The problem is, things get damaged. Sometimes very severely. It's not possible to keep everyone from acting irresponsibly, and as humans in a modern society, we have a lot of impact on nature. All it takes is one ship with a handful of zebra mussels to infest the entire Great Lakes ecosystem. All it takes is one person accidentally releasing Asian grass carp for it to infest the entire country. All it takes is one farmer using a bit too much fertilizer to form massive algae blooms in a nearby lake.

Conservation is an important preventative step. But that's like telling a bleeding person that they shouldn't have stepped on that nail. The damage is done, and the patient needs antibiotics and stitches now. It's important to prevent damage from being done, but it's just as important to fix it after the fact.

With both Asian carp and zebra mussels, (and lionfish and starlings etc etc etc) we are essentially powerless to stop their advance. To extend your metaphor, we literally have nothing in the way of antibiotics and stitches to salve this wound. We need to be trying orders of magnitude more than we actually are to prevent such calamities from happening in the first place. People have this misplaced faith the we as a race can engineer our way of out any problem, and it's just not true. Ban the nail, I say.