Mainlands are defined as:
“Original habitat of immigrant species” for mainlands are defined as. This relationship follows from the ecological mechanism represented by the terms in the item, not merely from an association between their names. Island biogeography explains species richness as a dynamic balance between immigration and extinction. Immigration generally declines as an island fills with species, whereas extinction rises as more species divide finite area and maintain smaller populations. The remaining alternatives—“Regions near oceanic islands”, “Desert ecosystems”, “High-altitude ecosystems”—refer to differe
Ref: Biogeography, Lomolino et al., 5th Ed., Ch. 1-4
