Myrmecochory of the exotic plant, Centaurea maculosa: A potential mechanism enhancing invasiveness

We hypothesized that the elaiosomes of seeds of the invasive plant, Centaurea maculosa (spotted knapweed), elicit seed-dispersing behavior by native ants (myrmecochory), potentially providing a mechanism enhancing its invasiveness into native plant communities. Elaiosomes are small nutrient-rich nodes produced on seeds of some plants. Ants collect and disperse elaiosome-bearing seeds, later consuming the elaiosome, but usually discarding the seed intact away from the parent plant. The seeds of C. maculosa possess elaiosomes, suggesting that the dispersal of its seeds and its ability to rapidly spread into native habitats may be enhanced by myrmecochory. We tested for myrmecochory at two Palouse prairie sites in Montana by presenting seeds of C. maculosa and two native dominants, Balsamorhiza sagittate (a forb) and Psuedoroegneria spicata (a bunchgrass). Seeds were presented in choice (seeds of all three plants) and no choice (seeds of only one plant) treatments. At both study sites, ants readily dispersed the seeds of the invasive plant, C. maculosa, while ignoring seeds of the native plants, B. sagittata and P. spicata. In addition, myrmecochory was not influenced when all three seed types were presented together. Different ant species were observed removing seeds from stations at each site, indicating that myrmecochory of C. maculosa seeds is not restricted to one ant species but rather may be a generalized response across ant communities. Our results indicate that myrmecochory may enhance dispersal of C. maculosa and facilitate its invasion into native plant communities.