"Mesopredator release effect" has been hypothesised as a possible indirect effect from eradications, where superpredator (e.g. 'cat') removal can generate a mesopredator (e.g. 'rat') increase which may increase the impact on their shared prey (e.g. 'birds'). Using more biologically realistic models for seabirds and introduced mammalian predators we show that mesopredator release only occurs in strongly top-down moderated (resource-abundant) systems. Even when mesopredator release can occur, the negative impact of more mesopredators is outweighed by the benefit of superpredator removal, allowing recovery of the prey population. The consideration of age-structured prey contradicts previous theoretical results for mesopredator release effect. Only superpredator eradication (not control) allows the population recovery of long-lived insular species.