As species accumulate on an island, immigration rates generally:
“Decrease” for as species accumulate on an island, immigration rates generally. 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—“Increase”, “Remain constant”, “Fluctuate randomly”—refer to different states, process
Ref: Biogeography, Lomolino et al., 5th Ed., Ch. 1-4