The Question is Blowing in the Wind
Like a cake covered in icing sugar, so powdery-white are the honey mushrooms and the ground around them. A typical one of these spores is only 1/ 3000 of an inch long. Every single mushroom in the clump sheds an estimated 15 billion of them but many don’t go far. Is there a single one which makes it high enough into the air to get transported away from the parent?
For any spore trying to travel a long distance, survival is the first challenge because conditions in the atmosphere are downright hostile; it is cold, very dry, and there is strong UV-light to break down essential molecules. Spores with thin walls don’t last long. Those with thick walls and a covering of pigment have the best chance of making it. But after its journey a spore faces other challenges. It has to land in a suitable place (sea or ice won’t do); it has to germinate (desert areas are usually too dry), and then it has to find a mate (species that do not need a partner are at an advantage), and ally itself with a compatible substrate (an ectomycorrhizal partner of pine will not survive in a South American monkey puzzle forest). Success only comes with escape from the parental ‘home’, survival of the journey and expansion into the new environment.
The wonder is that some species are very good at long distance dispersal and achieve wide distribution. Puffballs are notable examples and I have seen the same earthstar, Myriostoma coliforme, on the Big Island of Hawaii and in the Dutch dunes in Europe. The spores of these guys are not just thick-walled and well-pigmented, but they are hydrophobic and many have spines, both of which are excellent, additional adaptations to airborne transport. Pleurotus djamor, a close relative of the oyster mushroom, is widespread in the tropics, and yet, its spores have been detected as far north as Canada and Switzerland! But is this typical for most species? For only a few spore dispersal has been investigated, and here are highlights of what we have learned.
The Split-gill, Schizophyllum commune, is found all over the world, and is completely interfertile. In other words every Schizophyllum can mate with any other, regardless of its place of origin. In this way it is like humans, but just as with humans, there are regional differences due to geographical barriers such as seas, and high mountains. The South American Split-gill is different from the North American populations, so a natural question is: what happens in the Caribbean? Do both forms occur there and do spores from both groups get there? To find out, researchers set baits to trap the Schizophyllum spores. Each bait was a petri dish with mycelium from one spore of S. commune, on which other spores could land, and germinate. Subsequently the two different mycelia fuse and produce a third which can be distinguished from the original mycelium. What the researchers found was that spores could indeed migrate over long distances. There were many spores a mile out at sea, and not all of them came from the closest land. Yes, they did find South American spores in the Caribbean, but not further north. On the other hand, North American spores failed to turn up in the Caribbean. It was estimated that every hour around 18 spores land on every square meter of surface. This research suggests that seas do pose a spatial barrier, but one that is not absolute; with wind in the right direction and at the right time, spores may be transported over wide expanses of water.
A second study had a rather different emphasis and investigated not only how far spores can travel, but also how viable they are. Here the setting is Sweden, and the species in question are the old-growth forest dwellers, Fomitopsis rosea and Phlebia centrifuga. These species do not grow in areas where the forests have been chopped down, and are rare in places with small relicts of forest. Where the regions of forest are more extensive, in the north of the country, the two are more common. The question the study addressed was: how large do the forests need to be to sustain viable populations of these species? The same approach was used as in the Schizophyllum study, with the difference that wood discs were used to grow the mycelium, to better mimic the natural conditions. Baits were put out in seven locations in widely separated latitudes. Many more spores were found on the northern baits than on the southern ones (where scarcely any were found.) Significantly, the spores from the small, stressed populations tended to have more problems germinating than the ones from the large, healthy populations. It appears that the size of the population and the presence of old-growth forest both play a role in sustaining the fungi.
No one has investigated this kind of effect in California and it would be interesting to see whether there is direct contact between populations of, let’s say, Amanita lanei (formerly Amanita calyptroderma) in the coastal oak forests and in the Sierra foothills. Do spores travel through the air over the Central Valley between those populations, or is there only more local spore transport? Will the Sierra populations still be viable when all the coastal live oak has succumbed to sudden oak death?
Humans are very good at transporting all kinds of organisms, both inadvertently and deliberately. With their help, mushroom species have jumped, again and again, to new territory in modern times. The Octopus stinkhorn, Clathrus archeri, is a good example. It arrived in Europe from Australia at the end of the First World War, probably with military equipment or in bales of wool. It settled in the northeastern part of France and from there, helped by the local flies, it spread like ripples in a pond, until it is found in virtually every country in western Europe. Recently it has also appeared in California, undoubtedly with human help, though we do not know whether it came directly from Australia or via Europe. We should all keep our eyes and noses open to record its progress here.
However, the story of Armillaria mellea in South Africa shows that not all transplants are that successful. Dutch settlers in the 17th century planted a garden in Cape Town, to provide fresh provisions for the seafarers making the long voyages between Europe and the far east. With the roots of grape or citrus, or perhaps some other European plant, came the honey mushroom. Even now the original garden still harbours the very same honey mushrooms, all genetically identical. But the species has not made the jump into nature and has affected only the plants within the original garden.
Unraveling those stories, and determining what makes a species a successful long-distance flier or a perfect invader will keep amateur and professional mycologists busy for years to come.
Some good stories for further reading:
- Brown, J.K.M. & M.S. Hovmøller, 2002. Aerial dispersal of pathogens on the global and contintental scales and its impact on plant disease. Science 297: 537-541. (Abstract) (PDF)
- Gonthier, P., R. Warner, G. Nicolotti, A. Mazzaglia & M.M. Garbelotto, 2004. Pathogen introduction as a collateral effect of military activity. Mycological Research 108: 468- 470.
- Vilgalys, R. & B.L. Sun, 1994. Assessment of species distributions in Pleurotus based on trapping of airborne basidiospores. Mycologia 86: 270-274. (Abstract)