Of ants and seeds

I finally got around to reading the September issue of Natural History – possibly my favorite magazine – which features a marvelous article on how ants disperse seeds: “Jaws of Life,” by Robert R. Dunn. I knew this was an important topic for Appalachian forest ecology, because many of our spring wildflowers, such as bloodroot, Dutchman’s breeches and stinking Benjamin, depend on ants as the sole or primary agent of seed dispersal. This means, among other things, that once such wildflowers are eliminated from an area by several decades of overbrowsing by deer, or through large-scale clearcutting or other landscape alterations, they may take centuries to return on their own. As Dunn puts it, “Getting dispersed by ants… is like trying to get out of town on the local city bus. As often as not, you circle back to where you started. Seeds carried by ants are rarely taken more than a few feet from where they fall.”

So this obviously raises the question of why plants rely on ant dispersal in the first place: what advantage does it confer? Suggestions include protection from predators or from fire, or taking advantage of the rich nutrients available for the seeds’ growth in an ant nest’s midden. This last suggestion seems especially likely for the thin, acid soils of the Appalachians.

The plants go to the trouble of manufacturing a special ant lure, “a small, fatty appendage known as an elaiosome, from the Greek elaios, ‘oil,’ and soma, ‘body.'” The ants seem to find it more economical to carry the entire seed back to their nests so their larvae can eat the elaiosome, following which the seed gets chucked into their compost heap. “Elaiosomes… have evolved at least eighty-six and perhaps several hundred times around the globe…. In the Liliales (the group that includes the lilies) alone, ant dispersal may have evolved independently at least eight times.”

So it’s obvious that ants are highly valued partners for plants, even if we can’t determine the exact reasons yet. Nor is this rather extreme example of convergent evolution restricted to the plant kingdom, as a sidebar explains. This is worth quoting in full, I think. It accompanies an arresting photo of green and yellow, seed-like things with brown knobs on their ends.

The eggs of some stick insects, like the seeds of many plants, have nourishing appendages that encourage ants to pick them up and carry them away. The appendage of an insect egg is called a capitulum, and ants can remove it without damaging the egg. In the photograph above, for instance, eggs from the Central American stick-insect genus Bacteria are shown, magnified roughly fifteen diameters; the brown, knobby protuberances are the capitula.

The parallels between the elaiosomes of plant seeds and the capitula of insect eggs were first highlighted in 1992 by Mark Westoby and Lesley Hughes, both ecologists at MacQuarie University in Sydney, Australia. They gave seeds with elaiosomes and stick-insect eggs with capitula, along with several control items, to various ant species in southeastern Australia. The ants removed the seeds and eggs at a similar rate, treated them similarly, and threw them together into their garbage piles. The apparent advantage for the eggs is that, buried in the debris, they are less likely to be parasitized by wasps.

Perhaps it is appropriate that stick insects, which as adults mimic sticks, start out by living the lives of seeds.

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