Since challenges only allow new species, this makes them exponentially difficult depending on how long we’ve been using the app for. I don’t think this was the intention of the challenge. The challenges should allow resighting of a species. It should require that the species be resighted with the camera and not simply using an old photo.
To make challenges more difficult, you can allow for only one active challenge at a time. This prevents the user from simply accepting all challenges, and snapping species at random to get through most of them quickly. It would be good to be able to switch the active challenge. This would turn the challenges into a deliberate search. I think this was the original intention of challenges.