Where do brambles come from?

[Barry]

I have just moved to a new allotment site where many of the plots are overrun with brambles. In fact, at every allotment site I have been to, there are invariably brambles just waiting to take over abandoned plots. Now, why is it that brambles do this? Why, for example, don't we ever get raspberries doing the same thing?
Significantly, virtually every bramble I have ever seen is of the 'wild' blackberry variety, with murderous thorns, so these are clearly ancient residents. You never get plots taken over by thornless blackberries, do you? This would seem to indicate that these blackberries have been there for years! Was all of the UK once covered in blackberries?!!

[Johnboy]

Hi Barry,
Blackbirds are the main culprits and they eat Blackberries and excrete all over the place.
A seed with it's own starter fertilizer pack so to speak.
If you are growing thornless varieties they are normally protected against the birds so they do not get eaten to the same extent also I have no idea if thornless Blackberries breed true to type as they are asexually propagated.
JB.

[Allan]

I grew all the thornless varieties that I could get, result very poor flavour and no stamina, what didn't die on their own I dug out. Mine are now all from Ashton Cross or Fantasia, large berries, good flavour,unfortunately they haven't been managed and are taking over the land so I am having to clear them as fast as I can.
It isn't just seeds, the runners go a surprisingly long way in a year then the tip roots and starts another colony.
Allan