Journal Number 109
August 2008

ORIGINAL PAPERS

Australian Invaders Take to the Skies
By Mark Moorhouse

Is it reasonable to expect orchid seeds to spread by wind dispersal? Could Australian species possibly migrate across 1500 miles of ocean to start a new life in New Zealand using only wind to carry out this feat?

Lets examine a hypothetical scenario. This is necessary because it is literally impossible for science to calculate accurately the number of Australian sourced orchid seeds arriving in New Zealand. There are far too many variables. At best we can hope to prove that an airborne Trans-Tasman crossing is not only possible, but highly probable.

Consider some facts. Taking just one percent of the area of Tasmania, Victoria and New South Wales (those Australian States nearest and on a similar latitude to New Zealand) calculations show that just 0.1% of the orchids growing in this portion of Australia could conceivably produce 2 trillion seeds (using 500,000 seeds as an average capsule and 4 capsules as an average per plant and with plant distribution of only one plant per 100 sq m - see box).

The enormity of this figure raises an obvious question. Why do temperate orchids, in fact most orchids, produce so many seeds? Seeds so light that a manual shake of a ripe picked capsule, launches brown clouds of seeds floating about for a considerable time even on a virtually calm day.

It has been argued that so many seeds are set per capsule because so few flowers are pollinated and this is the way the plant compensates for this.

Dr Calaway Dodson [1] stated "In an acre of forest, there may be roughly 1,000 plants of Oncidium, and each plant may have 100 small flowers on it. Yet of these thousands of flowers, rarely do we find more than five seed capsules produced. This tells us pollination is a rare occurrence, yet since one seed capsule contains thousands of seeds, this is enough to keep the population going." He goes on to say " Some Cattleya capsules produce 30,000 seeds. In nature they would ride the wind."

Darwin [1] also pondered the enormity of orchid seed production. He counted the seed in a small European orchid and discovered that 6,200 seeds formed in each of the 30 or so capsules. He then calculated that if every seed grew, the offspring would cover the entire land mass of the earth in just three generations. He was mystified at what limited such rampant reproduction.

Interestingly this highlights two facts:

1. Even relatively large seed, like Cattleya are capable of becoming windborne.
2. Temperate orchids produce seed far more prolifically, but few grow.
   Just how few is a critical factor in producing a mathematical hypothesis.

A brief study of New Zealand orchids highlights that Pterostylis, Chiloglottis and Thelymitra spp have relatively high strike rates. This is evidenced by the fact that we can often find them growing in sizeable clumps containing two or more generations. Other spp such as Nematoceras also form clumps, but do so vegetatively. Most species of this family also extend the peduncle after pollination takes place to enhance seed distribution.

How effective is this? Take a walk through a Nelson forest and note the distance between colonies of Nematoceras. You may walk some 30 m or several hundred metres between colonies. How does seed travel from its mother capsule across several hundred metres of open bush to begin a new colony? The mother plant's adaptation relies on swirling wind currents, and this proves that some seeds do disperse successfully far further than the 10 m as suggested by the Korean experiment cited in Journal 108 [2].

Wind tunnel experiments strive for constant wind-flows. This in no way imitates the swirling broken air-flow of wind passing through forest or scrub. If the number of seed passing the 10m mark from the mother plant were just one 10,000th of a percent, and remember, our hypothetical scenario has 219 trillion seeds to begin with, that means a very large number of seeds still become airborne for significant distances.

Once launched, how do they remain airborne?
Thermals form about most sloping hillsides. These up-draughts can launch birds and fine airborne material to great heights. In our part of the globe the dominant wind is a westerly, so any Australian orchid seed picked up by thermals will gradually drift in an easterly direction towards New Zealand.

Many weather patterns cross the Tasman in less than 36 hours. Some seed could even reach jet-stream height and fast track the crossing of the Tasman sea. Whirlwinds could also play a significant part in launching seed into the air. It would only take a willy-willy to pass over a patch of dehiscing orchids to elevate hundreds of millions of seeds skywards in seconds.
In addition birds, bats and flying insects may be agents in getting seed airborne.

Why don't they just keep on flying right past New Zealand?
As the warm moisture-laden airflow approaches our coastal hills it is forced upwards and any minute particles of dust including our orchid seeds suddenly find themselves encapsulated in a droplet of water. Dust particles and our orchid seeds actually enucleate raindrops. Our beleaguered seeds are revitalized, and as the seed falls to earth within a raindrop, it finds a nice damp medium in which to start life.

But most of them would simply miss New Zealand.
While it is definitely true that some would pass to the north, if you draw a line from Cape Reinga through Gympie in Queensland, any orchid south of that line could disperse seed to New Zealand. This is based on the dispersal pattern of volcanic ash from eruptions in westerly belts. A simple example you can look at on a map. The eruption of Lake Taupo in AD 186 put a 2cm layer of ash on the Chatham Islands. Extrapolate a line westwards passing through Chathams and Taupo and you will see the possibility of seed also arriving from Queensland.

In summary, if just a staggering one billionth of one percent of the seed produced in one percent of the south eastern states of Australia arrived in New Zealand, then 20,000 plus seeds arrive here a few days after each decent windstorm in Australia. A few grow.

What species are most likely? Those species that strike most easily in their natural habitat, ie, are less fussy about where they grow, seem prime candidates. Have we discovered Australian Chiloglottis, Thelymitra or Pterostylis here in New Zealand? The evidence published in this journal speaks for itself.

References
1. Marden, Luis. 1971, National Geographic Magazine. Vol 139. No. 4 pp 485-513. Article:
    The Exquisite Orchids.
2. St George, I. 2008, New Zealand Native Orchid Journal 108 p.5 Ref as given there Chung, M.Y.