How does an exotic invasive insect pest impact island-endemic tree species?

Context: Forests worldwide are increasingly impacted by insect outbreaks, yet the mechanisms linking defoliation to tree mortality remain poorly quantified. Island ecosystems are particularly vulnerable, as endemic species often lack coevolved defences. On Réunion Island, the invasive psyllid Acizzia uncatoides has triggered severe defoliation of the endemic tree Acacia heterophylla. Questions: We ask how insect-driven defoliation influences the survival probability of A. heterophylla. Specifically, we test whether higher defoliation increases mortality risk and whether tree size and habitat modulate this effect. We also aim to quantify mortality risk over time under different combinations 1 of covariates. These questions address both ecological vulnerability and applied management concerns. Methods: We established and monitored eight permanent plots across two major habitats (mountain forest and subalpine vegetation) between 2018 and 2021. All trees were tagged, with repeated measurements of diameter, defoliation, and survival status. We applied a discrete-time Bayesian survival model to estimate survival probabilities. Predictions were generated across gradients of defoliation, tree size, and time, stratified by habitat. Results: Defoliation strongly increased tree mortality. Larger trees exhibited higher mortality risk than smaller ones, even under similar defoliation levels. Habitat also shaped outcomes: mortality rates were higher in the mountain forest than in the subalpine vegetation, after accounting for size and defoliation. Impact: This study provides additional evidence that exotic invasive insectdriven defoliation can rapidly increase mortality in endemic vegetation, highlighting the need for coordinated management and conservation strategies. Its insights can guide practices such as optimising monitoring frequency, refining the timing and scheduling of sanitary thinning, and supporting decisions on releasing a biological control agent.