What migration percentage is the cutoff for wildlife to be held in trust by the state vs managed federally? I’ve never heard of that requirement before.
The article you posted bemoaned the plight of scientists studying salamanders (among other species, in fairness). Do 90% of salamanders migrate across state lines?
If wildlife is indeed a resource held in trust by the states, it begs the question: should the USFWS even exist in the first place? It is not unreasonable to consider the possibility that states could (and should) fully manage wildlife in a better and more efficient manner.
*This post may contain my opinions.
There is no cutoff percentage wise. What you need to do is understand Natural Resource Policy and the various acts that landed us here, but, yes, absolutely the USFWS should exist.
The reasons are pretty cut and dried.
In the case of anadromous fish and migratory waterfowl, there has to be federal oversight. The Federal Government has negotiated agreements, treaties, etc. with other countries, tribes, etc. in regard to the aforementioned types of wildlife. It would be impossible, and impractical for every U.S. State to negotiate with Canada, Mexico, Indian Tribes, etc. in the case of migratory birds and anadromous fish. It makes total sense to collaborate and have the Federal Government negotiate with the other countries, entities, etc. Just imagine the nightmare of 50 different agreements with Mexico, 50 different agreements with Canada, 50 different agreements with the tribes. Plus, I'm not even sure a state would be allowed to legally form an agreement with another country.
As to endangered species, once again it makes total and complete sense. First of all it's a federal law, therefore it should have federal over-sight and a Department to administer. The way it works is that as a nation, we've decided to not allow endangered species to vanish. As such, it would put a tremendous burden on any one, or group of states, to entirely fund recovery efforts. Plus, if it were left up to States, some would want to take measures to help Endangered Species, some would just say F-it, we don't want to be bothered. If you're going to pass Federal law, which supersedes State law, then a Federal agency should be the managing agency. If you read the ESA that's spelled out that the Feds will work with the states and also support via funding, biologists, and other management.
So, that's the long version of yes, the USFWS does need to exist and yes it makes sense for certain species to be managed by the Federal Government.