Journal Number 108
May 2008

EDITORIAL

How Does That Australian Orchid Seed Get Here?
By Ian St George

Kevin Matthews raised an interesting point when he emailed early in the New Year, "I thought you would be interested in this record of the Australian Lesser Wanderer and my thoughts on Australian orchid seed arriving to our shores."  "I was down at Kaimaumau Village on 1 Jan 08 and saw what I believed to be a painted lady butterfly scoot past in a strong easterly wind. About half an hour later I saw a similar one about 100 metres further afield (it could have been the same one) and managed to capture it on camera after much running and patience: it finally settled for all of three seconds, giving me the chance to confirm it as a Lesser Wanderer".

"The Trans-Tasman passage time for the infrequent visits of butterflies from Australia in the right weather conditions has been calculated at 54-60 hours [1]. It's my opinion that these butterflies must stay at a comfortable altitude to arrive on our shores in such good condition."

"We know that Aussie bush fire smoke that's propelled skyward arrives here at low altitude but I wonder how feasible it is to assume orchid seed could arrive in a viable state if it were somehow propelled high enough to make the Trans-Tasman crossing? I have my doubts on this one. I think it's far more feasible for orchid seed to arrive here attached to an insect such as the Lesser Wanderer or birds making this relatively quick Trans-Tasman crossing."

"While chasing this elusive Lesser Wanderer to capture it on camera, I saw it visit  flowers fleetingly, fly
in amongst foliage, land on foliage and land on the ground. Given this behaviour I theorise that if these
butterflies were to come from a site in Australia with a high number of freshly seeding orchids there
would be a reasonable chance of seed getting caught up on/in these hairy bodied insects for transfer
to our shores; with perhaps the odd one establishing itself."

What a fascinating thought! This raises a number of issues…

1. Can orchid seed really cross the Tasman on the wind? or
2. Can orchid seed cross the Tasman on insects or birds?
3. If (1), can orchid seed survive high altitudes? Or might the wind bring it over at low altitude?
   How might it get into the air in the first place?

HMR Rupp concluded in 1932 that "It is conceivable, perhaps, that minute seeds of orchids have been conveyed by wind across the Tasman Sea, and that only those forms which have found suitable provision for their necessities in the new home have survived" [2].

The issues were reviewed in 2000: “Orchid seeds are very small, extremely light and produced in great numbers. Most range in length from c. 0.05 to 6.0 mm, with the difference between the longest and shortest known seeds in the family being 120-fold. The "widest" seed at 0.9 mm is 90-fold wider than the "thinnest" one, which measures 0.01 mm (because orchid seeds are tubular or balloon-like, “wide” and “thin” actually refer to diameter). Known seed weights extend from 0.31 lg to 24 lg (a 78-fold difference). Recorded numbers of seeds per fruit are as high as 4000000 and as low as 20 ± 50 (80000 ± 200000 - fold difference). Testae are usually transparent, with outer cell walls that may be smooth or reticulated. Ultrasonic treatments enhance germination, which suggests that the testae can be restrictive. Embryos are even smaller: their volume is substantially smaller than that of the testa.

As a result, orchid seeds have large internal air spaces that render them balloon-like. They can float in the air for long periods, a property that facilitates long-distance dispersal. The difficult-to-wet outer surfaces of the testa and large internal air spaces enable the seeds to float on water for prolonged periods. This facilitates distribution through tree effluates and/or small run-off rivulets that may follow rains. Due to their size and characteristics orchid seeds may also be transported in and on land animals and birds (in fur, feathers or hair, mud on feet, and perhaps also following ingestion) ” [3].

But what actual studies have been done? Korean researchers wrote, "These dust-like seeds are windborne and, thus, would seem to have the potential for long distance dispersal (a common perception); this perception has led to a prediction of near random spatial genetic structure within orchid populations. Mathematical models (e.g., a simple ballistic model) for wind dispersed seeds and wind tunnel experiments, in contrast, indicate that most seeds of orchids should fall close to the maternal plant (<6 m), supporting a prediction of significant fine scale genetic structure within populations. In reality we do not know much about seed dispersion in orchids” [4]. They studied the genetic structure of colonies of Cephalanthera longibracteata, and their results supported the prediction that the majority of seed dispersal occurs over distances of less than 10 m.

Using a simple ballistic mathematical model and wind tunnel experiments, Murren and Ellison showed that mean expected seed dispersal distances for the neotropical epiphytic orchid Brassavola nodosa were less than 6m under conditions approximating those found in its natural habitat [5].

Carlyle Luer considered Platanthera holochila, an orchid of mountaintop bogs on the older Hawaiian Islands, to be indistinguishable from Platanthera hyperborea var. viridiflora from the Aleutian Islands and the southern coast of Alaska far to the north. He wrote, "Seeds should not find transportation to the [Hawaiian] islands wanting, since the Pacific golden plover migrates annually between the cold bogs of Alaska and these high cool bogs of Hawaii. As evidence, a sun-dew (Drosera anglica), also common in the Alaskan bogs, probably also spanned the great distance in mud on the feet of birds" [6].

There is simply not a lot of science in all of this, but what science there is suggests long distance wind dispersal should be uncommon, so Kevin Matthews may well be right.

References
1. Early, JW , Parrish, GR , Ryan, PA. An invasion of Australian blue moth and blue tiger butterflies
    (Lepidoptera Nymphalidae) in New Zealand. Records of the Auckland Institute and Museum 1995; 32: 45-53.
2. Rupp HMR. Australian and New Zealand orchids. Victorian Naturalist 1932; 49: 151-2.
3. Arditti J, Abdul Karim Abdul Ghani. Tansley Review No. 110. Numerical and physical properties of orchid seeds
    and their biological implications. New Phytol. 2000; 145: 367-421.
4. Chung MY, Nason JD, Chung MG. Spatial genetic structure in populations of the terrestrial orchid Cephalanthera
    longibracteata (Orchidaceae). American Journal of Botany. 2004; 91:52-57.
5. Murren CJ, Ellison AM. Seed dispersal characteristics of Brassavola nodosa (Orchidaceae).
    American Journal of Botany 1998; 85: 675-680.
6. Luer Carlyle A. The Native Orchids of the United States and Canada excluding Florida.
    The New York Botanical Garden, 1975.