Date of Final Oral Examination (Defense)
Type of Culminating Activity
Master of Science in Biology
Ian C. Robertson, Ph.D.
Julie Heath, Ph.D.
Marcelo Serpe, Ph.D.
Slickspot peppergrass (Lepidium papilliferum) is a rare plant endemic to the sagebrush-steppe habitat in southwestern Idaho. Within sagebrush-steppe, the plant is restricted to microsites known as “slick spots”– shallow depressions of soil characterized by distinct clay layers and surface water retention that is higher than that of surrounding areas. Having one of the highest extirpation rates among rare plant taxa in Idaho, and considering its unique habitat requirements, limited range, and declining numbers, land managers and conservationists have voiced concern regarding the species’ long-term viability. While range-wide declines in slickspot peppergrass have been attributed largely to the loss of and disturbance to suitable habitat, seed predation by Owyhee harvester ants (Pogonomyrmex salinus) has recently been identified as another potential threat to L. papilliferum survival. However, the extent to which harvester ants remove seeds from slick spots is an unanswered question. To address this question, I conducted a field experiment to examine Owyhee harvester ant foraging behavior and to quantify the loss of seed by individual slickspot peppergrass plants. Additionally, I examined the potential for a dilution effect where the proportion of seeds lost per plant would be inversely related to the total number of flowering plants found in a slick spot. The study showed that seed predation by harvester ants represents a significant threat to seed recruitment in L. papilliferum populations, as individual plants sustained an average seed loss of 73.2% (N=20, range = 0‒97.7%). In slick spots with >150 flowering plants, seed loss was proportionally lower compared to slick spots that contained fewer plants, suggesting that harvester ant colonies may be reaching a threshold of consumption when the quantity of available seeds exceeds their capacity to collect and consume those seeds.
In a separate experiment, I examined the potential of seed introductions as a recovery tool for conservation and management efforts aimed at slickspot peppergrass. I demonstrated that L. papilliferum can successfully germinate, flower, and fruit when seeds are released into unoccupied slick spots. The total number of plants produced (N=9) was very low compared to the number of seeds released (N=19,800), although some of the seeds were exposed to seed predation by ants. Because poor climatic conditions in the year of study may have contributed to the low numbers of seedlings, further investigation into the use of seed introductions in recovery efforts of L. papilliferum is warranted. Overall, my research speaks to a plant species that living in a changing environment where the interactions between the plant and its natural enemies such as harvester ants are shifting, and it has highlighted the need for further investigations aimed at recovery tools such as introductions in the management and conservation of this rare plant species.
Brown, Jennifer A., "Seed Predation by Owyhee Harvester Ants and the Potential of Seed Introductions in Recovery Efforts for Slickspot Peppergrass" (2019). Boise State University Theses and Dissertations. 1569.
Available for download on Sunday, August 22, 2021