"Malcolm" wrote in message
...

snip

Is it
a "balance" to gain one introduced species at the expense of another
indigenous one? Is it a "balance" when mink supplant water voles on our
rivers?


Of course it's a 'balance' (if I may use that term in this context), since
the ecosystem will have been adjusting/adapting to changes in circumstances,
including the presence of 'new' species and the absence of 'old' ones. It
might not be the 'balance' some people prefer and are prepared to accept, of
course, but it's a 'balance' nonetheless.