Something entirely separate from the difficulty of individual islands which affects the core mechanics of the game on a save-by-save basis (or, if that's too difficult to work out, just in general).
I have three difficulty settings in mind thus far.
-Easy: makes things generally easier, reduces issues with genetic sickness (maybe make it so that having the same immunity gene doesn't make your animals sick?), predators aren't as strong, prey is easier to hunt, rogue males appear less often/have better genes, food supplies are more plentiful, cutting grass has a higher chance of dropping nesting material, recessive genes are easier to breed in
-Normal: standard Niche, no changes
-Hard: makes things generally more difficult, makes it so nichelings are more likely to fall ill, predators are stronger, food is scarcer, more blue birds/babies can be stolen even if they have an adult watching over them? (may be unreasonable, idk)
If anyone has any suggestions on this front, feel free to make them. I'm terrible with adjusting difficulty, especially when it comes to higher settings...