I've not actually seen one outside of the rules, but if I understand the spawning rules correctly creatures only spawn on undeveloped slabs. Which would mean a wildlife preserve actually prevents wildlife.
Would it be possible to have wildlife preserves coded to have an exception to the creature spawning rules? Not a coder so I haven't got clue.
From what remember creatures would spawn in cities a while ago. It just seems a bit crazy that building a wildlife reserve would guarantee you NOT seeing any creatures.