The Thistle and the Ant

By Christina Blebea, Project Lead

During my first week at Grassroots Ecology, Sara Witt, an ecologist on staff, took me to Stulsaft Park to pet a thistle. We ruined our shoes in a muddy seep and dodged poison oak, but it was worth it to touch the soft fuzz of an endangered plant.

The Crystal Springs Fountain thistle (Cirsium fontinale var. fontinale) is as woolly as it is mysterious. Although it is only found in serpentine soil that’s laden with heavy metals, studies have shown that it does great in nutrient-rich soil, too (Powell and Knight 2009). It still remains vulnerable to invasive species though, which led Grassroots Ecology to battle reeds (Arundo donax) in Stulsaft in 2010.

We’re also not quite sure how its seeds are dispersed, but it looks like the plant is trying to give us a clue. Its seeds have elaiosomes, bulges rich in fat and protein, which are targeted by ants who take the seed to their underground nests, eat the delicious part, and leave the seed intact to sprout a new thistle.

A Crystal Springs Fountain thistle in Stulsaft Park

A Crystal Springs Fountain thistle in Stulsaft Park

A few months after Sara first showed me the thistle, I wanted to share the plant’s cryptic charm with some local high school students. I took them on a hike to see the thistle and emphatically described its giant, wavy leaves and how they would soon see how fuzzy they were. But when we arrived at the spot Sara had shown me, the thistle that I had pet was gone. There are only five populations of the Crystal Springs Fountain thistle in the world, and it is believed that two of them no longer exist. I knew I had to act fast to conserve the rest of our thistle population at the park, so I proposed an ant survey to make sure the thistle’s seeds were being dispersed to ensure the plant’s longevity at Stulsaft Park.

The only problem was, I knew just two species of ants: the red ones and the black ones.

After a quick internet search (“how to survey ants”), I found the Jasper Ridge Biological Preserve and their ant survey. Soon, I was headed to their ant survey training where I felt my eyes would pop out of my head as I strained to look for some faint difference between two petri dishes containing small black ants. After holding them an inch from my face for several minutes, I felt I could finally pick out the faintest discrepancies between the two species: a steeper curve in the antennae, a slightly more matte abdomen. That is, until a colleague noticed my consternation and whispered into my ear, “those are the same species”.

I had almost given up hope on the ant survey at Stulsaft until I went hiking (many times over) with other ant-lovers and got to know the quirks behind each species: the field ant tastes awful if you suck it up into a vial, the odorous house ant smells like banana cream pie, the winter ant has a very shiny abdomen, and the Argentine ant scatters if you exhale on it.

The Argentine ant (Linepithema humile) is an invasive species that has been wreaking havoc across the world. They quickly displace native ants and can survive just about anywhere as long as humans are present to provide a source of water. We were worried that perhaps they found the seep where the thistle was at Stulsaft Park.

Volunteers and our Senior Ecologist Claire at the ant survey

Volunteers and our Senior Ecologist Claire at the ant survey

In June, a group of volunteers gathered at the park to survey 11 different points around the Fountain thistle population. Our goal was to find native, seed dispersing ants. After convincing our volunteers to brave the path to the thistle, we began our survey. Teams of two looked around their survey points for five minutes to see which species of ants they could find and then move on to the next point.

Argentine ants and aphids on a Crystal Springs Fountain thistle in Stulsaft Park

Argentine ants and aphids on a Crystal Springs Fountain thistle in Stulsaft Park

I was knee deep in wildflowers when the reality of our situation slowly crept up on me. Argentine ants were everywhere. They were on the thistle, the oak, the ground, they were even on my shirt. They had set up aphid farms on the thistle, harvesting the honeydew that the aphids produced as they sucked on the life juices of the plant. We found them at every survey point.

Only one species of native ant was found during the survey: the winter ant. Prenolepis impairs can compete well against the Argentine ant by foraging when it’s too cold for other ants and secreting a lethal substance upon encountering an Argentine ant. While we were happy to see the native ant, the winter ant is hardly known for seed dispersal. Their colony wouldn’t help our thistle much.

Winter ants in Stulsaft Park

Winter ants in Stulsaft Park

The ant survey volunteers and I plopped ourselves onto some picnic benches back at the park entrance, muddy, discouraged, and still flicking Argentine ants off of our bodies. Paul Heiple, our staff botanist, chirped away about his tried and true remedy for ridding his yard of Argentine ants (“just squish the queens, one by one!”) as I contemplated the thistle’s future. Could I set loose a bunch of lady beetles to attack the aphids? Can I sow thistle seeds? What about transplanting a few? Paul’s conversation penetrated my desperate thoughts, “of course, you’ll never know which ants are present in a region unless you come out at night!”

A night ant survey. A last foray before I start squishing Argentine ant queens one by one. Hope for the thistle’s natural dispersal at Stulsaft Park.

If you’re interested in joining the expedition, send an email to christina@grassrootsecology.org to participate in the event in late August. We’ll be happy to have any fans of thistles or ants who want to ensure a bright future for our local flora and fauna. In the meantime, take a moment to look out for the small ecological intricacies around you: the banana cream pie smell of an ant, the delicate elaiosomes on seeds, and the fuzziness of a local thistle.

Source Cited:

Powell, Kristin I., and Tiffany M. Knight. “Effects of Nutrient Addition and Competition on Biomass of Five Cirsium Species (Asteraceae), Including a Serpentine Endemic.” International Journal of Plant Sciences, vol. 170, no. 7, 2009, pp. 918–925., doi:10.1086/600140. https://pages.wustl.edu/files/pages/imce/knightlab/powell_and_knight_2009.pdf