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Keywords:

  • Acrocephalus warblers;
  • coexistence;
  • competition;
  • invasive species;
  • risk assessment

Not all non-native species have strong negative impacts on native species. It is desirable to assess whether a non-native species will have a negative impact at an early stage in the invasion process, when management options such as eradication are still available. Although it may be difficult to detect early impacts of non-native species, it is necessary to ensure that management decisions can be based on case-specific scientific evidence. We assess the impacts of a non-native bird, the Black-headed Weaver Ploceus melanocephalus, at an early stage in its invasion of the Iberian Peninsula. To do this we identify potential pathways by which competition for shared resources by Black-headed Weavers could lead to population declines in two ecologically similar native species, and generate hypotheses to test for evidence of competition along these pathways. Black-headed Weavers could potentially impact native species by displacing them from nesting habitat, or by reducing habitat quality. We found no evidence for either potential competition pathway, suggesting that Black-headed Weavers do not currently compete with the two native species. However, it is possible that mechanisms that currently allow coexistence may not operate once Black-headed Weavers reach higher population densities or different habitats.