Multi-layer networks reveal changes in plant-bird interactions driven by invasive species.

Biological invasions can strongly disrupt ecosystems, reshaping their structure and functioning. We investigate how two widespread invasive parrots -the rose-ringed parakeet Psittacula krameri and the monk parakeet Myiopsitta monachus- affect plant-bird interaction networks using a multilayer framework. Field data were collected over a full annual cycle in an area with both species, accumulating 288 h of observations and tracking 24,561 fruits from 576 plants. Parakeets modified networks by introducing novel interactions, increasing species turnover and altering modularity and nestedness. Acting as both seed predators and dispersers, they became central connectors, enabling native birds to access previously unavailable resources and increasing rare dispersal mechanisms. Their activities increased antagonisms and generated new interspecific interactions with numerous plant species. By exploiting plants not previously used by local birds, parakeets heightened the risk of secondary invasions and the spread of exotic plants. These findings underscore their dual roles in disrupting and restructuring ecological networks and stress the need to reassess their contributions in native and invaded ecosystems. Understanding their potential to facilitate exotic plant expansion is critical, as their ecological impacts will likely intensify with population growth and geographic spread. Comprehensive assessments are essential to predict and mitigate these far-reaching consequences.