Harvester Ants Reduce Seed Survivorship in Slickspot Peppergrass, a Rare Mustard Endemic to Idaho

Seed predation can significantly reduce the reproductive success of individual plants and their populations. The consequences of seed predation often are most pronounced for rare plant species, in which loss of seeds can have a disproportionate effect on populations. The present study examined the effects of seed predation by Owyhee harvester ants ( Pogonomyrmex salinus ) on seed survivorship in slickspot peppergrass ( Lepidium papilliferum ), a rare mustard endemic to sagebrush-steppe habitat in southwestern Idaho. Within sagebrush-steppe, L. papilliferum 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. Harvester ants frequently nest in L. papilliferum habitat and readily consume the plant’s seeds. We conducted a controlled field experiment at a population of L. papilliferum in 2012 to quantify seed loss to individual plants as a result of seed predation by harvester ants. Across 20 slick spots, plants exposed to harvester ants experienced a median reduction in seed survivorship of 89.2% (interquartile range, 69.3% to 93.9%) relative to plants in the same slick spot that were matched for size and shielded from ants. The proportion of seeds that plants lost to seed predation was more variable and significantly lower in slick spots with >150 plants than in those with fewer plants, suggesting that a threshold to the number of seeds that can be collected and consumed by ants may occur within the natural range of plant densities found in slick spots. Our results suggest that slick spots supporting large numbers of L. papilliferum in a given year may be buffered from the effects of predation, whereas those with relatively few plants are particularly vulnerable to high levels of seed loss to harvester ants.