On The Trails: Ballistic seed dispersal

A geranium flower before pollination and seed dispersal. (Photo by Deana Barajas)

By Mary F. Willson

Plants disperse their seeds in many ways. Some cast their seeds into breezes or water currents, some produce seeds with adaptations that facilitate transport on (or in) animals, and so on.

What might be the advantages of dispersing the seeds? It reduces the density of seeds near the parent plant, so potentially there is less competition among the siblings and less attraction for seed predators. There is also a reduced likelihood of inbreeding among sibs or with the parent.

On the other hand, the habitat in which the parent grew is (or was) clearly suitable for that species, so short dispersal and staying near the parent might be an advantage, while going far away is an uncertain gamble on finding a suitable place.

The "crane's bill" of a geranium flower will vigorously split open into five sections connected at the top, with seeds borne at the lower end of the split arms. So the seeds are flung in all directions. (Photo by Bob Armstrong)

With that background information, consider ballistic dispersal. Some plants explosively discharge their seeds: the capsule opens very rapidly, flinging or propelling the seeds away from the parent plant. The ballistically dispersed seeds may go somewhat beyond the canopy of the parent, but don’t usually go very far. Thus there could be all three kinds of problems (competition, predation, inbreeding).  However, the distribution of "suitable places" for a particular species is seldom known.

This mode of dispersal is not common, but it is widespread, having evolved independently in about 23 taxonomic families.

Here are some examples. Among violets (Viola), ballistic dispersal distances vary among species, but generally are up to several feet from the parent. Wood sorrel (Oxalis) plants are small herbs, usually less than a foot tall; explosive dispersal may send seeds about 15 feet away from the parent (that’s impressive, by such a small plant). Wild geranium flings its seeds for several meters, when the seed-bearing "crane’s bill" splits into five sections that curl back and forcibly catapult the seeds. Jewelweed (or touch-me-not, Impatiens) is a well-known herbaceous plant whose ripe pods explode at a touch. The seeds can be flung 20 feet or more from the parent. And then there is the famous tropical tree called Hura crepitans; when the seed pods explode with a bang, they can send seeds 100 or maybe 300 feet away. 

An early blue violet will disperse its seeds ballistically. (Photo by Denise Carroll)

Ballistic dispersal, by itself, is really short-range dispersal, compared to that accomplished by wind or vertebrates. Sometimes, however, that dispersal mode is linked to another one. Many species of Viola (including those in Alaska) have ballistic dispersal, but the scattered seeds are also equipped with elaiosomes, little oil-rich bodies that attract ants. The ants pick up the seeds and carry them farther away, sometimes taking them to the nest, where the colony feeds on the elaiosomes and the seed is then depositing in the colony’s trash pile, where germination conditions are often good. Thus the ants enhance the dispersal distance of these seeds and often put them in salubrious places. A similar arrangement is known in Acacia trees, a species of gorse (Ulex parviflora), and some other plants. In some (but not all) cases, ant dispersal may be facilitated by the initial ballistic scattering, as the ants seemed to avoid dense piles of seeds.

The bottom line here seems to be that, although the mechanics of explosive seed release have been well-studied in some species, the evolutionary ecology of ballistic dispersal is still relatively unknown. Is the cost of building a ballistic device in some way more costly than contrivances for other modes of dispersal?? Avoidance of competition, inbreeding, and predation are probably involved, but are those problems less serious for species dependent on ballistic dispersal? What constitutes suitable habitat for each species and are those suitable places patchy and scattered or perhaps nearby and contiguous? Is that distribution different for species dependent on ballistic dispersal than for species with other dispersal modes? Habitat suitability and distribution are very hard to assess. Lots more information is needed. 

• Mary F. Willson is a retired professor of ecology. "On The Trails" appears periodically in the Juneau Independent.