Complex sentence: In that period, the bulk of the island came to be used for cultivation, so that the native habitat of "Oryzomys" was exterminated and it was reduced to competition with introduced rats in man-made habitats, which the latter are well adapted to.
Split into simpler sentences: In that period, the bulk of the island came to be used for cultivation, so that the native habitat of "Oryzomys" was destroyed. Thus, "Oryzomys" was reduced to competition with introduced rats in man-made habitats, which the latter are well adapted to.