One argument that can be used is the example of the San Francisco red sided garter snake, Thermophilus infernalis. It comes from a very small range in the San Francisco bay area in an area that is now fabulously expensive and it's a question as to how long that wetland can stay out of the hands of condominium and strip mall developers.
Garter snakes have a cult-like following in the snake world, even in the US, where it's unusual a native animal gets any attention.
The snake was first put on the California state endangered list but when it went on the US Federal endangered list it became a crime to own one anywhere in the US without a hard to get and expensive permit.
The St. Louis zoo in Missouri reported that they could not sell or even give away their 300 captive bred specimens (and understand only 1200 exist in the wild) and ended up having to feed them to their cobras.
Captive populations exist in Germany and Denmark. If something awful happened to the wild population (understand that the spring count for the Devil's Hole pupfish - Cyprinodon diabolis revealed only 35 fish are left in the sole biotope - Devil's Hole in Ash Meadows, Nevada, down from 75 in the spring count; there used to be hundreds going back over 100 years, continuously and at this rate they'll be gone in a year and cannot be captive bred under any conditions. Well, they can be bred but the body morphs into something else when taken out of Devil's Hole) we would be reliant upon captive populations to re-stock the population in the wild.
So, any legislation of this type must balance the threat to native species with the threat to contributing to the extinction of species because of that legislation. In the worst case, saving 10 species but losing 100 because of the German propensity to maintain endangered species successfully can only be seen as "a win" from a very narrow perspective.
Richard J. Sexton