I disagree the "...humans introduced them..." part.
I disagree with ANY natrualist position that looks at the human and the human-introduction of anything as "bad" and "unnatural".
First of all, nature itself moves creatures around - plate tectonics, storms, hitching a ride of flotsam and jetsome - all natural ways that non-local creatures become local. So what if humans bring in some?!
Secondly, humans are of this planet, this ecosystem, so humans themselves ARE NATURAL, too.
Stop separating human actions from "natural" actions. Humans are a natural part of this planet. Human actions are natural. Nature created humans and humans do actions, therefore human actions are natural actions. No one calls beavers' making damns in rivers unnatural, do they? No one calls birds digging borrows (disturbing the earth!!!) unnatural, do we? Of course not. Stop calling human actions unnatural actions. Everything humans do - including plastics and greenhouse gases, ARE NATURAL processes of life!

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