Native harvester ant influence on invasive plant fitness and phenology in endangered coastal sage scrub

Background/Question/Methods

Harvester ants are a key seed predator and affect soil in several ways in coastal sage scrub (CSS), an endangered ecosystem found only in Southern California. Pogonomyrmex rugosus, a species of harvester ant found in CSS, has been previously found to forage the seeds of invasive plant species, and their nests are found to be associated with the invasive plant, Hirschfeldia incana. Harvester ant refuse piles are located around the nest and are where ants deposit dead workers, seed casings, and rejected seeds. It is known that plants that grow around ant nests can exhibit an increase in fitness, but this has yet to be proven in CSS, nor has it been tested in a context with invasive plant species. No studies to date have tested whether harvester ant nests affect the phenology of plants that grow around their nest. Here we show the effect P. rugosus nests have on H. incana phenology by measuring various life traits over time, including height, flowering, and when they set seed and senesce. These measurements were taken near nests and 5m away from nests. Fitness was measured by sampling and weighing seeds, and sampling entire plants, counting the total number of seeds produced.

Results/Conclusions

Preliminary results show that H. incana does grow taller and more densely when immediately around nests. If certain plant species do exhibit greater fitness or a phenology shift, this could potentially change the competition between plant species. For example, if H. incana experiences greater fitness when found around nests, it could outcompete surrounding native plants.