While I agree, the problem is that the Black footed ferret already is endangered AND the legal ferrets have not crept into the ecosystem from beyond the arbitra

While I agree, the problem is that the Black footed ferret already is endangered AND the legal ferrets have not crept into the ecosystem from beyond the arbitrary lines humans have drawn. So at both the micro and macro perspectives, you have no explosion of population. Taking a large population of ferrets and putting them in some California farmland and see them die off might be what your conscience requires as "proper evidence", but it's ironically illegal to even perform the test. Much like the CA F&G, it's an ideological problem, not an evidence based one for you.