The 'source pool' refers to:
“Mainland or nearby habitat” for the 'source pool' refers to. This relationship follows from the ecological mechanism represented by the terms in the item, not merely from an association between their names. At equilibrium, species identities can continue to turn over even when richness is approximately stable. The model predicts a balance of rates, not an absence of colonization or extinction. The remaining alternatives—“Island itself”, “Island vegetation”, “Predation pressures”—refer to different states, processes, or scales and therefore do not express the same causal relationship. Island b
Ref: The Theory of Island Biogeography, MacArthur & Wilson, Ch. 2-4