I would like to propose that iNaturalist allows users to assign multiple licenses to their contributions, whether they are observations, photos or sounds.
Like @murphyslab argued in an old comment, for contributions to be usable to both Wikipedia and GBIF, you’d have to use the rather liberal CC-BY-license. For some people, like semi-professional or professional photographers, that may not be acceptable.
One solution to this would be if iNaturalist allowed contributors to use (at least) dual licensing of photos (but it should apply to observations and sounds as well), as is relatively common practice in CC communities.
Personally, I’m generally fine with CC-BY-SA, if someone is able to make a business out of sharing stuff with my stuff in it, then more power to them. I am also fine with CC-BY-NC, but not alone, that’s too restrictive, and it also prevents my stuff from being used by Wikipedia, and I’d like to contribute there.
CC-BY is too liberal for some things, I have observations from old expeditions that may have some scientific value that I wouldn’t want to license with just CC-BY for others to make a monetary profit from without me having any rights to a share of that profit.
Dual licensing with both CC-BY-NC and CC-BY-SA is the ideal solution, as that would imply that users may comply with either license.
I am sure others can come up with other combination of licenses, thus the ideal solution would be a change to iNat that allows people to select multiple licenses.
Alternatively, I suppose that the dual license CC-BY-NC and CC-BY-SA could be added to the list of selections that one might make. It is not as flexible, but it would solve this exact issue right now.