Ant-plant Symbionts in Wisconsin Woods
Written originally for Wis Entomological Society, by Linda Curtis, botanist 2006
My research for a book about woodland sedges led me to a reddish spiny-backed ant species that hurries-away the seed sacs of long-stalked sedge before other animals can get them. That sedge, a Carex species, has small spikes with several sacs. Each sac is a wrapper that encloses a single seed-like achene. While the sacs are small, only 4 mm long, the symbiont ants are not much larger, about 5 mm long.
Do the ants consume the achenes for themselves? No, there is a hungry horde of pale ant larvae in the home nest that must be fed. The ant nests are usually in rotting logs in woodlands. Each nest consists of a few hundred ants and the ants tend to move every month or so. They forage for seeds, fruits, and small invertebrates during the day and rest at night. If a foraging ant finds long-stalked sedge with mature sacs, it will immediately leg back to the nest to get help. To mark the way, it leaks from its poison gland and so marks a continuous trail, ensuring that the patch of plants can be returned to at once. This is the same neuro-poison used to paralyze larger prey such as caterpillars, and so helps the horde of ants carry a rigid rather than squirming body back to the nest.
The significance of these ant “seed-snatchers” is that the seed-like achenes are barely munched on, only the outer parts are chewed off, and what is left is taken out as garbage. This “garbage” can still germinate to make new plants, whereas a bird or chipmunk would completely digest the achenes and so remove potential seedlings from their woodland environment.
This makes that ant a mutualistic symbiont of the plant because they hide the sacs away from other predators. There are other woodland plants that benefit in the same way. Because I am a “down and dirty” botanist, I have seen the same seed dispersal mutualism with trout lily and bloodroot, both spring ephemerals. For photographing low-growing plants, I often lie on the ground, knowing scout ants will check me out immediately, but the majority of ants are snatching the seeds from the plants as fast as they can.
That particular ant-dependant relationship is known as mymecochory. As long as the achenes are not totally consumed, and the inner part with plant embryo is not damaged, they can germinate either in the nest or on the ant trash pile. As a result, rotting tree trunks have tufts of this small grass-like sedge growing among the mosses.
The long-stalked sedge, Carex pedunculata, is perennial and can be grown as a native substitute for Eurasian lawn grass species. The trick is to gather the seed sacs before those rude ants, Aphaenogaster rudis, can get them before you.
Linda Curtis is a botanist that is retired from teaching, but not from writing. She apologizes for the tongue-in-cheek humor that implied “rudis” meant rude, when it really refers to ruddy, a reddish color. Her book Woodland Carex of the Upper Midwest was published in May 2006.
copyright © 2014 Linda Curtis, botanist