Meeting Abstract

P2.184  Monday, Jan. 5  Seed predation on slickspot peppergrass, Lepidium papilliferum (Brassicaceae), by the Owyhee harvester ant, Pogonomyrmex salinus (Hymenoptera: Formicidae) WHITE, Joshua P.*; ROBERTSON, Ian C.; Boise State University; Boise State University white@joimail.com

Owyhee harvester ants (Pogonomyrmex salinus) actively remove fruits and seeds from slickspot peppergrass (Lepidium papilliferum), a rare mustard endemic to southwest Idaho. We conducted a study to establish whether P. salinus is the only major seed predator of L. papilliferum, and to determine the importance of seed predation as a source of offspring mortality for this plant. We monitored a total of five L. papilliferum beyond the foraging distance of P. salinus and found no evidence of seed predation. We then conducted an experiment to determine the extent of seed predation for plants located within the foraging distance of P. salinus. A total of 13 pairs of plants were included; each pair was located within 10 m of an ant colony. One plant per pair was assigned as the treatment and the other as the control. Treatment plants were enclosed to bar any ant access. We visited the plants weekly; counting the number of attached fruits, depredated fruits, and naturally dehisced fruits on random inflorescences of each plant in the pair. Plants accessible to ants experienced a direct loss of at least 40% of its fruit and seed, while plants with no ant access suffered very little seed loss (ANOVA F_1,20=67.60 p < 0.0001). Likewise, ants removed over 90% of seeds placed on the ground, compared to almost no loss when ant access was denied (Mann-Whitney U test N=20 p < 0.0001). One hundred percent of seeds collected by ants (N=100) were returned to the nests and presumably eaten. A search of 30 midden piles revealed large amounts of empty fruit husks but no intact seeds. We conclude that harvester ants are the main seed predators of L. papilliferum, and possibly an important source of offspring mortality for this rare plant.