Evaluation of seed-dispersal services by ants at a temperate pasture: Results of direct observations in an ant suppression experiment

Ants disperse seeds of many plant species adapted to myrmecochory. While advantages of this ant-plant mutualism for myrmecochorous plants (myrmecochores) have been previously studied in temperate region mostly in forests, our study system was a pasture. Moreover, we used a unique combination of observing the effect of ant-activity suppression on ant dispersal and comparison of the contribution of ant and unassisted dispersal to the distance from mother plant. We established plots without and with ant-activity suppression (enclosures). We offered diaspores of a myrmecochorous (Knautia arvensis), and a non-myrmecochorous (Plantago lanceolata) species in a choice test and followed ants carrying diaspores during days and nights (focus of previous studies was on diurnal dispersal). We measured frequency and distances of ant dispersal and compared them with unassisted dispersal recorded using sticky trap method. The dispersal frequency was lower in enclosures (3.16 times). Ants strongly preferred diaspores of the myrmecochore to non-myrmecochore with 586 and 42 dispersal events, respectively (out of 6400 diaspores of each species offered). Ant dispersal resulted in more even and on average longer distances (maximum almost tenfold longer, 994 cm) in comparison to unassisted dispersal. Ant dispersal altered the distribution of distances of the myrmecochore from roughly symmetric for unassisted dispersal to positively skewed. Ants dispersed heavier diaspores farther. Ants dropped the majority of diaspores during the dispersal (which reduces clustering of seeds), while several (11%) were carried into anthills. Anthills are disturbed microsites presumably favorable for germination in competitive habitats. Ants provided non-negligible dispersal services to myrmecochorous K. arvensis but also, to a lesser extent, of non-myrmecochorous P. lanceolata. We followed the journey of seeds of a myrmecochorous and non-myrmecochorous species during diurnal and nocturnal ant dispersal. This unique approach with ant-activity suppression and unassisted dispersal quantification allowed us to show the key role of ants in dispersal of myrmecochorous species and demonstrate non-negligible contribution to non-myrmecochorous species dispersal.image