Conservation Status

I'd suggest modeling an "organism conservation status" type or something more inclusive sounding and then add it on a case by case basis. For example, I have imported Partners in Flight conservation status codes that are only applicable to birds in the Western Hemisphere. These should probably be added as part of CVTs with date and location properties. You could also add US conservation categories of threatened and endangered or subset it for state listings. 

I would create a CVT with status code (that links to the status classification system), date, and location at a minimum. For example, Bald Eagles are going through a US federal delisting period but may still be listed as endangered at the state level in some states and their status today in the US is different than it was a year ago. So, those three properties (code, date, location) should be sufficient but are also essential.  

I agree with Ed's suggestion about the model, but I would add a date for delisting as well. 

What I'd recommend is creating an "organism classification" type (or something similarly-named) in your private domain with the "conservation status" property, and try out some models there. Post back here (or on the data-modelers' list) when/if you want feedback. If it seems to work well, it could eventually get moved to the commons Biology domain.

I happened to be looking at my state's endangered & threatened wildlife list and they had both state and federal status.

So that might be something to consider.  Even if you only want to do federal level, it should be clear that it is. 

And something else I just thought of, is you'd probably need a country field, or source/location, so people knew where it was endangered/threatened. 

IUCN Red List http://www.iucnredlist.org/search/search-basic

 This site has a lots of useful info.