|From left: Wild Ginger, Nodding Trillium, and Bloodroot in early spring.|
Wild Ginger, Trillium and Bloodroot are done flowering, but that's not the end of their efforts. Now they must disseminate their seeds, and each has arrived at the same, six-legged solution to accomplish that task: Ants.
Myrmecochory (often pronounced MUR-mecco cor-ee), the dispersal of seeds by ants, is a convenient invention. Ants are found all over the world, from the tropics to the Arctic, so they are a ready resource. Similarly, plants that employ ants for seed dispersal live in diverse habitats, including the tropical rainforests of Latin America, the dry shrub communities of South Africa and Australia, and the eastern deciduous forests of Europe and the U.S. (1). Myrmecochory is thought to have developed independently more than 100 times, with more than 11,000 species of plants relying on these insects to spread their seeds (2).
That nature converged on the same solution in different, and distant, plants suggests that it works. Like any method of seed dispersal, though, myrmecochory has a cost. It demands adaptations, and in one group of ant-dispersed plants, that adaptation is in the form of a bribe.
To lure ants, myrmecochores attach small, fatty bodies called elaiosomes (e-lay-o-somes) to their seeds or fruits. Depending on the species, these mini nutritional packets are clear, white, brown, or other colors and shaped like worms, flags or amorphous dollops. Some may emit an odor like rotting insect carcasses, a trick to attract ants to take the seeds back to their nest, remove and feed the elaiosomes to the colony, and leave the seeds to germinate (1).
|From left: Wild Ginger, Nodding Trillium, and Bloodroot seeds with their elaiosomes.|
Both plants and ants are thought to benefit from this relationship. Plants benefit by reducing competition for light and nutrients between parent plants and their offspring. Moving seeds away from the parent plant also lessens the risk of local extinction: If one part of a population dies, another, more distant, part may survive. Another potential benefit is reduced seed predation. Perhaps better than any other animal, ants can disperse a cache of seeds meters away from the parent before mice, birds or other seed eaters find them.
One more potential benefit is improved seed germination. Seeds discarded in or near ant nests may end up in refuse piles, nutrient-rich microenvironments that can aid germination and seedling growth. The medium in ant nests may also retain more water or be better aerated, another potential aid to germination and growth.
Ants benefit from myrmecochory, too, and it's likely they're adapted to the interaction. Not all species of ants forage for seeds that have elaiosomes, but those that do may have some yet-unknown characteristics that lead them to that behavior. Widespread though it is, this mutually beneficial relationship still has some secrets to share.
Myrmecochores of Minnesota
The ant-dispersed plants in this region tend to be early-blooming herbs of deciduous forests. Here are a few, based on personal observation or mention in references.
Bloodroot (Sanguinaria canadensis)
Nodding Trillium (Trillium cernuum)
Wild Ginger (Asarum canadense)
Yellow Violet (Viola pubescens)
Spring Beauty (Claytonia virginica)
White Trout Lily (Erythronium albidum)
(1) Handel, S.N., and Beattie, A.J. (1990). Seed dispersal by ants. Scientific American 263 (2): 76-83B.
(2) Lengyel, S., Gove, A.D., Latimer, A. M., et al. (2010). Convergent evolution of seed dispersal by ants, and phylogeny and biogeography in flowering plants: A global survey. Perspectives in Plant Ecology, Evolution and Systematics 12: 43-55.