Complex sentence: They found the tragedy of the commons not as prevalent or as difficult to solve as Hardin maintained, since locals have often come up with solutions to the commons problem themselves; when the commons is taken over by non-locals, those solutions can no longer be used.
Split into simpler sentences: They found the tragedy of the commons not as prevalent or as difficult to solve as Hardin maintained, since locals have often come up with solutions to the commons problem themselves. When the commons is taken over by non-locals, those solutions can no longer be used.