Priority–green island synergy: A testable hypothesis for insect-mediated invasional facilitation in Eucalyptus plantations

Non-native forest plantations can concentrate susceptible hosts, recurrently receive alien organisms through movements of planting material and thereby become focal points for biological invasions. Yet, whether an alien insect can directly alter host tissues in ways that facilitate a later alien insect remains poorly understood. We propose priority–green island synergy as a testable hypothesis for insect-mediated invasional facilitation. In southern China, the earlier-detected gall inducer Leptocybe invasa and the later-detected leaf gallers Ophelimus bipolaris and O. maskelli provide a candidate system. Leptocybe invasa galls alter nutrient, phenolic and phytohormone profiles and may generate transient physiological islands in galled or connected tissues. The hypothesis predicts that, after host genotype, tissue age, microclimate and propagule pressure are controlled, prior L. invasa attack should increase Ophelimus host acceptance, gall initiation or survival; that any benefit should co-vary with measurable physiological traits; that locally mediated effects should decline with increasing distance from the primary L. invasa gall; that the direction and magnitude of the response should vary with primary gall density and the interval between primary and secondary attack; and that any facilitative response should co-vary with realised gall development and associated physiological changes. Taken together, these predictions are designed to distinguish gall-mediated facilitation from alternative explanations for co-occurrence, including shared host susceptibility, propagule pressure, environmental filtering and variation in local adult abundance. We outline matched field surveys, controlled arrival-order experiments and physiological mediation tests that can evaluate the hypothesis and distinguish facilitation from alternative explanations. If supported, priority–green island synergy would show how an alien insect can engineer host tissues in ways that increase plantation invasibility. If unsupported, the same tests will delimit when plant-mediated interactions amongst alien herbivores are neutral or antagonistic rather than facilitative.