By Meredith Swett Walker
Harvester ants live up to their name. Like a farmer bringing in a crop of grain, the ants are busy seed collectors. In some habitats they are the dominant seed predators. But do harvester ants play favorites? When faced with a cornucopia of seeds, do they select certain species over others like a kid picking all of the chocolate chips out of the trail mix?
Researchers at the University of California, Riverside set out to find out if one harvester ant species, Pogonomyrmex rugosus, selects seeds in proportion to their availability or if it has preferences. And if so, could the ants’ seed preferences affect which plants’ seeds go on to sprout and which end up as an ant’s meal? The answer to the seed survival question is important because exotic invasive plant species are a big problem in southern California’s coastal sage scrub where they conducted their study. Their paper, “Seed Selection by the Harvester Ant Pogonomyrmex rugosus (Hymenoptera: Formicidae) in Coastal Sage Scrub: Interactions with Invasive Plant Species” is published in the latest issue of the journal Environmental Entomology.
The research started with some basic observations. What types of seeds were the ants harvesting and bringing back to the colony? Harvester ants typically travel to and from the nest in a line or “trunk trail” several meters long and then scatter into an area of a few square meters where they forage. This is called the “foraging patch.” Lead author Christopher Briggs observed ants returning to the nest along the trunk trail and recorded which types of seeds they were carrying. Briggs also estimated seed availability in the foraging patch soon after it was established and after it was abandoned by the ants. This allowed the researchers to determine if the ants had had an effect on the seed bank (defined here as seeds on the surface of the soil.)
Getting a good look at what the ants carried back to the nest required Briggs to get up close and personal, with his face about 30 cm or so from the trunk trail. Luckily, the ants did not appear to mind. They did object when his feet and other body parts got in their way. Unfortunately for Briggs, Pogonomyrmex venom is believed to have evolved for defense against vertebrate predators. Briggs says the sting he received from one P. rugosus that found its way inside his shoe “felt like a large screw being driven in slowly for the next several hours.” His account is backed up by the “King of Sting,” entomologist Justin Schmidt, who rates the sting of a related Pogonomyrmex species at a 3 out of 4 on his pain index — worse than a hornet. Schmidt describes the pain as “bold and unrelenting.”
Briggs and his co-author, Dr. Richard Redak, found that the ants were preferentially collecting seeds of the invasive species filaree (Erodium cicutarium) in greater than expected proportions given their availability. A majority of foraging ants returned with either filaree seeds or another exotic species, mustard (Brassica tournefortii). However, this disproportionate harvesting resulted in no changes to the seed bank in the foraging patch.
Next the researchers tested whether the ants exhibited this preference when presented with different species of seed in equal availability — a “cafeteria-style” experiment. They allowed P. rugosus colonies to choose between four species of seeds: brittlebush (native), buckwheat (native), filaree (exotic), and mustard (exotic). The seeds were presented in covered petri dishes which allowed ants access through holes cut in the sides, but prevented birds and other vertebrates from eating the seeds. Unfortunately for Briggs, one non-seed eating vertebrate felt compelled to interfere with the petri dishes just to stick it to the meddling primates. Despite being weighted down, several of the petri dishes were overturned and were found to “smell strongly of urine.” Briggs suspects coyotes.
P. rugosus colonies at one of the field sites selected filaree seeds over mustard and buckwheat. At the second field site, brittlebush, filaree, and mustard were selected over buckwheat. These experiments again demonstrated the ants’ preference for filaree seeds — despite the fact that these seeds have “awns” or bristle-like appendages that frequently become entangled with surrounding vegetation, making them difficult for the ants to carry. This is also the first study to document P. rugosus feeding on mustard.
Several previous studies have shown that seed predation by ants can affect the composition of plant communities, and in one case the abundance of invasive plant species. However, this was not the case here. Despite extensive foraging by P. rugosus on the exotic invasive species filaree and mustard, the researchers found that the seed bank did not change in this Coastal Sage Scrub habitat. Briggs and Redak speculate that this may be due to the high abundance of seeds in general and the fact that the ants appeared to abandon foraging patches before the seed bank was significantly depleted.
Unfortunately for the native plant species of the Coastal Sage Scrub, P. rugosus’s taste for the exotic flavors of non-native plant seeds does not seem to be making a dent in the population of invasive seeds. That does not mean that P. rugosus isn’t affecting the distribution or success in more subtle ways that this study was not designed to test. But for now, it appears that the arrival of invasive plants like filaree and mustard have simply added variety (and perhaps a little spice?) to the diet of the hardworking harvester ant.
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Meredith Swett Walker is a former avian endocrinologist who now studies the development and behavior of two juvenile humans in the high desert of western Colorado. When she is not handling her research subjects, she writes about science and nature. You can read her work on her blogs http://picahudsonia.com and https://citizenbiologist.com or follow her on Twitter at @mswettwalker.