Publication Date


Date of Final Oral Examination (Defense)


Type of Culminating Activity


Degree Title

Master of Science in Biology



Major Advisor

Ian C. Robertson, Ph.D.


Julie Heath, Ph.D.


Marcelo Serpe, Ph.D.


This study was completed during the summers of 2014 and 2015 to continue research on the relationship between harvester ants (Pogonomyrmex salinus) and slickspot peppergrass (Lepidium papilliferum). Slickspot peppergrass is a rare mustard endemic to south western Idaho. Over the past century, the species has declined in abundance, largely as a result of habitat degradation and fragmentation. In addition to these reasons for decline, seed foraging by harvester ants and plant destruction by harvester ants and small mammal herbivores have been recently indicated as potential factors. This study confirmed that harvester ants remove large numbers of L. papilliferum seeds. Specifically, we found that 90–100% of the seeds produced by individual plants were often removed by ants, regardless of the number of seeds within the foraging range of a colony. Nevertheless, although intensive seed foraging was common, we estimated that the number of seeds available to individual ant colonies often exceeded the colony’s capacity for seed removal. Thus, when seed numbers are high, predator satiation may be a viable mechanism for L. papilliferum seeds to escape seed removal by harvester ants.

In addition to documenting the occurrence and intensity of seed removal by ants, we confirmed that small mammals and harvester ants sometimes act as herbivores on leaf and (in the case of mammals) root tissues. Mortality as a result of herbivory was at times high, although understanding the causes of variation in patterns of herbivory will require further study. In addition to investigating seed predation and herbivory on naturally occurring plants, we evaluated whether seed introductions represent a viable tool for recovery of slickspot peppergrass. We were successful in germinating L. papilliferum seeds, although our efforts were severely hampered by seed predation by harvester ants. In order for seed introductions to become a feasible approach to recovery efforts for L. papilliferum, a method to mitigate the effects of seed predation will be needed.

Finally, because this document was written as publishable chapters that reflect the contributions of multiple authors, it has been written in first person plural (i.e., we) rather than first personal singular (i.e., I).