SELECTIVE ADVANTAGES TO HEMI-PARASITIC ANNUALS, GENUS MELAMPYRUM, OF A SEED-DISPERSAL MUTUALISM INVOLVING ANTS .2. SEED-PREDATOR AVOIDANCE

Field experiments were used to assess the importance of third species antagonists, in this case rodent seed-predators, in driving the evolution of seed dispersal by ants in the genus Melampyrum. Daytime and nighttime seed-removal frequencies were determined by placing seeds so that 1) ants and rodents had access, 2) only ants had access, 3) only rodents had access, and 4) neither ants nor rodents had access. Experiments were conducted with Melampyrum populations of three different species at four sites, two in the U.S. (M. lineare in Maine and Michigan) and two in Switzerland (M. arvense and M. silvaticum). Morning versus evening fruit dehiscence and the effect of elaiosome removal on seed-attractiveness to ants were also investigated. Though seed predators can have an effect in reducing seed pool numbers (50-70% in ant exclosures), it is suggested that because of the timing of seed release and the thorough nature of dispersal by ants (85-100% in rodent exclosures), third species antagonists are not currently as important as environmental constraints in maintaining this mutualism. Historically, as suggested by the prevalence of morning (versus evening, p < 0.001) seed release, the effect of rodents may have been larger. Removal of the elaiosome decreases removal rate by ants (p<0.0001); organisms such as carabid beetles that do so may alter the effectiveness of the mutualism.