People like animals. I'd certainly be willing to buy Coke rather than Pepsi if I thought it was going to save an owl or a possum or something. I also don't think Coke would be above putting an owl on some of their packaging to remind people that buying Coke will save owls.
I'd much prefer this approach to legislation.
I can't think of a single company off the top of my head that is a 'known endangered species killer' so I'd suspect the mechanism you suggest does not actually function.
No legislation will stop species extermination if information about the extermination is unattainable. If it's not available to activists and competitors, it's not available to regulators.
If the information is attainable, then the aforementioned activists and competing companies are both incentivized, morally and financially, to find it.
And, assuming you somehow can get that information (which you can't), how you would you boycott some oil exploration contractor eight levels removed from the consumer? How do you boycott Monsanto/Bayer, which sell to farmers who sell to wholesalers who sell to every mill in the country who sell to every single bakery?
You organize the farmers to boycott.
If you can raise $1mm from various environmental activist communities to lobby congress, you could just as easily raise the same million and organize a boycott that directly targets the revenue source of the bad actor.
I don't need to argue that the environment and endangered species have value. They clearly do. I'm arguing that activists should act more like entrepreneurs and find a way to use market forces to extract that value - in this case, protecting species and their environments.
I've thrown out hypothetical examples, but I'm not an environmental activist, I don't pretend to understand the market. But I know the market is there, because this thread is dedicated to discussing the market opportunity.
This is a very real problem for communities in Africa and the Amazon basin; they are unable to build the infrastructure necessary for further development because privileged Western animal rights activists are more concerned with protecting animals than other humans.
There's a very good reason to be extremely conservative in extinguishing a species, with the benefit of hindsight it won't be worth it. The extinction of the mammoths and dodos also increased economic growth for someone at the time.
The protection of obscure species concerns a small portion of activists very much, and they are very loud in their activism, but there are other large portions of the population that don't care nearly as much and would prioritize economic growth over protecting endangered species. The activists' personal convictions are not more important or more valuable than those of people who don't care as much for endangered species. They're just different, and the activists tend to be louder and more passionate in voicing them.