Publication Date

5-2015

Date of Final Oral Examination (Defense)

12-2-2014

Type of Culminating Activity

Thesis - Boise State University Access Only

Degree Title

Master of Science in Biology

Department

Biology

Major Advisor

Ian C. Robertson, Ph.D.

Advisor

Jennifer Forbey, Ph.D.

Advisor

Julie Heath, Ph.D.

Abstract

Selective foraging by granivores can have important consequences for the structure and composition of plant communities and may have particularly severe costs for offspring recruitment in rare plant species. To understand how granivore foraging behavior affects soil seed pools of both common and rare plant species, preferences of ants for seeds need to be measured as a function of seed availability in the environment as well as take into account the individual attributes of seeds available as forage. In the current study, I examined the foraging decisions of the Owyhee harvester ant (Pogonomyrmex salinus), a generalist granivore native to sagebrush-steppe habitat throughout the Pacific Northwest that includes areas occupied by slickspot peppergrass (Lepidium papilliferum), a threatened mustard endemic to southwestern Idaho. P. salinus can be a voracious seed predator of slickspot peppergrass in areas where their habitats overlap and, as such, these ants may pose a threat to the long-term viability of the species. Although several studies describe the basic diet of P. salinus, none have examined their foraging decisions as a function of natural seed availability, nutrient content, or seed morphology. Moreover, the importance of L. papilliferum seeds in the diet of P. salinus has not been adequately assessed. Therefore, I compared the diet of P. salinus relative to the availability of major seed types (i.e., Bromus tectorum, Poa secunda, Sisymbrium altissimum) in addition to L. papilliferum located within 12 m of ant colonies, a typical foraging range for individual ants, and analyzed the nutritional attributes and travel time associated with each seed species. I found that harvester ants exhibited distinct seasonal patterns in resource use each year of the study. In general, small seeded species (i.e., P. secunda, S. altissimum and L. papilliferum) were overrepresented in the diet of ants relative to their availability, whereas B. tectorum (despite being abundant throughout the study) was largely avoided and a small component of the diet of P. salinus. My results also suggest that the reduced travel time associated with smaller seed types and the greater energy and protein content of L. papilliferum relative to other seed types may play a role in its selection by P. salinus.

In light of these results, I examined whether ants would alter seed selection in response to an experimental removal of a preferred food type (P. secunda) while maintaining the abundance of a less preferred food type (B. tectorum). My removal technique at treatment colonies significantly reduced P. secunda seeds on the ground relative to the controls and altered the foraging behavior of ants at those colonies. Specifically, I found that ants from treatment colonies (i.e., colonies where I removed P. secunda from within 12 m of the nest entrance) collected significantly more B. tectorum seeds than those from control colonies. Moreover, ants from treatment colonies travelled further to collect P. secunda seeds than those from control colonies, as indicated by a significant increase in the establishment of trunk trails at distances beyond 12 m from the nest in treatment colonies. Thus, my experimental results suggest that preferences for particular seed types have important influences on seed selection by seed harvesting ants, even when their value is altered by increasing the costs of search time to find them.

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