Center for Biological Diversity is a front group for the "Humane Society of The United States" (note this is NOT the same as your local Humane Society).
The HSUS is notoriously anti-hunting and pro-animal 'rights'; it's highliy likely that many of their members are also anti-2A, since anti-hunting/pro-animal-rights and anti-2A sentiments nearly always go hand-in-hand.
Cam Edwards has been talking about this on NRA news.
As I recall, there has never actually specified a solid upper OR lower threshold for lead exposure based on anything that could be called 'hard science'.
In fact, how much lead have shooters have been exposed to for decades? It seems the shooting community and hunters would have an abnormally high number of cases of lead poisoning. That doesn't appear to be the case.
There is more here than just wanting to save animals. Ask them for the studies and methodology, along with the actual data used to establish '10-20 million animals dead from lead poisoning'. I'm betting you won't get any of it, because it doesn't exist, or at least it's not anything that could be called science. More likely they find some dead animals, assume they died from lead poisoning, and then extrapolate that data out.
They don't need data because they know they can go to Congress and legislatures like California and New York and repeat this; the anti-gunners won't care about facts or truthfulness and will gladly take whatever these groups say at face value.
If you don't think these groups will just make things up based on the flimsiest of observations, see: DDT and Alar.