Invasive Strawberry Guava weakens avian frugivory in native forests of an oceanic island

Frugivorous birds play a vital role in tropical forests by dispersing seeds and facilitating plant regeneration. The effectiveness of frugivores, including birds, as seed dispersers depends in part on their fruit removal rates. Although invasive alien plants (IAP) may influence the seed-dispersal role of birds in varied ways, few comparative field studies have dealt with avian frugivore-plant interactions, particularly in settings of advanced invasion on oceanic islands. We assess how IAP influences birds’ attack rates on fruit models, to explore the ecological and conservation implications for avian seed dispersers and the plants that they disperse. We studied and compared two protected native forests on the oceanic island of Mauritius (Mascarenes), each comprising an area weeded of IAP (since 1990s) and an adjacent area invaded primarily by Strawberry Guava ( Psidium cattleyanum ), the overwhelmingly dominant invasive woody plant in Mauritian lowland wet forests. We quantified avian frugivory in weeded and invaded forests using artificial fruit models made from non-toxic modelling clay, which were collected 72 h later to quantify avian feeding attempts (based on beak marks on the models). Moreover, passive acoustic monitoring was used to assess frugivorous bird assemblages and overall activity across forest types. This procedure was repeated 11 times over 1 year. Bird species richness was comparable across the two forest types, but total acoustic detection rate was higher in the weeded forest at both sites, with a moderate shift in bird assemblages in the invaded forests. Native bird species tend to be most negatively impacted by IAP. Avian attack rates on fruit models were about thrice higher in weeded than in invaded forests at both sites, and were positively associated with increased Acoustic Complexity Index (ACI) values, suggesting greater bird activity and foraging effort in weeded forest. Invasion by the dominant fleshy-fruited IAP reduces habitat quality for native frugivorous birds and weakens bird–plant interactions, potentially disrupting bird-mediated seed dispersal. This may be driven by two mechanisms: native fleshy-fruited plant species produce fewer fruits in invaded forests and/or forest penetrability by birds may be lowered by an extremely high stem density of understorey weeds. Conservation strategies should prioritise in situ control of IAP to reverse the lowered carrying capacity of birds, particularly given that only 8% of surviving natural habitats have been weeded, while the remaining 92% continue to be degraded by IAP. Weeding larger native habitat areas would promote natural regeneration of native plants, while fostering the recovery of threatened native bird species. Over time, this would enhance ecosystem resilience and reduce support for currently intensive and costly management interventions like planting, which largely constrain areas that can be managed for conservation.