To find out, I spent two wonderful summers in southern Appalachian forests, quietly observing, ants, a half a centimeter long, carrying (or not!) round yellow trillium seeds through the damp leaf litter. Sitting on the forest floor, silent and still for hours at a time, makes one realize how any noise in the forest seems amplified. Two chipmunks wrestling in the leaves yards away sounded like a charging bear. A deer carefully picking its way through the brambles was like a jet plane. And though I routinely ended up covered in mosquito bites, sore and cramped, with soggy knees and backside from crouching on the forest floor all day, those quiet days spent among the trees and woodland creatures will forever be some of my favorite memories.

Back to the science: I watched to see how often, and how far, ants dispersed seeds from both widespread and rare trillium species. I found that the probability of a given seed being dispersed was indeed lower for the rare species, even when the same number of ants were nearby. And I found that ants showed a clear preference for the seeds of the more widespread species.

My colleagues and I confirmed these findings with controlled experiments in the lab. The conclusion was clear: ants weren’t dispersing the rare species as much¹.

If you give an ant an elaiosome

This was a fascinating finding, and I wanted to know more. Back in the lab, I analyzed the seeds and elaiosomes of the widespread and rare species to see if there were differences in their chemistry or structures. There were. For one, the seeds the ants preferred had higher elaiosome-seed mass ratios, meaning the seeds themselves were lighter and the ant-snack add-on larger. What’s more, ants preferred elaiosomes with higher concentrations of fatty acids, which are important nutrients for ants.

While I was exploring the seed chemistry, I also discovered something completely new: There were chemicals in some of the elaiosomes that the ants didn’t like². Researchers had previously identified chemical compounds that were thought to be attractive to ants, but none that might serve as deterrents. Why would elaiosomes contain chemicals that deter ants if they rely on them for seed dispersal?  

My guess was that these range-restricted trilliums, which have less-attractive seeds, might actually benefit from avoiding too much dispersal. Why? It’s possible they rely on specific habitat requirements, like specific soil types or plant communities, that just don’t exist farther away. If that were the case, these plants would benefit from less extensive dispersal away from the suitable habitats.

Zooming out

Before wrapping up my research, I had to check on one more piece of the puzzle: that the rarer, range-restricted trilliums were indeed not occupying the full extent of their potential range, based on general habitat and climate characteristics, compared to their widespread relatives. If this were the case, it would confirm that these rarer trilliums aren’t just limited by something like the climate, but rather by something like microhabitat characteristics or dispersal ability.

Potential (pink) versus actual (green) ranges of a. T. grandiflorum and b. T. discolor. Miller et al., 2021.

1. Miller, C. N., & Kwit, C. (2018). Overall seed dispersal effectiveness is lower in endemic Trillium species than in their widespread congeners. American Journal of Botany, 105(11), 1847-1857.
2. Miller, C. N., Whitehead, S. R., & Kwit, C. (2020). Effects of seed morphology and elaiosome chemical composition on attractiveness of five Trillium species to seed‐dispersing ants. Ecology and evolution, 10(6), 2860-2873.
3. Miller, C. N., Papeş, M., Schilling, E. E., & Kwit, C. (2021). Reproductive traits explain occupancy of predicted distributions in a genus of eastern North American understory herbs. Diversity and Distributions, 27(8), 1489-1506.