Lasiophanes valdiviensis. Interaction levels (# stations with interacting ants) of ants with Ct seeds in L sites were the highest, with D. tener and D. antarcticus being responsible for most interactions. Ps seeds presented fewer interactions and P. carbonarius and D. tener were the species most frequently involved. In conclusion, the composition of the ant assemblage interacting with native seeds did not differ with invasion level. In contrast, it did differ for exotic seeds, which also presented higher interaction levels in less invaded areas. These results suggest that ants could be holding back the arrival of exotic seeds in areas with low invasion levels.