I am currently spending 20+ hours a week in search of fungi, in three hour sessions dotted all over Sussex, Surrey and West Kent. And I do see some problems, and they are getting worse, but they are actually quite specific rather than general.
Problem #1 is the "Polish Problem", and it is cultural. People from all sorts of European countries pick fungi in the British countryside, but whenever I see a combination of (1) people with carrier bags stuffed to the brim with all manner of fungi in all sorts of condition and (2) nothing even remotely edible left in the forest, I'm afraid the people involved are Polish more often than they are not. Other people tend to leave some stuff for nature, or other collectors, or even leave little ones so they can grow and be collected next week maybe. The poles take everything.
Problem #2 is the "London Problem." The closer you get to London, the more people there are trying to find edible fungi, and by the time you get anywhere near the M25 then you might as well not bother. And it is no mystery why this is. Picking fungi has already been prohibited in Epping Forest and Richmond Park, and Hampstead Heath isn't going to supply much of London with wild fungi (although I did see an impressive collection of beefsteaks very high up in a tree in Battersea Park on Wednesday, on my way to a talk in Chelsea.) It looks to me like we have a domino toppling procedure ahead of us. The next place that fungi picking will be prohibited by law will be somewhere like Ockham Common (which is quartered by the junction of the M25 and A3). But this will just shift the problem to the next easiest place for Londoners to get to, such as Limpsfield Chart. I'm not sure what the answer is. If we could just replace everything inside the M25 with what was there before humans turned up then we'd be sorted!
