"Kay" wrote in message
...
snip
Had Opuntia arrived in Oz as chance seeds in pre-human times, they would
have established more slowly, other things would have evolved around
them. They would not have become the problem that they did.


Not at all, it spread because it was a great environment for it and there
were no natural predators, not because of any specific human program to
deliberately spread it.

Whether they were a 'problem' or not is a human value judgement, the reason
they were regarded as a 'problem' is that it interfered with human
requirements for that land.

--
Tumbleweed

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