Saturday, March 17, 2007

Seed Dispersing Ants and "Woodpecker Trees"

Western Trilliums (Trillium ovatum), the beautiful early Spring wildflowers, are now blooming in the nearby woods. As you can see the white flowers (sometimes pinkish or purplish as they age) are arranged above three offset leaves or giving the plant a pleasing symmetry. This set of "threes" is reflected in their name, latin for "threes." The arrangement of threes is also reflected not only in their petals and leaves but also the flower parts--sepals, stamens (6) and stigma. They often occur in moist, shaded woodlands in this area. I first encountered Trilliums in Northeastern Kansas and have also observed them in Northeastern Iowa, and Maryland. Another common name is "Wake Robin" since they appear in early spring about the time that Robins become more active.
Apparently, ants carry the seeds back to their nest, where they eat an oil-rich appendage (elaiosome) that is on the seeds. They then discard what's left of the seeds and thus disperse the seeds in the quiet forest floor. Some believe that this structure produces a pheromone that elicits a "dead corpse response" in the ants. This interesting hypothesis states that the fatty acids in the oils of the elaiosomes of certain plants have undergone convergent evolution to resemble those of arthropods resulting in them being more attractive to carnivorous and omnivorous ants (Hughes et al. 1994) . Apparently this mutualistic relationship is common in eastern north America where ants disperse (myrmecohory) as much as 30% of the spring flowering herbaceous plant seeds in the deciduous forests. The more I learn about the ecology of ant seed dispersal the more interesting it gets. This great site discusses the entire subject and talks about how certain stick insects lay eggs that look like seeds and are taken back to the ant nest and cared for. The hatchlings of some species of these stick insects even look and behave like the ants!
I like Hansen's site for it's interesting descriptions of his plants of the Northwest that he offers for sale.

I went out and cleaned out some more of the dead bracken fern from the flower beds this morning. The bracken fern is one of those annual ferns that is so very common here. I'll take some photos soon of the edible fiddle heads that are just now beginning to emerge from their winter dormancy. They are beautiful when they first emerge and in the fall when they turn a golden yellow, but they quickly become leggy and take over your gardens and flower beds if you let them. Also they die back in the winter and leave their unsightly brown foliage which has to be cleared out.
I also encountered on one of my walks a dead tree covered with woodpecker holes. The common Pileated Woodpecker appears to have made most of the holes, judging by their large rectangular appearance. These are the types of trees that foresters, working for the most part for the large timber companies such as Weyhauser, want to eliminate. They talk of the diseases that they carry and advocate cleansing the forests of such "trash." In a tree farm this might be the thing to do, but in a balanced forest ecosystem, such dead trees provide an invaluable source of food and living places for numerous species of insects, fungi, amphibians, birds, etc.

Hughes,L; Westoby,M; Jurado,E (1994): Convergence of elaiosomes and insect prey: evidence from ant foraging behaviour and fatty acid composition. Funct. Ecol. 8, 358-365.

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