Seed predation and herbivory on slickspot peppergrass

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How Lepidium papilliferum persists, or even whether it can persist in the face of intense seed predation by harvester ants remains an open question. Despite the voracity with which harvester ants consume L. papilliferum seeds, there may be naturally occurring situations that mitigate seed loss and facilitate the maintenance and replenishment of seed banks. For example, in years of high L. papilliferum seed production the number of seeds available to harvester ants may exceed the ants’ collective capacity to consume them (i.e., satiation effect), leading to replenishment of the seed bank for future generations of the plant. By contrast, in years of low seed production the proportion of seeds consumed by ants, if not the overall number of seeds, may be much higher, leading to the impression of untenable levels of predation for the plant to survive. Periodic relief from high levels of seed predation through satiation may be essential to L. papilliferum survival in areas where they overlap with harvester ants (which is extensive). To address this possibility we examined whether the proportion of seeds taken from individual L. papilliferum plants declines as a function of increasing seed abundance within slick spots. In addition to possible satiation effects, the vulnerability of L. papilliferum seeds to seed predators may be influenced by soil surface texture. From a distance slick spots often appear uniform and bare. However, closer inspection reveals their surfaces often to be quite variable. Some areas within slick spots have smooth, fine-grain surfaces, whereas others are coarser because of small stones, fissures, lichens and mosses. Coarse soils may provide seeds with physical refuge from detection by ants. Moreover, studies have shown that seed harvesting ants prefer to traverse soils of finer texture, and that they travel slower and drop seeds more frequently in coarse soils than in fine soils. Thus, the second objective of our study was to determine whether L. papilliferum seeds deposited on coarse soils in slick spots are more likely to escape predation than those deposited on fine soils. While most of the research on L. papilliferum herbivory has focused on seed predation by ants, field observations reveal that these plants are also susceptible to defoliation by harvester ants and small mammals. Because little is known about the occurrence and consequences of defoliation on L. papilliferum, our third objective was to document in greater detail the occurrence of non-seed related herbivory by harvester ants and small mammals in order to develop a more complete picture of the threats that contribute to L. papilliferum mortality.

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Reference Code: U16ROB01IDUS 

Full Citation: Robertson, I. and M. Jeffries. 2016. Seed predation and herbivory on slickspot peppergrass. Report prepared for U.S. Fish and Wildlife Service. FWS Agreement No.: F15AS00038. Department of Biological Sciences, Boise State University, Boise, ID. 17 pp.

Location: ELECTRONIC FILE BOTANY: PLANT SPECIES: {Lepidium papilliferum}
Also located in: ELECTRONIC FILE ZOOLOGY: MULTIPLE_SPECIES