a legitimate farmer wouldn't wreck his land. The large gardens on federal land are not even close to legal, and certainly wouldn't be even if the pot prohibition ended. They are nearly all mexican cartel grows for the black market outside of california.
Legalization would probably be the end of most of that activity.
As to harming of wildlife... i think the poisons dumped from meth labs are a bigger target to go after... many of the chemicals are mutagenic and/or carcinogenic.
But yes, the mess from big illegal pot grows is certainly significant in the localized areas where they occur. Their massive use of pesticides, particularly insecticides, can do major damage to the food web in streams... with few stream insects, trout populations suffer but some make it and concentrate the pesticides from weakened and infused insects. Which means that your creel may contain some hot fish.
Chemical fertilizers don't do any good things to stream systems either.
But under legal cultivation, runoff of such things just means someone is wasting money, as farming is all about efficiency. Organic farming, which used to be "snicker snicker" is way more common now, and certainly reduces any off-site risks. And people seem to be willing to absorb the minimal difference in production costs.
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