The Dark Abyss Mouth is indeed not suitable for living; there's no light above, dark attribute energy everywhere, and bare rocks, with only a few unknown plants stubbornly surviving in the crevices.
The only place with light there is Fanshan Town.
But Fanshan Town relies on barriers and plants mutated sunflowers to provide light.
Once the energy barrier no longer blocks the dark attribute energy, even the mutated sunflowers will be swallowed by darkness and eventually wilt.
Yet, raising mutated creatures doesn't require much light, just daily feeding suffices.
Furthermore, the slaughterhouse can be built within Fanshan Town to form a breeding-to-slaughter industry chain.
These industries are not suitable for the populous Little Straw Mountain; separating humans and livestock is the best approach.
Of course, this also entails logistics and transportation issues.
However, with the two islands so close, it's not too much trouble.
