Why do mushrooms grow in rings?
Well, it's not because fairys did a dance there. As far as we know, it is also not down to natural selection - there is no competitive advantage given by growing in rings, so no reason for evolution to select this trait. The explanation is more mundane than that.
Mushroom "fairy rings" persist for a long time, getting slightly bigger each year. The mushrooms we see are just the fruiting bodies of the fungus - the organism itself lives in the substrate (which in these cases is the ground) as a network of filaments called a mycelium. That organism started growing near the centre of the rings we see, and initially it was just a little patch of mushrooms, or perhaps just a single mushroom. It soon exhausted all the available nutrients in that area, and at that point it only had one direction to go: outwards. The result was a very small ring. Year after year the same thing happens again, each time the mycelium being forced to grow outwards and away from the area it has exhausted of nutrients.
Another commonly-believed myth is that only one sort of mushroom grows in rings. In reality there are hundreds of them, although some do it much more frequently than others, and some don't do it at all. One species (Marasmius oreades) so likes to grow in rings that it has been given the common name "fairy ring mushroom" (or "fairy ring champignon"), but this name has caused a lot of trouble because some people assume that whenever they see a mushroom growing in rings it must be this species. Unfortunately there is another species of mushroom, about the same size and nearly the same colour, which also tends to grow in rings in grassland, and this other species is very poisonous. Clitocybe rivulosa (syn. C. dealbata) contains potentially-lethal amounts of muscarine, and causes severe heart/lung problems.
Some species grow in rings only when the conditions for their development is near to perfect. The blusher (Amanita rubescens) is a very common mushroom in the UK. I find it pretty much every time I go looking for mushrooms from August right through until November, and for many years I never saw a single ring of them. Then one day I came across not just one ring, but a whole chain of them (see picture). I've not seen single a ring of blushers since then.
I have, however, seen a much bigger ring - so big that it was impossible take a photo of the whole thing, so I had to video it. This is another edible species, quite tasty when young, which goes by the name of trooping funnel (Clitocybe geotropa - literally "the funnel that walks across the ground", and here it lives up to its name).
A number of people asked me how long I thought it had taken for this ring to get that big, and my honest answer was that I had little idea - more than thirty years but how much more I did not know. So I asked mycologist Michael Jordan (founder and president of the Association of British Fungus Groups), and this was his response:
"I would hazard a guess at the mycelium starting life way, way before any of those trees were planted. That's a BIG ring. Mycelia expand at different rates according to species, but there are well-established 'fairy rings' on the lawns of some historic houses that are referred to in records going back 400 years. If the wood has seen a fairly undisturbed natural progression, you may be looking at centuries, not decades."
(1) The true "fairy ring mushroom" (Marasmius oreades). Note the grass is lusher in the vicinity of the ring. The grass actually benefits from the presence of the fungal mycelium. This picture was taken yesterday (25/09/2013).
(2) A chain of rings of The Blusher (Amanita rubescens). Very unusual for this species.
(3) The biggest ring of mushrooms I've ever seen. A ring of trooping funnel (Clitocybe geotropa) more than twenty metres in diameter.
(4) Closeup of the C. geotropa ring. Trooping trooping funnels...